Troubleshooting old Century autopilots

Quite a few older light aircraft have old analogue autopilots installed, such as the Century III, or the Piper Altimatic IIIc that is installed in my own aircraft. These can be troublesome, so here are some tips on how they can be mended, based on my own experience and talking to others. DISCLAIMERS: (1) There are several variants of these autopilots, so the information here may be plain wrong for the version you have. I accept no liability for any errors in this information. (2) If this information helps you diagnose your autopilot problem, get the necessary remedial work done and signed off by a licensed avionics engineer. (3) Although the electronics in these autopilots are relatively simple and built with 1970s technology, due to over-zealous regulation only a few outfits are actually approved to repair them. Don’t take anything you read here as a suggestion to do it yourself.

First, some general advice:

1. Make sure you know at least 2 ways of turning the autopilot off in a hurry. Hopefully, you will have a quick disconnect button on the yoke. You can also use the switches on the autopilot control console, or the autopilot circuit breaker (it should be pullable), or in an emergency, the master switch.

2. Be wary of using automatic pitch trim. The Century manual says it is important, but it can also be dangerous. If you suffer a reduction in engine power when you have pitch mode or altitude hold mode selected, the autopilot will trim back. The same can also happen if you get a failure in the autopilot system (or it could trim full forward). Then, when the autopilot disconnects, you are in a stall which you may have difficulty recovering from, because the aircraft is so far out of trim. See this AAIB report for a sad example (although the autopilot involved in that case was not made by Century). Some modern autopilots don’t have automatic trim, instead they have LEDs telling you which way they want you to trim.

3. Don’t expect these old autopilots to perform as well as a modern digital autopilot – they are much less sophisticated.

Now, some specific problems:

1. Older variants of the Century III range have a 1D395 amplifier that uses a “gripple board” PCB (not fibreglass, and the vias are made from small rivets, and from soldering the component leads on both side of the board). They also use carbon composition resistors.  They can be very unreliable because of bad joints. If you are tempted to resolder some joints, bear in mind that (a) only a few repair stations are authorised to mend these boards, and (b) the PCB may be varnished – the varnish has to be removed before any soldering (it’s not the modern self-fluxing sort), and replaced afterwards using a suitable PCB coating spray, to protect against condensation.

Newer variants (e.g. Century IIIc and Alimatic IIIc) use a 1C515-1 or 1C515-2 amplifier. This has a fibreglass PCB with proper vias, and carbon film resistors, and is much more reliable than the older type.

2. If the autopilot basically works but tends to over-control, producing small oscillations in pitch or roll, then one possible cause is that the flying controls are sticky or stiff. As a result, the autopilot has to apply too much force to move the controls; and then the controls move too far. Having the flying control systems lubricated may fix the problem. If it’s overcontrolling in pitch, and you have automatic pitch trim turned on, try it with automatic trim turned off, in case it’s over-trimming. If it’s oscillating in roll when set to heading mode but not when  set to roll or nav mode, it could be caused by backlash in the DG or HSI (i.e. the DG or HSI doesn’t respond to small changes in heading).

3. Some of the connections between the different components are made using small round blue connectors with 4, 5, 7 or 9 pins. These connectors can become unreliable after many years, because the gold-plated split receptacles lose their grip on the pins. In my case, this problem occurred in a connection between the glideslope coupler loom and the main loom. When no glideslope coupler is installed, the main loom connects directly to the AI; but if a glideslope coupler in present, the glideslope loom plugs into the AI instead, and the main loom AI connector plugs into a connector that the glideslope loom provides. This connection is constrained by the glideslope loom to be only a few inches away from the AI, so it has to be made behind the instrument panel, where it is subject to heat generated by the avionics.

The original connectors were made by Amphenol, who no longer manufactures them; however there are substitutes made by Cooper Interconnect. One supplier in the UK of these is Farnell (Newark in the USA). The part numbers all begin with 126-, for example 126-214-1000 is the 4-pin inline locking plug, and 126-215 is the 4-pin inline socket.

4. You can do a number of autopilot tests on the ground. The AI needs to be steady, so you’ll probably need to start the engine, assuming a vacuum driven AI. If you engage Roll mode, then you should be able to get the yoke to turn left and right by adjusting the roll knob, and there should be a position of the roll knob somewhere near the centre where the yoke doesn’t turn. Likewise, if you engage Pitch mode, you should be able to get the yoke to move forwards and backwards by adjusting the pitch wheel, and stop when the pitch wheel is near the centre position.

5. If there is no yoke movement at all in one plane, the fault probably lies in the amplifier. I’ve heard of blown output transistors (which are easily replaced), and a blown transformer. If there’s no movement in either plane, then either there is no power to the amplifier, or the oscillator in the amplifier probably isn’t working – this could be due to a fault in the amplifier or in the wiring to the DG (the coil in the DG is part of the oscillator, unless you have a Bendix-King HSI, in which case the autopilot adapter takes its place).

6. What those tests won’t tell you is whether the autopilot is receiving and processing the pitch and roll signals from the AI. To do this normally requires a flight test. However, it may be possible to remove the screws securing the AI so that the AI can be rotated and pitched a little (get your engineer to advise whether this is safe to do). If so, then with the roll knob and pitch wheel set to produce no yoke movement, you should be able to make the yoke move appropriately if you rotate or pitch the AI.

7. If the problem is lack of pitch feedback, then the cause could be a faulty amplifier or a faulty connection in the wiring (it’s unlikely to be the AI, the sensor is just a coil). You can check this with a multimeter. I’ve measured the resistance of the sensor in the AI at 120 ohms in one case, and 80 ohms in another. [EDIT: 80 ohms is correct. The 120 ohms measurement was due to a bad Amphenol connector in the circuit.] You should be able to measure this between pins 14 and 15 of the 24-way Centronics plug that connects the 1C493 glide slope coupler (if present), or between pins 8 and 12 of the amplifier connector. [Note: (1) the Century documentation suggests that some models with older amplifiers may have a pitch filter unit installed between the AI and the amplifier, in which case this won’t apply. (2) Pin numbers on the amplifier connector, looking into its open end, are A thru S from bottom to top on the left side, and 1 thru 15 from bottom to top on the right hand side.] The other possibility is lack of pitch excitation signal to the AI.

8. If the problem is with pitch mode or altitude hold mode, could the fault lie in the 1C493 glideslope coupler? There is a relay in the coupler that passes the pitch signal from the AI and the altitude sensors straight through when the glideslope has not been captured, so there’s not much to go wrong. However, from the circuit diagram, it looks that the designers made the elementary mistake of not putting a diode in parallel with the relay to catch the back EMF when the transistor controlling the relay switches off. Unless there’s some sort of protection not shown on the circuit diagram, that transistor is liable to blow. If it goes short-circuit then the relay will be permanently on – although in that case, the glideslope engaged lamp should light. If it goes open-circuit, the glideslope will never engage. If you want to eliminate the glideslope coupler from the equation, you can replace it with a 24-pin Centronics socket – you need to link pins 13&15, 19&21, and 20&22.

9. There are 6 helical potentiometers in the control console to adjust autopilot performance (left bank angle, roll centering, right bank angle, down limit, pitch centering, up limit). These can be accessed by removing the front panel. See the maintenance manual for the in-flight adjustment procedures, and be sure to take a safety pilot or lookout with you. The pitch centering adjustment is also the one used to get the autopilot to hold the altitude you had when you selected altitude hold, rather than an altitude a little higher or lower.

9. If you’re planning on keeping the aircraft for a while, and your analogue autopilot isn’t working, you may be better off having a modern digital autopilot fitted rather than paying money to fix the old one.

That’s all for today. Please post a comment if you wish to share any experience of getting these old autopilots working.

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491 Responses to Troubleshooting old Century autopilots

  1. C Michael Hoover says:

    RE: Your Century troubleshooting post:

    Thank you! I was not aware of where to find the connectors that Century used until you gave me the info. Thank you again. Another tip that I may add: Century, and Mitchell before them, used a lot of electrolytic capacitors made by a manufacturer named Callins, (a is correct, not c). These were poorly sealed and tend to dry out internally and change capacitance and ultimately fail. I know of one reputable avionics shop that starts a repair by immediately replacing all of those capacitors with new high quality, high temperature (ne. Panasonic NHG or similar) of the appropriate value. I solved a couple of issues many years ago by doing the same thing.

    • davidcrocker says:

      Thanks for your feedback! Regarding the capacitors, on the amplifiers I have seen there are a couple of axial non-polar electrolytic capacitors, but the rest are blue and disc-shaped, tantalum electrolytic according to the parts list. Is it the blue ones that are liable to dry out? I’ve seen an amplifier in which one of them failed and apparently took one of the transformers with it.

      • silentflier says:

        Re Capacitors… The main culprits for leaks or drying out are usually black coloured and mounted “end on ” (axial or radial? I can never remember!) Sometimes it is possible to see a brownish ring that almost looks like a water stain on the PCB around the base of the capacitor. It’s nothing new – I’ve been changing these since the late 70’s !

    • rcall1ns says:

      I wish to differ with you regarding the quality of the Callins electrolytic capacitor. As a former member of the manufacturing corporation, I personally known that the seals where state-of-the art for the time period ! Please contact me if you wish details on their quality control test. Bendix would not have used the Callins brand if it was not the highest quality available for the time period.

      • Rick Dozier says:

        The important part you mentioned was “for the time period” These are old units. Parts used were probably never meant for, nor tested for as long as they have been in service..Hell Apollo moon shots were “state of the art” in the day. Now my watch has more power!! Things change. There are only so many ways to test things before you put them out to production. I do not think anyone was inferring inferior quality of your product but rather today’s products do the same job as well if not better with all the new attributes such as longevity of materials of a new part. No one expects your stuff to last forever. It was good for as long as it did last . Be proud of that. I would not take offense at that track record, but rather think of it as a compliment to your old product that lasted so long.

  2. Vern Eliason says:

    I have a 1973 E model Aztec with an Altimatic iii B 1 and I am having fits with my auto trim. It will trim up but not down. The pitch servo works with the yoke control and pitch wheel on the auto pilot but in flight, it will not adjust to bring the nose down. Up works just fine. Any suggestions would be appreciated. Or call toll free 866 999-0949.
    Email me at

    • davidcrocker says:

      The auto trim system comprises a trim sensor and trim motor. There may also be a relay and/or a trim amplifier. The trim sensor is a switch that is moved by the difference between tensions in the up and down elevator cables (see fig. 1-17 in the service manual). It has separate contacts for up and down. I suggest you get an engineer to check whether the switch is operating correctly in both directions, and whether the cables and connectors between it and the other components are in good order. According to the manual, the trim sensor on the PA23 is “under floorboard and middle seats, pilot’s side”. If your system includes a pitch trim amplifier, the fault could also be caused by a blown output transistor in that amplifier.

  3. Ronnie Hughes says:

    Nice to find this thread. I’ve brought a Piper Altimatic II back to life. This is the fore runner of the III and used RF signals between the AI and the amplifer. However later versions used the 52D54 DG and a 5 KHz signal between it and the 1C292 radio coupler. My system works great and hopefully I have enough spares to keep it running for a good long time yet. I’ve been searching to see if Mitchell ever made a glide slope couple for this autopilot, but so far, I can’t find any reference to one. This system differs from the III in that there is a DC signal between the 1C292 radio coupler and the 1D293 amplifier. If a glide slope coupler ever existed, it would also have a DC output to drive into the 1D293 amplifer.

    Thanks again for your blog.

    Ronnie Hughes
    Austin, TX

    • davidcrocker says:

      Hi Ronnie, thanks for your comment, and congrats on bringing your Altimatic II back to life. The Century II and III autopilot manual I have doesn’t mention a pitch channel option for the Century II at all, so unfortunately I can’t say whether a glideslope option ever existed. Regards – David

      • Jerry schwartz says:

        My alternator light won’t go on when alternator switch goes off–if I push check button all warning lights go on when alternator engaged. Need help. 1977 turboarrow

      • davidcrocker says:

        Sounds like either the 50 ohm resistor has failed, or the diode between the 50 ohm resistor and the alternator warning light has failed. Most likely, it’s the resistor. When I had a 1980 Turbo Arrow, instead of a 50 ohm power resistor with a heatsink, it had five 220 ohm 2W resistors in parallel, in a bundle. Not surprisingly, several of them had failed. I replaced it by a 50 ohm power resistor (which is what the only illustration I found in the Piper documentation showed), rated at 6W and riveted to the aluminium plate.

    • Larry Wilson says:

      Ronnie I have a Altimatic 2 but the autopilot will not follow the direction gyro I sent it off to be rebuilt but did not help it. The autopilot it self works fine I just cant get it to follow the direction gyro. Any ideas
      Thanks Larry Wilson

      • Ronnie Hughes says:

        Laarry, which directional gyro do you have in your plane? The Altimatic II used two different types over the years. The first was the old large format DG with the heading hold or heading dial (Mitchell PN 52D27). This style has an RF capacitive pickup which feeds into the bank resonant circuit of the amplifier. If the autopilot responds to the tirm knob and banks left or right and you have this type of DG, then thr problem is likely an open cable between the DG and the amplifer.

        Later models used the Mitchell PN 52D54 which looks like a modern 3″ DG with heading bug. I am guessing you have that DG since your comment sort of implies that you have a heading bug which the autopilot does not follow.

        If my assumption about you having the 52D54 DG is correct, then you also have a Mitchell 1C292 radio coupler. This radio coupler contains an oscillator circuit which works in conjuction with an inductor and capacitor in the DG to resonant and generate a sine wave signal of approximately 5KHz. The heading bug in the DG moves a pick off coil allowing the DG to feed back to the radio coupler a modified signal which indicates where the heading bug is in relation to the 12 o’clock position of the DG. The radio coupler compares the amplitude and phase of this signal to the original signal to determine if the heading bug is centered or left or right of center and if so by how much. The radio coupler converts this into a DC signal which is fed to the 1D293 amplifer of the autopilot. When the radio coupler is in heading mode, this signal will range from +2V to 0V to -2V as the heading bug is rotated from 90 degrees left of center to center to 90 degrees right of center. When the radio coupler is in NAV, OMNI, LOC NORM or LOC REV, the coupler will also combine the left-right error signal from the nav radio source with the heading error and generate a left-right DC signal for the autopilot amplifier which includes an intercept heading. Once on course, it will also develop and hold a wind correction angle over time to keep the airplane tracking the nav source. In short, the radio coupler is a very niffy analog computer which was cleverly designed using minimum components.

        Also, you mentioned that you had “it” overhauled, but I’m not clear what “it” is. Was it the DG that was overhauled or some component of the autopilot? If the DG was overhauled and you still have an issue with the autopilot not responding to changes in the heading bug, then the issue is either in the 1C292 radio coupler or in the wiring between coupler and the DG or between the coupler and the amplifier. If the autopilot is working as a wing leveler and responds to the bank trim knob and banks the wings left and right, the likelyhood of it being a problem in the 1D293 amplifier are slim.

        The connections between the radio coupler and the DG and amplifier are on connector CD-31 as follows:

        Pins A & B = heading input from DG
        Pins D & E = exitation to DG
        Pin E & K = ground
        Pin J = +12V
        Pin F & H = +/-2V signal to amplifier

        (Connector CD-34 in the input from the nav radio).

        If possible, check for a 5KHz signal between pins D&E. Look for a varible amplitude signal of 5KHz between pins A&B. The amplitude should vary as the heading bug is moved left and right. These checks must be done with the DG connected. If you can’t see any signal here, expect that the oscillator transistor (Q-1, Q-2) or the transformer T1 in the radio coupler is bad. (Again, this assume a known good DG and good wiring between the DG and radio coupler). Also check that CD-33 is connected at the DG.

        If you see good 5KHz signals between DG and coupler, check the DC output between pins F&H. With the coupler in HDG mode, you should see a DC level between -2V to +2V as the heading bug is moved left and right of center. If not, then there is a failure in the radio coupler. Possibly a transistor, diode or failure of transformer T-1. Also, the electrolytic capacitors I’ve looked at in a couple of 1C292 couplers had failed.

        Also check this signal at the end of the cable going to the amplifer, connector CD-32 pins A&B. If the signal is at the amplifier end of CD-32, then the issue is in the autopilot amplifier.

      • Larry Wilson says:

        I tried to email you at is that a good email are could we swap emails where I could talk to you. It looks like you are in Austin tx I am out of Pearland,tx. My email is Phone number 509-420-6976, Where could I get a wiring diagram for this autopilot at. I appreciate the come back I had Bob Furgerson in Tulsa, OKC rebuilt my computer a long time ago before they quit working on the Altimatic 2 and he said he had sold the test rig to somebody but that panned out no good. Is there a way of buying a used Altimatic 3 and connect the servos for the Altimatic 2 to the altimatic 3.
        Have a great day
        Larry Wilson

  4. Keith says:

    Great write up! Any thoughts on a CIII which seems to grab the glideslope, but goes below it (a lot)?



    • davidcrocker says:

      I would first check that altitude hold is fully working. At a safe height, not in LOC mode and with autotrim off, push the yoke forward to override the a/p until you lose say 200ft, to simulate going below the g/s. Does the a/p then recover to the original level? If no then it’s a problem in the pitch channel, probably the amplifier. Otherwise it’s likely to be a problem in the glideslope coupler, so borrow a known good unit if you can and try it instead of yours.

  5. Ferdinand leygonie says:

    Does any one have maintenance manual for the Altimatic IIIc, my Piper lance also dont want to do pitch control and takes a dive on altitude hold. Here in the African Sticks its a major problem getting any information or service done.
    Ferdi Leygonie
    Swakopmund Namibia

    • davidcrocker says:

      You can buy paper maintenance manuals for the Altimatic IIIc (part no. 761-602) and the trim amplifier (68S94) from Essco and elsewhere. However, the Altimatic IIIc is basically a Century III with a different autotrim system, and I’ve see the Century III Maintenance Manual (part no. 68S54) freely downloadable from various sites in the past. The 68S54 includes circuit diagrams of the amplifier and other components, unlike the 761-602.

    • Damion Frasier says:

      Did you solve your problem? I have the same issue with my Altimatic IIc in a Piper Seneca II. Thanks.

  6. Troy Zatorski says:

    I have a Century III that has lost the altitiude hold. I tried to check the wiring between the controller and the Alt Can, but it doesn’t match the install manual. What confuses me is the fact the wiring doesn’t match (there are no broken wires) but it worked at one time, just recently.

    • davidcrocker says:

      I would first check that pitch mode is fully working, to rule out the amplifier and the connections between the amplifier and the AI. If pitch mode is OK, then try to borrow another altitude capsule. When checking the wiring between the altitude can and the controller, bear in mind that if you have a glideslope coupler installed as well, then the basic wiring is modified by the insertion of the glideslope harness – which is shown in a separate wiring diagram..

  7. It’s great to find this post, and this blog. Thank you, David, for all the feedback here.

    My Century III is original equipment (I think) in my 1966 PA-24-260, and has been only intermittently functional for many years. When it has worked, it does a very stabile job of holding course and pitch, less so altitude. It is not coupled to the glideslope, and I am not interested in ever doing so.

    When it does NOT work, the rocker switches on the controller simply won’t activate the autopilot. I’ve examined and cleaned the connections on the back of the unit, and have discovered that if I simply slap the bottom of the 1C404 controller (as it dangles in place on a ground check) that I could make the unit “wake up” and work just fine. On reassembling the panel, it would even continue to work for the rest of the day’s flying (albeit, VFR only, and with a very distrustful pilot watching over it).

    My best guess for making this fully reliable would be to replace one or both of the relays (Century part number 80S13) that I’ve found inside the controller (1C404) , or one, some, or all of the four micro-switches in the rocker switch assembly (Century part number 40C42). I say so because this kind of “percussive maintenance” most often points at some electro-mechanical component, and those are the only ones inside the box.

    My avionics shop has suggested that I send in both the 1C404 and the computer (we think it is likely a 1D395, probably hiding behind the baggage compartment wall), and they will do what they can, and bench calibrate the two units working together, but as I’ve asked them about putting in NEW relays or micro-switches they’ve been honest about how hamstrung they are. They can neither locate original parts, nor make substitutions. They could scavenge old parts from other failed units, but those could be as faulty as what’s in there right now, short-lived and unreliable. They will only warranty their work for 60 days, and only for the specific components which they replace while they have it. That’s fair, but not what I’m looking for. I want this thing fixed for longer than that.

    Of course, I could be barking up the wrong tree with my electro-mechanical component theory. It might also be a cold or corroded joint, or an intermittent junction inside a solid-state part. I’m not in a position to chill or heat test individual parts on a bench rig, as my shop would be. It’s essential to have a good relationship with an ethical and fair shop. But it does bug me no end that they are stuck with having to scavenge a dwindling supply of archaic components in the repair of a system that should be working its very best for safest flight.

    To boil this screed down to a conclusion, I’d be very grateful for any feedback on the generalities of this story, and I would lick clean the boots of the wizard who has the right insights or answers for solving my specific problem.

    Terrific blog, David. Thanks again for having posted this thread…

    • davidcrocker says:

      Hi Elliott, good to hear from you. I’ve not heard of a 1C404 failing before. Have you tried waggling the cable to see if you can make the unit start or stop working? That could show up a problem with the connector or with one of the soldered joints to the connector. Otherwise, I agree with you, it is most likely a problem with the main relay in that unit. Intermittent connections inside solid state parts are very rare, and there are very few solid state parts inside the 1C404. If the relay is no longer manufactured, then it should be possible to find a modern replacement for it, although getting approval for such a replacement may be a different matter. I have access to a harness for bench testing these autopilot components, but unless you live in the UK then that won’t be any help to you.

      • I did find and fix my intermittent controller problem, and it was ridiculous how simple it was: One of the lugs on the CIRCUIT BREAKER for the autopilot was loose, so power to the entire autopilot was intermittent from the main bus. I retightened the connections on the back of the breaker, and voila, it all worked perfectly for about 8 months.

        It hurts to think of all the years I’ve flown that airplane with a flaky autopilot, due to such a simple problem, but not to worry, a new problem has just cropped up! This one is downright dangerous. I’ll write about it in a new thread.

        Thanks again!

  8. Steve Von Gruben says:

    Hello, David, thanks for taking your time to work autopilot problems via internet.
    I have an Altimatic III in a 1967 Twin Comanche that has been working fine until recently. The heading function has stopped working entirely, even when selected to nav. The pitch functions are working normally. Can you suggest a starting point?

    • davidcrocker says:

      First check whether it works in roll mode. Does it response at all to the roll knob? (you can check this on the ground, provided the AI is approximately level). If the answer is no, and your amplifier is the 1D395 type (which I think is almost certain for that vintage autopilot), the fault probably lies in the amplifier.

  9. Jim Davis says:

    Hello. I have a 78 PA32RT-300 Lance with a Altimatic IIIc autopilot. and a Garmin 430 GPS. If I have the AP in “Nav” mode, and the heading bug on the DG is nowhere near the course I am flying, the AP will try and catch up to the bug and THEN try and correct for the course… If I put the bug close to the heading, it will fly the GPS (or VOR) just fine. Also, it seems the ILS mode doesn’t work at all. Any ideas? And thanks for your time!

    -Jim Davis

    • davidcrocker says:

      Hi Jim, I believe that’s expected behaviour on aircraft not fitted with an HSI, but check with the autopilot user manual. The autopilot needs to know both the current CDI deviation and the angle between the current heading and the desired track. On aircraft without an HSI, you need to set the heading bug the same as the OBS if you are tracking a VOR, or to the ILS course you want to intercept and follow. It’s only if you have an HSI fitted that the course selection information is available directly and the heading bug is ignored when not in HDG mode.

  10. John Fletcher says:

    I have a ceentury IIB auotpilot and it works fine except in the nav mode. It wont track the VOR or the localizer. Any suggestion where to start looking?


    • davidcrocker says:

      Assuming it works OK in heading mode, I would check the wiring from the VOR/LOC receiver to the radio coupler. If that looks OK then I would try swapping the radio coupler with a known good unit.

      • John Fletcher says:

        Thank you. Yes it works fine in the heading mode so will check wiring and radio coupler.


  11. Dwight Wilcox says:

    I have a Century 31 A/P system in my Mooney J model (built 11/81). The problem described is intemittent – sometimes all is well, most times the problem exists – either all the flight, or suddenly starts happening. Symptoms – turn on A/P, yoke moves back, few seconds later, the trim wheel starts a rapid nose up. Sometimes it is ‘yoke moves down, then trim moves nose down’. This is without engaging Alt Hld on the A/P computer. Other occasions, in cruise, A/P on with Alt Hld, and maybe after an hour or two, then the described movement of yoke and trim wheel will occur.

    The computer has been checked, the AI has been replaced – two times – and the AI connector has been rebuilt. Pins in the computer frame have been checked for tightness.

    Century calls the complete system ‘Model AK 884’.
    Any suggestions for a fix would be appreciated.


    • davidcrocker says:

      I haven’t any experience with the Century 31. I see that it has a completely different control console from the Century III series, so I suspect that it is a completely different autopilot. Assuming it is in pitch hold mode when the trouble occurs, then it sounds like a problem with pitch feedback from the AI to the computer. As you have had the AI and computer checked, I would suspect the wiring between the two. If it were a Century III variant, I would first check whether the receptacles in the two round blue connectors (one on the back of the AI, the other where the GS harness connects to the main harness) are gripping the pins tightly. Then I would bypass the GS coupler in case the fault lies there. This is exactly the problem I experienced in my own aircraft, and the problem was the connection between the GS harness and main harness. But in a Century 31 installation there might be a single harness. The maintenance manual should tell you.

      • Dwight Wilcox says:

        Thanks. I’m asking the factory to send me a system diagram for my installation. Hope to get more into it next week with an avionics ET. Will let you know if we find the problem.

      • ray says:

        can you suggest something to do,,,on a century 31 it wont hold heading
        holds altitude fine,the controls all lock in,,,just wont recognise the heading bug of the hsi
        any help is appreciated

      • davidcrocker says:

        Sorry, I have no experience of the Century 31.

      • Dwight says:

        I have a Century 31 that caused a lot of grief for a long time. Finally sent the computer to AutoPilots Central in Tulsa, OK. No problems since. Not suggesting your problem is in the computer (I’m guessing HSI output problem, or connection issue between HSI & A/P). However, the Tulsa folks seem very ‘up on things’ and I suggest a call to them.

        Dwight w/31 in ’82 M20J

  12. Eric Davis says:


    Thanks for this post. Wondering if you have any thoughts regarding a problem I’m having with my Century 2000AP. All functions work except altitude hold. When engaged, the indicator lamp illuminates and sometimes the system will maintain the desired altitude. When the failure occurs, the AP will command a slight climb or descent. If the failure mode is to climb, I will let the system go to see if it captures an altitude that is near the desired altitude. Eventually it will command (via the panel lights) some up trim.

    I have had the static system checked for leaks (none) and we had the AI overhauled (it was time anyway). In fact we’ve tried three AIs all with the same result. The problem has only gotten more frequent. Our avionics shop is at a loss and wants us to send the unit to Century.

    I decided to crack the unit and have a look at the pressure transducer. I was excited to find that the brass nipple that connects the static line through the rear of the unit was completely clogged. The hose that connects from this nipple to the top of the pressure transducer was also full of green gunk. After clearing these lines I flight tested and found the problem still exists.

    Based on the presence of this material in the static line within the AP chassis and my symptoms, I am still of the opinion that the pressure transducer is at fault. The unit is a Foxboro ICT (pn 1800-L644-03A), which is now manufactured by Honeywell. Because the part is so easy to get to, I am considering de-soldering and replacing the unit. Do you have any thoughts on this?

    Thanks and Regards,


    • davidcrocker says:

      I’m not familiar with the Century 2000, however from your description I think the problem may well lie in the pressure transducer. On the Century III range without altitude preselect, I believe there is a valve which shuts off one side of the transducer from the static vent when altitude hold is selected. If your system also uses such a valve, then a leak in this component could also cause the symptoms you describe.

      • Eric Davis says:

        Thanks. I thought you and your readers might like to know that I did find the problem here. Actually there seemed to be two problems. The static line blockage was probably a contributor, but the more interesting find was a cracked power transistor on the pressure transducer PCB. The part is an 2N6045 power transistor used to amplify the signal coming from the transducer itself, which is a piezo-electric device.

        I removed the defective transistor and soldered on a shiny new one. I test flew today and now the plane flys like it’s on rails. Altitude hold functionality is fully restored.

        I hope others find this information useful as no one, including my avionics guy, had any idea what could be causing the problem. I have read of similar problems online with the same symptom and the focus seems to be on pitch and bank signal errors coming from the attitude indicator and/or slipping clutches followed by expensive board swaps in Mineral Wells.

        For me, the telling clues that none of these were the problem were the following facts:
        – attitude mode worked perfectly so the servos were not the issue.
        – when in altitude hold, wings would stay level so likely not a problem with the AI
        – when in altitude hold and the plane was climbing, the AP would ask for up trim indicating that its logic state was in fact commanding a climb.
        – the previous fact led me to believe that the AP was getting an incorrect signal from the pressure transducer, or its state machine was suffering from a defective logic gate elsewhere
        – because the pressure transducer is an electro-mechanical device it seemed like the likely place to look for a failure first, as opposed to faulty logic gates elsewhere.

        Admittedly, I got pretty lucky as the problems with the transducer subsystem were visibly discernible – contaminated static line and cracked power transistor case.

        Best Regards,


    • Steve Carter says:

      Is your 2000 a trim prompt, no electric trim version?

      Steve Carter

      • Eric Davis says:

        Hi David,

        Yes it is a trim prompt version, no electric trim motor. By the way, since applying the fixes noted in my last post here the altitude hold function has been rock solid.


      • steve carter says:

        Steve Carter here, thanks for the reply. Do you know if the trim prompt version has the same board as the eletric trim version. I recently added an Aspen EFD 1000 with digital to analog converter but seems a little slow to capture/hold altitude. Any ideas would help.


      • Eric Davis says:


        According to the service manual, the PCBs are different for installations with trim servos and those without. The former is referred to as the “AUTOTRIM OPTION PCBA” and the later is “TRIM PROMPTING PCBA”. Either PCB plugs into the same connectors within the chassis.


      • steve carter says:

        Do you know if mine has the transistor you refer to in your post as well? May need to check it out. Aspen output voltage is per spec. from Aspen for all other commands. The altitude hold works but the trim servo is just not responding as quick as it was (with Analog unit installed). I could be ascending 1500 FPM and push Alt. Hold and aircraft would nose over immediately. Now it flys past that point and returns slowly to selected altitude.


      • Eric Davis says:


        I should probably let you know at this point that I am not an Avionics Technician. I am a pilot and Electrical Engineer so I cannot advise with authority.

        Your unit will also have the transistor within the pressure transducer sub-assembly. However, it does not sound to me like that is your problem. The signal from the pressure transducer is only used to maintain altitude. When you’re flying attitude mode on the AP, it uses the pitch signals from the AI to maintain a constant pitch. In your case, the digital (ARINC) signals from the Aspen are converted to analog signals, using a digital-to-analog converter (DAC), simulating the analog signals that would otherwise come from the AI.

        All this is to say that if altitude hold is working, the pressure transducer on your AP is fine. If attitude mode is working strictly in terms of pitch up/down as commanded, not necessarily response time, then the signals from the DAC are good and so is your autopilot.

        Does the transition from a climb or descent to level attitude result in oscillations through the intended altitude and then eventually dampen to zero deviation from the desired altitude? If so, there could be some partial blockage of the pressure line that serves the AP. In my case, the line inside the AP case was partially blocked with what I think was mold (I live in the humid south). I could blow air through it but only with much force. This partial blockage will cause the rate of change on the transducer side of the blockage to equalize with static pressure outside the aircraft more slowly resulting in the oscillating behavior until the pressure on both sides of the blockage have equalized. In effect, it converts the pressure transducer from an altimeter into a vertical speed indicator.

        If there is no oscillation like I have described here, but instead only a delay in AP response to a pitch change from straight and level, or vice-versa, then there is clearly some latency being introduced by the Aspen unit, the DAC, or both. If the response time was instantaneous when your AI was installed, then the AP is not the cause. How about changes to bank angle, are they also delayed using the Aspen for control? I don’t have an Aspen so I don’t know if it has a heading mode to drive the autopilot, but if it does is there a noticeable lag in AP bank angle response to changes to heading via the Aspen?

        According to the Century Maint. Manual, the pitch reference signal from the AI is a 5kHz sine wave that is scaled at 6 mV RMS per degree at nominal excitation. I believe you would see the RMS amplitude increase from nominal for positive pitch deviations and decrease from nominal for negative pitch deviations. It may not be practical to measure, but this is what the output of the Aspen DAC should look like as an input to the AP.

        Good luck and let me know how it goes. I am planning to have the Aspen installed and would like to hear about your results.

      • steve carter says:

        I’m not an avionics person either, did not sleep at Holiday Inn last night but the info you have given is good stuff. I have had century equipment in various aircraft over the last 35 years or so and have been to their shop numerous times(money squared) for service (some good some not so good) The 2000 is in an experimental SX 300 so no worries on our qualifications or experiences. I removed the NSD 360A for the Aspen for upgrade. An avionics shop did the new harness and tied the Aspen in. When it wouldn’t do what it was supposed to I took it back and found 3 bent pins the static line blocked, and the computer not seated in the tray. So much for avionics techs LOL.Thanks again for the input and help.


      • steve carter says:

        I will get back to you after I fly it some more.


  13. davidcrocker says:

    Glad you got it working!

    • Steve Carter says:


      Any experience with the century 2000 yaw damper and fixing it? Mine is not moving the rudder in flight when I adjust the pot. CW/CCW. It has been to century once already for this and they found the tube/vile that carries the fluid that causes a signal to out put to the servo was dry! No Fluid in it! It’s an IC 753 Yaw damper amplifier. I am tempted to just take it out and apart to see if this has happened again. Wonder what kind of fluid these things use? Got any ideas?

      Thanks Dave !

  14. Steve Carter says:

    Have Century 111 with electric trim in 1979 Bonanza. Just flew 7 hours flawlessley and took out today only to find heading bug not working. Re cycle breaker and AP and try to command roll with just roll engaged and nothing. Servo grabbed it from me and right wing down here we go. Dis engage AP after overpowering roll upset and fly back to base. Go through ground checks and find servo motor running/wherring for roll but no movement on yoke left or right with bug or roll command knob. This just appeared! Any ideas?

    Steve Carter

  15. Steve Carter says:

    Thanks David

    Checked roll servo and found broken bridal cable.Post that prevents cable from coming off on forward side bent. Maybe bridal cable too loose? Servo still works. Bridal cable looks like pully must come off to be replaced. Suggestions?


  16. Hi david
    I have a automatic 111B in a 69 PA30. I think it came from Piper with 1D395 amplifier and it has been changed to 1C515-1 amplifier. It has worked great up until now. In the altitude hold mode it works good,but when you turn off the altitude sel, and try to decend with pitch commond knob the altitude sel. dose not disgage.
    What’s the difference between the 1D395 and 1C515-1.

    • davidcrocker says:

      Does the pitch mode work if you engage it by itself from the start? When you turn the altitude hold off and on again, does it feel as if it is engaging and disengaging positively on the control panel?

      The main improvements of the 1c515-1 over the 1d395 are the use of an epoxy glass pcb with plated through holes (which is much more reliable than the old gripple board 1d395) and the addition of a lag filter and rate circuit in each channel for better performance.

  17. Steve Carter says:


    what is the install procedure for roll servo bridle cable on C111? don’t find it anywhere in the book.

    steve carter

  18. Ian Wilson says:

    David have you any ideas or suggestions?

    I have an Altimatic IIIc in a PA23. It has worked flawlessly until a recent annual inspection…
    It has the later 1C515-1 amp with radio coupler & glideslope coupler.

    It was initially passing all ground checks fine but on flying would roll left every time- we tried making an adjustment on pot B on the console but it made no difference (and oddly turns without an obvious stop to it – is this normal?) Anyhow after endless hours of searching I traced it back to the roll signal wire broken at the AI amphenol plug (probably after the AI was taken out for a routine overhaul). Oscilloscope measurement confirms excitation wave between pins B & C.

    Anyhow, today thinking I’d fixed it I checked it again, once again passes all ground tests. In flight it is possible by keep moving the roll switch left and right in roll mode, or in heading mode keep moving the heading bug left and right of the lubber line to roughly fly straight. If I set a heading on the DG and wait, the aircraft eventually deviates slightly then over banks to try to correct – forming an increasing oscillation – which would get out of hand if left. (I know the amp has not been touched). Once again potentiometer B on the console makes no apparent difference to the tracking of heading, and I have not altered potentiometer A (left bank) or C (right bank) as they were set fine before.

    Any ideas? It seems I’m getting signal, but I think there is something wrong with the comparator somehow. How much adjustment and how obvious should the roll centring adjustment on pot C be (should I pull the console and inspect the potentiometer?) Could the radio couple be U/S and affecting the system?

    Please help, I’m at the end of my tether with it – and its sad as prior to the annual it was a great, perfect autopilot I used to love using to reduce workload IFR.


    • davidcrocker says:

      It sounds to me like lack of feedback from the AI to the amplifier. You say you found a problem with the Amphenol socket where the harness connects to the AI, but did you check the Amphenol plug/socket where the GS harness connects to the main harness? If you trace the cable from the AI, you will find this connector after about 15cm of cable. You will find the roll centring adjustment very easy to make after you’ve fixed the problem, just put the autopilot in roll mode and adjust the pot to centre the yoke so that the aircraft flies level.

      • Ian Wilson says:

        I’ll check that plug out. Signal is getting to the amp though now since fixing the AI plug, because It doesn’t roll left all the time- I can roll it left or right or balance it by slightly jiggling the roll knob left and right of centre!.

        Does the roll signal go through the glide slope coupler? I thought it bypassed it straight to the 1st summing point in the amp?

        How many turns can one make on those pots usually? How many are normally needed to centre? Am I being too careful with it? I’ve tried 2 turns. Is the pot broken?

      • davidcrocker says:

        If that plug is loose, the signal may be getting through intermittently. The roll signal does not go through the glideslope coupler, but it does go through a short part of the glideslope harness (the bit between that plug and the AI). The adjustment pots on the console are 10 turn or 20 turn, I can’t remember which. Once you’ve reached one of the ends of the pot, it will continue to turn but the pot slider won’t travel any further, and on the ground you may be able to hear a faint click once per turn as the mechanism jumps a screw thread (as it is designed to do).

  19. Ian Wilson says:

    Cheers David- I’ll take all those thoughts with me at the weekend when I next attack the system. I’ll pull the console as well I think and check the pots are working fine as well. Thanks for advice on the harness, it really is like spaghetti junction in there- all held away from the moving bits by cable ties!
    Do you know anyone who happens to have the 66D141 test set? I bet that would soon help identify the errant cable or component for some of the guys here.

    Is it possible to email you so I can share some video of what is going on when I flight test perhaps?
    You seem to have a good working knowledge of the system- are you an Avionics guy?

    My mail is ian dot wilson at me dot com

    Best wishes

  20. Heliwest says:

    Good article, thanks!

    I’ve got a Seneca II with Autocontrol IIIB. My problem is that the correlation between the heading bug on the HSI (KI 525A) and the autopilot inputs is off by a few degrees. For example: to fly a heading of 090, i would set the bug to 097 (off to the right of my desired heading to be flown). I took off the faceplate to see if there were adjustments that could be made and found 4 inset screws above the P/N: 1C338-3. Any idea on weather this is possible?

    Thanks, much appreciated.

  21. Jay Sterrett says:

    Hi Ian,

    I just purchased a ’73 Aerostar with CIII. is it possible to add Alt pre-select?



    • Ian Wilson says:

      I’m not sure there’s an easy way to do this. The cheapest way might be to add a STEC unit to what you already have – not Ideal because you then are using one unit for roll and one for pitch – bit messy.

      There is a Century unit I have seen in my friends Commanche which has altitude preselect but I’m not sure even of the model no. I’ll try and investigate. My guess is that you would end up with a more extensive install to do as well as at least a 337 field approved mod paperwork or STC.

      I’ll look into it more and let you know what I can find out.

  22. Ian Wilson says:

    Okay we checked out the autopilot in the Comanche. It’s a Piper Altimatic III. Looking at the data collected so far by the A&P in his notes it looks like most of the components are similar to an Altimatic IIIc installation, but the console is different. Presumably the altitude hold and preselect trickery is carried out in the console unit. Sorry no part no, for the console discovered as yet.

    On my own autopilot front, we eventually traced a broken wire in the glide slope loom and two fractured wires in the AI plug – problem was they were still stuck in the insulation sleeve covering the wire and pin so visually all looked well. It was only on continuity testing that the breaks became obvious on wiggling the wires.

    Thanks so much to everyone for their help and encouragement – I can confirm problems are fixed and I’m up and running again!

  23. Pierre Tschumi says:

    Cheers gentlemen,
    Testing my Altimatic III on the ground with engines running, the roll comman and pitch mode are testing correctly. In heading mode, with the heading bug centered on the HSI, the yoke stays still. Moving the heading bug leftward, the yoke follows just fine. However I cannot stop it, it moves to its stop, eventhough I move the bug back to the opposite side.
    In flight, roll command, Altitude Hold, pitch command fonction properly. In heading mode, the Seneca 2-200T keeps turning left. I’m not sure about the NAV function.
    Does any one have an idea where to look for?
    Thank you

    • davidcrocker says:

      I think those symptoms are most likely caused by a problem in the amplifier, so I suggest you try to borrow a known good amplifier and see if that fixes the problem.

      • Ian Wilson says:

        Is the autopilot working fine in roll mode? I’m sure David will correct me if I’m wrong but surely if his AP works fine in roll mode his amplifier is okay, its more likely a wiring, radiocoupler or DG issue?

        Also when you say in heading mode it rolls left in flight, does it stop at approximately 20 degrees left bank and just keep turning or would it continue rolling past 45 degrees if you don’t disengage the AP?

        If so, I wonder if the radiocoupler or wiring may be more likely the issue here?

        It might be worth bypassing the radiocoupler – it comes in between the DG and the console if I remember right, I think you can remove it from the harness which should allow you to test the unit in heading mode. It might also be worth checking the wiring from the DG to the console for continuity also.

        While testing in heading mode in this manner, see if the roll command knob has any action – it shouldn’t and so if it does cause any change the console is faulty.

      • davidcrocker says:

        Hi Ian, the autopilot amplifier has separate inputs for AI roll input and DG input, and the DG input is processed by a separate coherent filter. That, and the fact that the aircraft will turn in one direction but not the other, leads me to think that the problem may be in that filter. Most wiring issues I can think of would affect both directions equally, although a console or wiring fault connected with one of the roll limit adjustment pots is a possibility. The radiocoupler isn’t involved in heading mode, other than to pass the signal from the DG directly to the amplifier DG input, and the fact that the autopilot shows some sort of response to the heading bug suggests that the signal is getting through.

      • Ian Wilson says:

        Thanks for clarifying David, my misunderstanding of the system.

        If its of any interest I came across a 1C515-1 amp recently, I think it was eBay if I remember correctly.

  24. Terry Tibbitts says:

    Can you provide instructions on how to remove the Century IIB console? I need to check the connector on the back and it is not obvious to me how to remove it from the tray.

  25. Vicente Collazo-Davila (Vince) says:

    Hi David. My Navajo has an Altimatic IIIB that is functioning perfectly except for the GS capture which quit working about 4 months ago. Even though I’m in altitude hold 60% below the GS for the the required 20 or more seconds, the green capture light won’t come on and the the plane holds level flight right through the GS. Based on what I’ve been reading here I suspect the GS Coupler, and maybe that relay that you mentioned. Any ideas on how to troubleshoot this? Also another question: on top of my GS Coupler there is another little rectangular box with a wiring harness coming out of it about 3×2 inches rectangular, any idea what it could be? It doesn’t show in my manual and it was also made by EDo Aire.

    • davidcrocker says:

      Your problem sounds like either a fault in the glideslope coupler or in the wiring from the CDI or HSI to the glideslope coupler. From the glideslope coupler schematic, it appears that the designers made the elementary mistake of failing to provide a diode to catch the back emf when the relay is turned off. Maybe it’s a mistake on the schematic, but if not, then I would suspect the transistor that drives the relay. Try to borrow a known working glideslope unit to see if that fixes the problem. As for the other box you mention, my Arrow had a box near the glideslope coupler that contained the relay for connecting the electric trim control to the trim amplifier, so maybe that’s what it is.

      • Vicente Collazo-Davila says:

        Hi David, you were right, the other box is the trim relay. I haven’t been able to get my hands on another coupler, but I will try checking the connectors. Thanks again.

  26. Brad Haslett says:

    I have a Century IV & flight director in my Baron coupled to an Aspen, but is still driven by the vacuum EdoAire A/I and Century IV F/D “brain”. It performs well in all functions but altitude hold. Every so often the “chicken wings” (pitch mode) will jump to about 10 degrees on both the Edo & Aspen attitude indicator. Of course the A/P tries to follow the pitch command. I can immediately disconnect the A/P and reconnect and sometimes it repeats the process 5 minutes later, and on some flights not at all. This problem was there before the Aspen install but seeing the “jump” in the flight director cue is much easier to spot on the Aspen. I haven’t had the opportunity to test it on a VFR flight plan to see how long the F/D cue remains at 10 degrees before correcting, assuming it does correct itself.


  27. Scott Grant says:

    I have a Century IIB in my 1969 Arrow, and shortly after I bought it, the wings began to rock with a 3 second period and about 15-20 degrees of bank with the AP engaged. I had an avionics shop look at it a week ago, and after a $200 center adjustment (the bill sounds high to me, but I digress…), the mechanic pronounced my plane “repaired”….until I took it up in the air and the problem remained! It rocks when tracking any input, heading bug, LOC, or Nav. It passes all the ground checks. Anyone out there that had the same problem brought to successful resolution? Thanks.

  28. Robert R. Crociati says:

    I have PA-23-250 Aztec w/ the Altimatic 111B autopilot. It works great when you can get it to engage. While it was shop having a new radio installed I had the technician look at the problem.
    He worked on the roll servo to clean connections and to check out its operations. He claims he
    tested it 100 times and that it never missed once. Flying home it work great. Flew 1 week later
    and back to square one. There have been times when we would engage the autopilot and it won’t
    engage for a while. Then all of a sudden it would catch???

  29. Douglas Crociati says:

    I want to leave a little more Information. only the roll portion of the AP fails. No response from the roll command knob, or any capture of the heading bug. All the pitch functions seem to be working properly.

  30. sam aljundi says:

    hi david
    i have piper 28-180
    i have mitchell 1c359 it works but i dont know the full details on tracking or anything else
    i turned it on and it was holdind the wings so i disengage it because i dont have any info on it
    can you guide we to look for manuel
    thank you

    • davidcrocker says:

      What I think you have is a Piper Autoflite. If so, you can buy the operating manual from Essco for $12 or $14, depending on whether your system includes the nav tracker.

  31. Daniel Spurlock says:

    Our Piper Lance 1976, with Altimatic III C autopilot, will not keep a Slaved HSI inflight. Seems like a 20-30 degree lock out in less than 10 min. After manual DG allign with Wet Compass. How do we isolate if we have a bad DG, etc. Bottom line, heading precession 15-25 degree in less than 10 minutes in flight.

    • davidcrocker says:

      If I read your comment correctly, you are saying that after aligning the HSI with the compass, it drifts (nothing to do with the autopilot). What type of HSI is it? Is it vacuum driven, or electric? As you are aligning it manually, I guess it isn’t slaved to an electronic compass.

  32. On a recent series of flights, my Mitchell Century III autopilot exhibited an increased tendency to oscillate when following the DG heading bug, so I would shut it off. On a subsequent air check, it again started by oscillating, but with a strong banking movement to the right, and very weak to zero corrections to the left. An excessive right bank would happen quickly, requiring a forceful manual override, and pulling the breaker to re-establish level flight. Not fun.

    On the ground cockpit check of the system, using either Roll or HDG functions, I could replicate the error, ie, I could initiate a right aileron command using the Roll function with the Roll command knob on the master controller, or using the HDG function with the DG heading bug. Either of these would cause a right bank to initiate, but with no ability for the opposite aileron movement to initiate. I think I can hear the servo running, but it won’t roll left.

    If, with the yoke centered, I commanded a left roll or heading change, and if I “assisted” the yoke in turning left, I could feel a very weak servo movement in that direction occurring, but it was unable to roll the yoke left by itself. That weak servo movement is intermittent. The pitch control system seems to be working just fine.

    So far, that is all I have tried. My hope is that it is a poor connection on the back of the 1C404, or possibly an adjustment of the “left bank” or “roll centering” gain potentiometers on its face, because those are things I can easily access.Where ina Comanche260would I locate the roll servo? I’m guessing the amplifier is behindthe rear bulkhead, but I could be wrong.

    What else should I be considering on this new problem? Thanks again for hosting this valuable blog, and for monitoring it.

  33. davidcrocker says:

    In my opinion it’s almost certainly an amplifier problem. Try swapping the amplifier with a known good unit if you can. Most likely causes are a blown output transistor, or (especially if you you have the older 1C395 amplifier) a bad connection on the PCB.

  34. Do you know if the 1C515-1 or 1C515-2 is a direct swap-out for the 1D395, ie, with the same connectors and pin patterns? Or does it entail substantial re-wiring to use the newer amplifier? Just wondering… Thanks!

    • davidcrocker says:

      All 3 amplifiers have the same pin connections and signal voltages, so for a ground test you can use them interchangeably (I have). But I don’t know whether replacing a 1C395 by a 1C515-1 or -2 and using it in flight is approved.

  35. Hi, David: Well, I’ve pulled and examined the 1D395, and thought I’d share my notes as I go.

    The device is an easy “pull”, in the comanche located behind the baggage compartment on a shelf within easy reach of the access panel, next to the ELT. Two screws to free the Amphenol connector, and two fasteners to release it from the shelf.

    On the bench, disassembled, the two sides of the amplifier have four-each of the power transistors. Correct me if I’m wrong, but I think one side is entirely concerned with the roll control circuit, and the other manages pitch control. In my unit, it’s obvious that some or all of the transistors have been changed out on the “roll side”, which had labels for all the pins recorded in pencil on it. I removed the two push-pins from all the transistors on that side, and used a digital multimeter to check them. One checked bad, ie, with an open condition on one side.

    All the power transistors are the same Motorola equivalent of the 2N3055. I picked up a couple of 2N3055’s from Radio Shack, and replaced it. With the amplified back in place, alas, it was no better; still incapable of rolling left.

    I removed the amp, and rechecked my work, including a bench check of the “pitch side” power transistors, as well. This is where things got squirrly. The new transistor still checks good, but I should mention that the junction voltage on the component is a bit higher than the one I see on the Motorola transistors (about .502 v, compared to .495 v). Would that make a significant difference in some way?

    On the re-checks, I found a transistor on the “pitch side” that seems to test bad, so I replaced that one, too with a Radio Shack 2N3055.

    I’m pretty sure that someone in the past has changed out ALL four “roll side” transistors at once, because they were brighter, and all installed with a white thermal grease. Seeing that, I used thermal grease, too, on the parts I changed out. I was meticulous about the correct placement of the insulating mica plates and bolt insulators, and am certain of the transistor wiring, having labelled the pins on the pitch-side, too.

    So, what happened: (a drum roll)… Interesting but incorrect things, as it happened: The roll function no longer rolls right, nor left. The pitch functions now flutter up and down a bit, then goes into a run-away pitch up condition on ground checks. I am less discouraged by all this than you might think, because it tells me that I am at least working on the right things, to begin with.

    More thoughts: I lucked into obtaining a 1C515-1 amplifier on eBay for a very fair price (well under $200 USD). No “yellow-tag”, and no guarantees, but on initial installation and ground checks in the plane, it rolls right and left, follows the heading bug right and left, and initiates appropriate pitch up/pitch down changes as it should. That assures me that the balance of the autopilot components are pretty much working, and my issues have likely all been in the 1D395.

    What I’d like to try next with the older amplifier is to try changing out all eight power transistors with the same Motorola transistors I see in them. I’m thinking that there may be circuit “balancing” issues that are being caused by mixing components.

    Alternatively, it has also occured to me that I might be having problems caused by thermal grease contaminating some of the power transistors pins, causing poor connections. I think there was too much used by the last person working on this unit, and I likely repeated that error with my work, making things worse by not controlling where the grease ended up.

    I’d appreciate anyone’s comments and ideas, because I firmly believe the older amplifier can be made to work reliably. I think I’m getting closer…

    Thanks, Dave, as ever for starting and hosting this forum!

    • davidcrocker says:

      Hi, 2n3055 is the usual replacement for those transistors. If in doubt replace all of them. Also check the resistors connected between them. One side is pitch, the other is roll. The 1d395 circuit boards are very prone to bad connections between the rivets on them and the tracks they connect on opposite sides of the board.

    • davidcrocker says:

      Ps silicone grease will not cause any problem as long as the soldered joints are good.

  36. Hi, Dave,

    More notes on my amplifier issues with the Century III, and I hope they are helpful to others. I’m open to suggestions, advice and questions.

    First: with the 1C515-1 (which I found on eBay) , it ground checked pretty well, with strong roll response, but with somewhat weak pitch responses. On the first flight test, I found that it rolled too aggressively, exceeding 30 degrees in Roll mode, and exceeding 20 degrees in HDG mode. (It was as steep as 40+ degs in both).

    Also, and of more concern, it would over-correct on HDG mode in both directions, leading to a series of S-turns with increasing bank across the intended flight path. The Roll function was “touchy” to center, and would bank too steeply and rapidly. On centering the Roll function, and checking Pitch and ALT functions, it was pretty sluggish at responding to changes of pitch or altitude.

    It’s worth noting that this amplifier was tagged as having come out of a Piper Seneca, and my plane is a single engine Comanche 260.

    To try to remedy the ground checks, I found and adjusted two variable resistors located on the pc board, marked R51 and R111 respectively in the Century manual. They are labelled Roll and Pitch Threshold, respectively, both on the schematic, and, blessedly, on the pc board, too. I took note of where they were set by counting the number of turns CW and CCW, listening for the soft “tick” noise the pot makes at the end of its travel. After a few trials, I ended up adding four full turns CW to the pitch threshold pot, and backing off (CCW) six full turns on the roll threshold pot from their original positions. (The full travel, end to end, of these potentiometers seemed to be between 16 and 18 full turns.)

    With these changes on ground tests the pitch indicator needle on the controller was much improved in its “certainty” and smoothness when I adjusted the pitch wheel up and down, and the pitch servo initiated changes more energetically. The action of the roll servo seemed to slow somewhat as I rolled the heading bug right and left of course, and it seemed to dampen better when close to center.

    I also tried adjusting the three settings concerned with maximum bank angle and centering on the controller, but because of gyro noise, I couldn’t be sure of hearing when or if I’d reached the stop. I ended up deciding to just back those three settings off 1 full turn while still on the ground.

    I carefully reassembed everything, ran up the engine, and repeated my ground checks. It all felt pretty good, so I taxied out, took off, and climbed it up to 3000 ft. The pitch and altitude hold functions were flawless as near as I could tell; better than the 1D395 ever provided. The roll function worked better in that it seemed less aggressive at initiating a roll when commanded, and would center with less “twitchiness”, but it was still over-banking at maximum, leaning over to 40 degs, or more. The HDG function, too, would over-bank, but no longer over-corrected to the same degree. The instability was much better, but it would still over-shoot it’s heading, and take considerable time settling on to a course on the DG, or would gently oscillate back and forth across the heading. The cables and controls are well lubricated, so I would discount that as a factor.

    I didn’t have a safety pilot along with me, so didn’t attempt to reset the roll limit or centering pots in flight.

    So, the remaining issue is limiting the angle of bank: how many turns on the controller should be about right to reduce the maximum bank from 40 degrees to 25 on HDG mode. I am guessing the CCW is the right way to turn on pots A and C, but how much? Should I also do anything more to B, the centering pot? I don’t really understand how that pot influences the AP’s handling.

    Also, I’m tempted to try backing off the Roll Threshold pot on the amplifier mainboard a bit more. Again, I’m uncertain of that setting’s overall effect on AP handling, and would welcome advice from anyone with more Century III wrangling experience. Should I adjust it or leave it alone?

    If I have understood what I’ve read in the service manual, having a 1C515-1 instead of the 1D395 is the definition between having a Century III(c), instead of a Century III, and should, when I get it all properly tweaked, provide smoother and more accurate performance if all works as intended.

    I hope these notes help others. Thanks again, Dave, for this forum.

    • davidcrocker says:

      The 6 console pots need to be adjusted in flight. Its easy to do if you have a safety pilot. Set the L and R bank pots to limit the roll to 25 or 30 degrees. Set the roll centring pot to centre the needle when tracking the hsi. Set the pitch centring pot so that when you engage alt hold it settles on the correct altitude. The maintenance manual lists the whole procedure.

  37. MARK SHIPLEY says:


    • davidcrocker says:

      I suggest doing a ground test. First, ensure that electric trim is turned off and the AI is steady. Engage roll mode and check that you can get the yoke to turn in both directions using the roll knob. Do this for a while to see if the circuit breaker cuts out. If it does, you have a problem either in the common circuitry or in the roll channel, most likely in the amplifier. If it doesn’t, engage pitch mode and try moving the yoke forwards and back using the pitch wheel. If you can make the breaker cut out that way, you have a problem in the pitch channel.

      If the breaker still doesn’t cut out, try turning on electric trim, and make sure that the trim wheel moves after a short delay if you command full pitch down or up. If that makes the breaker cut out, suspect a problem in the trim amplifier.

      The most likely problem to cause the breaker to cut out is a shorted output transistor, although this will generally affect the ability to move the corresponding servo in one direction as well. Another possibility is a short in the wiring to one of the servos, or a short within the servo winding.

      • MARK SHIPLEY says:


      • davidcrocker says:

        I would try swapping the amplifier with a known good unit. The blue tantalum disc capacitors in the amplifier sometimes go short-circuit.

  38. Jim says:

    Hi Dave,
    79 Piper Aztec F with Altimatic X with electric trim and F/D
    All pitch, roll, glideslope modes work perfect, but in level flight the altitude hold hunts continuously, sometimes deviating from desired altitude by 100 feet pluss or minus if I let it.
    Thanks for any ideas, I can’t find anything on the “X” on the web!

    • davidcrocker says:

      Hi Jim, I’ve not come across the Altimatic X, but here are some suggestions:

      1. Are you certain there is no sign of a similar problem in pitch hold mode? If it occurs in pitch hold mode as well, then it could be caused by stickiness in the elevator controls or in the pitch servo mechanism.
      2. Does the problem still occur if the auto trim is turned off? Over-trimmimg can be a cause of pitch hunting.
      3. Have the static line feeding the altitude capsule cleaned out. A blockage will cause a delay in the altitude feedback to the autopilot, which may cause those symptoms.

      • Jim says:

        Thanks Dave, we checked it again today and, yes, it also occurs in pitch attitude hold mode. In either mode I can turn off the electric trim, trim it by hand so the airplane is stable, turn on the electric trim then select either attitude or altitude hold and eventually it will trim one direction or another and as it tries to maintain either attitude or altitude the electric trim induced oscillations increase, even in an attitude controlled climb or decent I can hold the rate but the trim continues to move first one direction, then the other while the rate is stable.

        No stickiness is noticed on the ground, everything has just been lightly lubed and cable tensions checked during the annual inspection. In the air however, after the electric trim or autopilot has held then disconnected there seems to be additional effort required to “unstick” the elevator as if the servo is still putting pressure on the cable.

        The oddest thing is that it holds both glideslope and pitch wheel controlled climbs and desents perfectly, but the trim is constantly moving one direction and then another just as it does in altitude hold mode…. once it starts moving, it never stops, first one direction, then the other.

      • davidcrocker says:

        What happens if you engage pitch or altitude hold mode and leave the electric trim turned off?

      • Jim says:

        Nothing, no trim, no altitude or pitch hold.
        The electric Auto-trim switch is on the flight director sub panel with the AP/FD on master switch. Altitude and pitch hold seem to be dependent upon both the AP/FD and the Autotrim switches being on.

      • Jim says:

        Found the Altimatic X maintenence manual online, checked the stabilator, pitch bridle, and trim cable tensions. Stabilator was within specs, but the pitch bridle and the trim cable tensions were very very loose. Re-tensioned everything per the MM, test flew (but only at 3000-3500 ft) and the autopilot directed trim movements were minimal both in ATT and ALT modes. Hope this solved the problem.

  39. davidcrocker says:

    Assuming it used to work OK, I think something in the mechanical pitch control system (including pitch servo) or trim system (including trim motor) have become sticky. I can’t think of any likely electrical or electronic faults that would cause that symptom.

  40. Hello David and thank you for this most interesting blog. I have a G-reg PA-28-161 with the Piper autocontrol II aka Century III. This worked until a couple of years ago but often needed the connector at the back of the DI cycling. Finally it gave up the ghost altogether and is now only capable of turning the servo one way regardless of the heading bug. I’ve probably made things worse by fiddling with it, bussing through the wiring, dismantling connectors etc..

    My engineers don’t have much knowledge of these and would rather not get involved

    You mentioned having a bench harness and i wondered about testing the control unit …. I’d be happy to bring it to you if you if you were willing to have a look at it. I hope you don’t mind my asking. Thanks in anticipation! David

    • davidcrocker says:

      Hi David, yes we can do that. I’ll drop you an email. I suggest you have your avionics technician replace that connector with the Cooper Interconnect equivalent that is available from Farnell, although as a temporary measure the receptacles can be tightened up with a needle.

  41. Dwight Wilcox says:

    After maybe 15 years of intermittent trim problems on my Century 31 autopilot (in Mooney M20J), and an avionics shop changing some computer components, reworking ribbon cables, back to Century nine times, and so forth, I heard about a company called “Auto Pilots Central” in Tulsa, OK. Sent my AP computer to them, and after about 3 weeks received the computer back. This was about 3 months ago, maybe 20 flight hours, and the unit works like I remembered from years ago. They seem to have a very knowledgable staff, have a lot of spare parts for these old things, and were enjoyable to work with. I talked with a Barry, and a Bob Ferguson. Phone 512.251.5322

    Dwight Wilcox in MD

  42. Vicente Collazo-Davila says:

    Hi David,

    A couple of days ago my Altimatic IIIB started tripping its circuit breaker 6 to 8 seconds after turning it on. I pulled the amplifier as you suggested and our tech checked the 8 power transistors. The 4 on the roll side checked good, but 1 on the pitch side was totally shorted and 2 were fairly low resistance. Will be replacing all 4. My question: With that one totally shorted across all 3 interfaces (b, c, e) what are the odds that it fried its related resistors and diodes? Do you have a schematic of the I 515-1 that you could share?

    Once again, thanks for your time and dedication to helping your fellow aviators.

    Vince Collazo

    • davidcrocker says:

      Replacing all 4 power transistors on the pitch side is definitely a good idea, because when one of them goes short circuit, it overloads either 1 or all 3 of the others. You can use type 2N3055 even if the originals are a different type. When these transistors fail, they don’t usually take any other components with them. However, I would also check the two 68 ohm 2W resistors that are attached directly to those transistors. If you wish, you could also check the driver transistors for shorts. These are the 4 large metal can transistors (2 for pitch, 2 for roll) at one end of the circuit board.

      The schematic is included in manual 68S54. I don’t know of any free downloads of this at present, however is offering it for $13.95.

      • Vicente Collazo-Davila says:

        Hi David,

        Thanks for that link. I bought the manual, and for $13.95 it’s a great bargain. Not only do I like the schematics, but the troubleshooting section is outstanding. It’s a much superior service manual than the Piper Altimatic one that we already had.Today we put the amplifier back in the plane, and it readily passed all of the ground checks. I’m scheduled to fly it in about a week so I’ll hold off on flight checking it till then. I find it interesting that it was the transistors on the pitch side that were bad, We had those replaced 3 years ago to correct an abrupt pitch down tendency. The roll power transistors are still the original Motorola ones.

        Take care,

  43. Another vote of thanks for that aero-stuff link. I’ve also ordered the manual from Essco, but that’s not instantaneous like the the Aero-stuff site. Brilliant!

    • davidcrocker says:

      It’s also much cheaper from Aero-Stuff, Essco charges $90.

      • Gustavo Andreoli says:

        I have a 1977 Baron with Century IV AP, with independent Trim and AP switch on/off . I just discover the ap and tested it, heading and nav mode work fine, when use ALT mode there is a lot of altitude oscilation and in ATT mode the trim go so up while control remain quiet so, when disengage the huge atitude change let me a dangerous corrections control action and a rapid turn off the sistem. Do you think is a mechanical malfuntion or my incorrect operation stile. Thanks a lot for your opinions…

      • davidcrocker says:

        What does it do if you leave the auto trim switched off!

  44. Gustavo Andreoli says:

    Ok i ll do it thanks a lot. And telll what happen
    Thnak you.

    • Gustavo Andreoli says:


      Thanks for your advise my Century IV work better with trim off, but now have another prolem, only heading and Rev mode are available, when push to change to ATT or ALT mode dont admit, only rev mode.
      I wait your advise,
      Gustavo Andreoli

  45. Vicente Collazo-Davila says:

    Hi David,

    The Altimatic IIIB worked good for about 20 flight hours after we replaced the 4 pitch power transistors. Now it wants to climb as soon as you engage it with altitude mode or so says my partner. I don’t know if he tried it in pitch mode alone so I’ll check that when I fly it Friday. I suspect one of those pitch power transistors is gone again. My question is, what is more likely to cause the power transistor to fail: the pitch servo drawing too much current, or some component in the IC 505 amplifier itself?
    Thanks for your kind attention,
    Vince Collazo-Davila

    • davidcrocker says:

      You need to test it in pitch mode with the auto-trim turned off. If it only climbs in that mode, then most likely one or two of those transistors have failed again. When fitting the transistors, did you use new mica washers and smear thermal paste on them? I would also check whether the wires to the pitch servo are in good condition and there is no possibility of them shorting to ground or anywhere else. Also measure the 2 power resistors attached to the transistors in case they are exhibiting greatly increased resistance, which could make the transistors overheat.

      • bill dino says:

        David, I am not sure if my first message got through so please excuse me if this is a duplicate. You mention above that if the two power resistors exhibit an increased resistance it could make the transistors overheat. My impression was that an increased resistance of those two 68 ohm resistors would result in a decreased current through the power resistors, could you explain? This is important to us because we are seeing blown power transistors and we thought it might be the servo but some of the resistors are way out of tolerance. One is around 90 ohms.

        Also, I have heard that the shield on the servo wires is “hot” and not at aircraft ground and shorting of the shield to ground can cause these transistors to blow. However the schematics seem to indicate the shield is at aircraft ground and is only related to the clutch. Can you comment on the shield?

        Thanks very much ! – Bill

  46. Vicente Collazo-Davila says:

    Hi David,
    Thanks again for all your help. I have a few more questions before I dive into this project. In case one of the pitch driver transistors 2N3116 turns out to be bad I checked on the availability of a replacement, but can’t find them. However it looks like NTE123A would be a suitable replacement. Do you concur.

    My second set of questions have to do with the pitch servo. I realized that I’m not really sure that I understand how it operates. There are 4 leads going to it: solenoid, pitch common, pitch motor and ground. The way I believe it works is the following: The solenoid signal comes directly from the console and it merely engages the clutch when pitch mode is selected; the motor is DC bidirectional and the positive voltage will come over the pitch motor line or the pitch common line depending on whether it’s a pitch up or a pitch down command to drive the capstan in opposite rotation; the current returns through the ground lead ( or does it return over the pitch motor and pitch common lines depending on which way you’re driving the servo?)

    • davidcrocker says:

      I’ve never had to replace a driver transistor, but a quick check of the transistor characteristics suggests 2N3700 as a suitable replacement that I can get hold of. I’ve looked at the data for NTE123A and that looks suitable to me too.

      The pitch motor current returns through the pitch motor common lead, not the ground lead.

  47. Vicente Collazo-Davila says:

    Hello again David,

    I forgot to mention in that last post that we did the test you suggested with the autotrim off, and it still wanted to climb as soon as we engaged pitch. Another thing I forgot to mention was that on the ground checks, as soon as you turn on the autopilot, you hear what seems to be one of the servos running in the back. I assume it’s the pitch one since roll was engaged and there was no movement of the yoke.

    • davidcrocker says:

      Yes, I expect one or more of the output transistors has failed again, although that isn’t the only possibility.

      • Carl W says:

        I fly a Piper Challenger with the Autocontrol III. I belive it is a Century IIB. My Piper is a 1973. The problem I have is the cable that is connected to the AI via the Amphenol connector was pulled out during a recent annual. This of course was discivered when I engaged the autopilot on a recent flight. Looking under the panel I saw the gray wire that has four colored small gauge wires dangling loose. The wires are colored green, white black and red. All four are to be soldered onto the four solder pot socket contacts. My question is what colored lead goes to that pin on the connector. The conector has alpha designators, not numerical. The grey wire bundle leads to the Autocontrol connector. Can you please help?

  48. davidcrocker says:

    Carl, if you buy the service manual ($13.95 download from, there is a wiring diagram for the Century II on page 3-6. It doesn’t give the colours of the wires to the AI, but it does show the pin letters and what each of those 4 wires connects to.

    The socket receptacles on those connectors wear out over time, so you might consider replacing the connector at the same time, especially if it is a loose fit on the pins on the AI.

    • Carl says:

      David, I downloaded the manual, and for the price it is a bargain. I should be able to resolve the issue now. As for that connector, I spent a number of years in the aerospace connector industry, and that Amphenol connector was a poor choice in a life critical aerospace application. The strain relief on my connector was not properly tightened, or over time either losened or the harness wire diameter changed in time as a result of the plastisizer in the jacket aging. I would suggest that the strain reliefs are tightened when the instruments are serviced. In my case, there was enough stress put on the harness during removal of the AI and HI to pull the cable out of the connector. I wonder how many people incure major expenses at the avionics shop having these autopilots worked on when the source is probably just poor terminations at the comnector?
      Thanks much for the help!

  49. Jack S. White says:

    Hi Dave:

    My 1966 twin comanche has a factory installed Altimatic 111 and a few years ago I had an avionics shop install a Century glide slope coupler/wiring harness which is coupled to a Garmin 530W and Century 1000 HSI . The autopilot has performed solidly until Sunday when, upon engaging either pitch or alt hold button, the plane would pitch up violently (which is same direction as the trim needle pointed). Normally, I would engage pitch button, adjust the altitude knob to center the trim needle, and then press alt hold to maintain level flight. Now nothing seems to adjust the trim needle, and engaging either button just pitches the airplane into a high degree climb. No avionics work has to been done to the plane or the avionics recently; and it flew flawlessly for four hours on a flight two weeks ago. All other features of autopilot work correctly. Any thoughts or suggestions?

    Thank you in advance.

    Jack S. White

    • davidcrocker says:

      Concentrate on getting pitch mode working before you try altitude mode. When you say that nothing seems to adjust the pitch needle, does that include the pitch wheel when you are in pitch mode and the AI is showing level flight? You can test that on the ground. If the pitch wheel does affect the trim indicator but the aircraft attitude doesn’t, then I would suspect the wiring between the AI and the amplifier – probably the little blue connector on the back of the AI, or the one close to it that connects it to the glideslope harness. If the pitch wheel does not affect the trim indicator, then most likely your autopilot amplifier (which I assume is a 1D395 in that vintage aircraft) has developed a bad connection between one of the rivets and the PCB tracks. Try swapping it for a known good amplifier.

      • Jack S. White says:

        Hi Dave:

        I will try your suggestions to confirm the issue. My recollection from my recent flight is that when the pitch button was activated, operating the pitch wheel did not make any difference to the trim needle. Can the amplifier be updated to a newer style as I believe this one was repaired approx 6 years ago for the same problem?

        Thanks, and I truly appreciate your guidance.


  50. davidcrocker says:

    Jack, the newer amplifiers (1C515-1 or -2) are electrically compatible with the old ones (1D395), however even amplifiers of the same model can be set up differently to cope with the particular aircraft characteristics such as roll and pitch sensitivity. You could ask Autopilots Central whether there are any STCs or other approvals to replace a 1D395 by a 1C515 in your aircraft type.

    • Jack S. White says:

      Hi Dave:
      I had my autopilot checked by Conrad at The Radio Shop at KORH. He is on of few shops on the East Coast that still has Century breakout boxes, test equipment, etc. for the Altimatic autopilot. The computer was fine. It turned out to be a short in the attitude indicator pickoff pin(s). We have ordered an exchange unit from Mid-Continent Instruments; not cheap but I have heard good things about the quality of their repairs. Any thoughts/comments?
      Thanks again,

      • davidcrocker says:

        Interesting, I’ve not heard of the AI pickoff being faulty before.

      • Jack S. White says:

        Hi Dave:
        Conrad diagnosed the attitude indicator as the issue by, after confirming that the computer bench tested correctly with the breakout boxes, servos, etc, substituting a known working attitude indicator. It correctly moved the pitch whereas mine would not do anything with the pitch. He concluded something “shorted” within the instrument.
        I truly thank you for altruism in maintaining this terrific website/blog.

  51. Vicente Collazo-Davila says:

    Hi David,

    We pulled the 4 pitch power transistors. One was good, one was questionable, one was totally shorted, and the last was good base to collector but open base to emitter. We replaced all 4 and used dielectric grease on both sides of the mica insulators. As you suggested, we checked the two 68 ohm resistors. One was showing 78.7 ohms and the other 62 ohms. Both well out of the 5 percent tolerance even when cold. Was this enough to have caused our problem with the transistors? Would there be a problem with replacing the 2 carbon resistors with 2 carbon film 68 ohm 2 watt resistors?

    Best wishes,
    Vince Collazo

  52. davidcrocker says:

    Hi Vince, I don’t think those resistor values are bad enough to cause the problems, but I would replace them anyway. The parts list says they should be 68 ohm 2W and I would use metal film resistors. Check the cable from the amplifier to the pitch servo, I’ve heard of them chafing on edges of sheet metal, resulting in an intermittent short to ground.

  53. Rocketrider says:


    Excellent write-up.
    I have a 78 Piper Turbo Lance with Altimatic III C, which has some problems.
    1e: The pitch wheel needs to be rotated almost all the way to the up position to maintain level flight, so I can climb using the pitch wheel only 1500ft/min.
    2e, when engaging the altitude hold, the plane takes a dive with 500 ft/m, then it will almost level, but still descend with 100ft/min, never to maintain altitude.
    3e, I have a slight wing rock, which appears to get worse over the last 3 years.



    • Rocketrider says:

      Something went wrong in my reply.
      I meant to state climbing using the pitch wheel max 200 ft/min, and descending with over 1500 ft/min

      • davidcrocker says:

        Let’s split problem 2 into 2(a) (diving before levelling off) and 2(b) (descending at 100ft/min). Problems 1 and 2(a) have the same cause: you have a large offset in the pitch channel. First, try adjusting the pitch centering potentiometer. To do this you need to remove the faceplate of the console, This reveals 6 helical potentiometers, which you can adjust using a small screwdriver. The pitch centering one is the 5th from the left. Start by doing a ground test. On level ground, start the engine to level the AI. Engage the autopilot, set the pitch wheel to level the trim indicator, and try adjusting the pot. It should affect the trim indicator. See if you can adjust it so that the trim indicator is level when the pitch wheel is centred. If you can’t, then suspect a problem in the amplifier, the pitch centering potentiometer (unlikely), or the wiring problem between the two. If you can, then adjust the same pot in-flight to eliminate problem 2(a).

        Problem 2(b) might be caused by a leak in the altitude capsule, but I suggest you fix problems 1 and 2(a) first in case it is related.

        Problem (3) might be cause by stiffness in the aileron controls (when were they last lubricated?), or by backlash in the DI or HSI.

  54. Rocketrider says:


    Thank you very much for this very fast reply.
    I will start doing what you suggested with the potentiometer, at the end of the week.
    I will keep you updated on the proceedings, this would also help other people with the same problem.

    Thanks again



    • Rocketrider says:

      Done the adjustment last week. Took a slight movement of the potentiometer to correct.
      more movement did not do as much as the initial small adjustment. Could it be possible that the contacts were fouled and that with a little twist it clean was again.



      • davidcrocker says:

        Yes, that’s entirely possible.

      • Rocketrider says:


        Thanks for the reply,
        So now on to the altitude hold problem.
        I have removed and dis-assembled the altitude hold module. I have tested the aneroid in a small vacuum chamber. It expands while vacuum applied, so might be OK, but I do not know the amount of expansion vs pressure. I find the expansion very small, especially if it needs to move the rotor towards the contacts with altitude differences smaller then 100 ft or so.
        The rotor locks when energized, and both contacts and spools are OK.
        Is it worthwhile to reassemble, or should I buy a new aneroid to make sure.
        Cost of the aneroid is $300 us

      • davidcrocker says:

        Did you establish that the fault was still present after adjusting the pitch centering pot?

        The way I believe it is supposed to work (unless your autopilot has altitude preselect) is that when altitude hold is engaged, a valve closes that isolates one side of the aneroid. Then changes in static pressure on the other side cause the aneroid to expand or contract, and this is sensed by a coil (not by contacts). If all of this is correct, then a leak in the valve or a leak on the isolated side of the aneroid would cause the pressure to equalize over time, making the autopilot insensitive to slow changes in altitude. However, I’ve never dismantled an altitude hold capsule so I can’t really advise you.

  55. Vicente Collazo-Davila says:

    Hi David,

    Got to fly two 1 1/2 hour legs today, and the pitch mode worked flawlessly in pitch mode and altitude mode including altitude capture both descending and climbing. Thanks for the advice

    Take care,

  56. Sean says:

    Hi David,
    I have a Century 111 in an older Bonanza. The Alt hold, HDG and Nav work well however the pitch trim doesnt seem to work well and only runs in one direction. I thought the trim sensor may need to be adjusted but I dont see a sensor in the plane anywhere. I did check all the round connectors and they are tight. I read somewhere that some of them got the trim signal off the pitch servo. I saw in one of your posts that you mentioned the transistor on the trim amplifier, would that be a good place to start since it runs in one direction?
    I also have recently had a GS coupler installed. It captures the GS about two dots high then slowly centers up. The Green GS never comes on. Any thoughts?


    • davidcrocker says:

      Hi Sean, it is only the later versions of the Century III (e.g. Altimatic IIIc) that get the pitch trim signal from the amplifier output. All the others have a mechanical sensor which senses differential tension in the elevator cables. This sensor is usually under the floor towards the rear of the aircraft. Page 2-34 of manual 68S54 shows where it is in a Bonanza. It is common for this sensor to break, which would cause the autotrim problem you report. Some aircraft also have a trim amplifier, and if yours has one then a failed output transistor in the trim amplifier is also a possibility.

      If the green GS lamp never comes on yet the glideslope is captured, then either the lamp has failed or there is a problem in the wiring between the lamp and the glideslope coupler. You need to have the lamp working because as soon as it lights, you are supposed to make a power reduction. Manual 68S54 includes the in-flight setup procedure for the GS coupler.

      • Sean says:

        Hi David,
        Thanks for your input. I looked into things a bit more. I have the pitch trim amplifier setup. I got the proper instructions to test it on the ground with the pitch wheel and doing this it works in both directions. However it still seems out of trim when I disconect it after an approach.
        On the issue with the GS coupler. The light tests and wiring the wiring checks fine back to the coupler. I was reading somewhere that when interfaced with the Garmin 430 some installers incorrectly wire it to the ILS energize pin which has a 20mA sink instead of the ILS/GPS pin which apparently has a 500mA sink for the arm signal. Would that cause this issue or would it not work at all if it was wired to the wrong one. Again the coupler will capture the GS but it over shoots it and seems very lazy about catching it but then centers up after a bit. The light never comes on.
        Thanks again for all your help.


    • glenn thomasson says:

      Where were you able to get a GS coupler from? Im looking for one

  57. Sean says:

    Hi David,
    I need to clarify. My manual pitch trim work fine i was refering to the autotrim.

    Thanks Again,

    • davidcrocker says:

      Hi Sean, if the lamp self test is OK but it doesn’t come on when the GS is intercepted then that suggests a fault in either the GS coupler or in the wiring to it. From the diagrams, it appears to me that the relay contact and wiring that lights the GS lamp is also responsible for switching the autopilot out of ALT mode, so that could explain the slow capture. I suggest you check the wiring with reference to figure 5-3 in manual 68S54. I don’t think the ARM pin needs to be connected, the GS coupler arms itself when the autopilot is in ALT mode, the radio coupler is in LOC NORM mode, and the GS deviation is full up for 30 seconds or so. However, a positive signal on the ARM input (pin 9) will arm the coupler provided the aircraft is not above the glideslope at the time. The current sink capacity of the device driving this pin is not relevant, the pin takes only a tiny amount of current.

      • Sean says:

        Thanks Dave,

        The wiring all checks out so Im going to take a closer look at the relay inside, from what you said that explains both symptoms. Ill keep you posted.

  58. George Glackin says:

    What a great post! We’re having many of the above problems with our Century III autopilot that’s installed in a 1972 Baron. We lost the altitude hold entirely. Our avionics shop has sent the unit out twice to an outside shop and now the altitude hold works, but the plane pitches up violently and climbs 120 feet, then levels off. Pitch mode works normally. We have also now lost good tracking in Nav mode (continuous oscillations left & right), but acceptable tracking in Omni mode with the needle centering. The Localizer now tracks consistently right of course when the Omni runs centered. There is also some debate about whether our unit has Glideslope capability – the Techs think that some S/N’s do not have that capability. The next step is to send all the components & cans out together to the outside shop.

    We are losing faith in our shop’s ability to figure this out and have wondered about good options for new or newer units. The shop quoted us approximately $40K for an S-Tec unit installed, but that seems out of line for what we need. What options would this group recommend? many thanks.

    • davidcrocker says:

      Hi George, have you adjusted the helical potentiometers in the control console as per the service manual since the altitude hold was fixed? In particular, one of the pots sets the maximum pitch up angle, and another one is adjusted to avoid the very problem you are having i.e. a climb or descent when alt hold is engaged. Another of the pots adjusts the tracking when in Omni, Loc or Nav mode. I’ve not come across a unit that tracks off centre in one mode and not in another, and I can’t think of any fault that would cause that symptom.

      If your aircraft has the glideslope coupler installed then there should be a green “Glideslope coupler engaged” light on the instrument panel.

      I can’t comment on the S-Tec autopilot as I’ve never flown with one.

      Good luck – David

  59. Garren S says:

    What a wealth of information. I’ve got an old Century 1 and hopefully you can help me out with it. It works fine when the battery is switched on, however once the engine starts it stops working. Whenever the alternator is the power source it shuts off. I don’t have an A/C ripple noise in the radios, so I’m assuming that isn’t it. Any ideas? Thanks

  60. Hello, I have a 1977 Mooney 201 with a Century III autopilot. It worked ok until recently. It stopped working after a shop changed out the engine, installed an engine monitor and adjusted both ailerons since they were out of rig. Now I find the Roll function acts backward. Testing on the ground with “roll only” caused the control wheel to go the wrong direction when I turn the roll selector one way or another. Same symptom occurs if I use the DG heading bug and turn it. It works but completely backwards. Could the connector going to the roll servo be installed 180 degree out (upside down)? I don’t know where the roll servo is located but suspect it’s near where the ailerons were adjusted. If this isn’t the case, I suspect the amplifier has a transistor defective. What do you think? Thanks!

  61. On my earlier post, my 77 Mooney 201 with Century III autopilot was not working right on roll movements. I found out that the shop that adjusted my ailerons accidently connected the roll servo incorrectly. The rod between the aileron bell crank to the servo was set to the top servo position and should have been at the bottom servo position. This seems to be one of those things that can be hooked up either way. The servo swings one way or another, controlled by electrical voltage polarity. and the servo rod connections causes the swing to push or pull on the aileron bell crank. If someone would have drawn a line showing the orientation when the wings are level the problem wouldn’t have come up. (it wasn’t designed sailor proof)..

  62. astra47 says:

    Thank you, sir, for this great resource. I have an Altimatic IIC in a ’76 Seneca II. Fortunately, the AP has been functioning well – until yesterday. Following AP disengagement, the Roll switch on the console no longer returns to the “full out” or down position. The roll function does terminate, but the switch rocker remains in what appears to be a “neutral” position (similar to center-off)unlike the other three switches that are fully down. Does this mean a switch assembly replacement is necessary or am I missing something operationally. Thanks much.

  63. Matt Jacomos says:

    I tried to post this to the Mooney Tech site but got nounced back.

    I have a C41 in an M20J and since having some radio work and a 100hrly carried out the AC thinks its a dive bomber when the ATT Hold is engaged.

    The pitch down is pretty aggressive, so much so that you can’t wait for it to level out, you have to over ride the AP before you hit Vne. This also happens when ALT Hold is engaged as well.

    I’m trying to find a copy of the Century 41 Adjustment Drawing online to make sure my pitch centering adjustment is set correctly, but I am loathe to spend $90 for one diagram.

    but I suspect there’s been a disconnection in the input wiring somewhere during the other work that is the issue, or my aneroid has issues.

    I would appreciate any tips on diagnosing this problem if you have them.
    Cheers Matt

    • Dwight Wilcox says:

      I have a C31 in my ’82J Mooney. Same problem, but intermittent, for many years. One ‘fix’ involved a bad connector between A/P computer and A/H – mentioned on this site. Other fixes involved circutry inside the A/P computer. Try with & without electric trim ‘on’ as a diagnosis tool. Bottom line – if you are not able to resolve problem as being external to the computer ( bad connection type thing), call AutoPilot Central in Tulsa, OK. Send computer to them for diagnosis and repair. This is what I did, after dealing for over 10 years with an intermittent problem like yours – and it has worked great since (recommended by someone on ‘Mooney-tech). And, don’t bother with the factory in Mineral Wells for computer repair – mine was there 9 times over the years, and their ‘fixes’ did not last
      PS – the mooney tech list is:

  64. Kristin Winter says:

    Do you know what components are common between an Altimatic IIIB and a Century IIIC? I would like to upgrade my 1969 PA-30 to the IIIC and get rid of the troublesome altitude preselect function.


  65. Doug Crociati says:

    I believe I am having trouble with my Amp on my Altimatic IIIB autopilot, as it is working intermittently, I currently have the IC515 amp in my Aztec, Can you please tell me if the IC515 is interchangeable with the IC515-1 or IC515-2 Amplifiers as there seems to be easier to find a serviced on of those.

    Thanks a lot
    Doug Crociati

    • davidcrocker says:

      In principle they should be interchangeable, although in practice some have a sticker on them saying “This amplifier is set up for Altimatic IIIc autopilot” (or similar). I should point out that I am not qualified to give a legally valid opinion.

  66. Lee Mcdaniel says:

    Pitch or roll oscillation can also be caused by a high starting voltage on the servos causing the a/p to correct, then over correct. As a service tech back in the 70’s, about 75% of problems with Century a/p’s could be “repaired” by unplugging the connectors bending the pins slightly to make a better connection.

  67. Martin Marinaro says:

    Hi Dave: I have a Mitchell Autoroll autopilot’wing leveler in my 1968 PA28-180 and the system is not working properly. It has been wandering 5-10 degrees left and right, and now it just rolls hard right and stays there. Autopilot Central overhauled the gyro and servo some years back. The gyro is #IC359, S# 57228, and the servo is #ID 363, S# 5217C. It is not connected to a heading indicator or AI. Any idea what might be wrong and is it worth it to send the unit out for repair?

    Thanks much, Martin

    • davidcrocker says:

      Sorry Martin, I have no experience of that unit. If it uses the same round blue Amphenol connectors as the Century III, then I would start by looking for connectors that de-mate too easily when you unlock them, especially ones in places that get warm such as behind the instrument panel.

      • Martin Marinaro says:

        Thanks for the follow-up Dave. I’ll look at the connectors. It does have those round blue types.


      • silentflier says:

        Might also be the gyro bearings… as they start to fail they cause slow running which would make the gyro oversensitive and cause the wandering. If they have now failed completely and the gyro is not running at all you would get no correction and therefore a constant turn output unless the gyro is exactly centred when stationary… Have a listen. when you switch off – can you hear it running? “Normal” run-down time is 3 minutes or more

      • Martin Marinaro says:

        Thanks for that input silentflier. I’ll check to see if it’s running. I’m beginning to question whether this old a system is worth fixing.? Sure wish I had a manual for it.

  68. Michael Apodaca says:

    There is some great information here thanks.

    My Century IV is solid in all modes except Alt Hold. It will work fine for hours then suddenly kick off int Attitude mode. Sometimes i can just push Alt again and I’m fine other times it will not go on at all. When it is giving me trouble i can still capture the GS and all is fine. Thought i might have a problem with the CS button on the Control yoke but nothing seems to be wrong. We have cleansed all the connections. I had the controller recently sent out for another problem and it was repaired so I’m guessing all is well with that. This is very intermittent so i just don’t know where to go next short of going to Autopilots West.

  69. Lazaros Greece says:

    Hi to all.
    I’m an electronic technician and my friend had a Piper with an Altimatic III.So he ask me if i can something with this unit.The problem is with the temperature.When the weather is cold the unit is working fine. After 15-20 min when the temperature is up inside the unit or when the weather is hot the unit is not working properly. So when i get the unit i plan to replace all electrolytic capacitors and try to find maybe resistor out of order. I need if you know the pin out of the unit.Where can i apply the (+) and (-) of the power supply.
    Thanks for any help.
    Lazaros Greece.

  70. davidcrocker says:

    If you are going to do any work on your Altimatic III then you need to purchase manual If the amplifier is the 1D395 type (PCB with rivets instead of plated through holes) then the problem is probably not a capacitor, rather a bad joint between one of the rivets and the PCB trace.

  71. Scott Grant says:

    Question: Can a 1C363-1-161R servo be cannibalized or modified by an avionics tech to be a 1C363-1-183R roll sevo? I’m asking because these servos are becoming harder to find. What do the last three digits in the part number indicate? Trivial or major difference? Thanks much.

  72. John D. Ellenberg says:

    I have a Century IIB with radio coupler installed in my Beech Debonair. I’m having a little problem getting the “centering adjustment” correct. If I select a heading of 330 degrees, it flies around 334 degrees (flies a little right of selected headingd). In adjusting, do I turn the centering pot clockwise of counterclockwise?

    Thanks for any help you can give.

    John Ellenberg (

  73. Daniel Tortolani says:

    Hello David, i was reading your blog and i see that you know a lot about this autopilots, i have recently purchased a piper turbo lance II equiped with an altimatic IIIb (century II) autopilot, i have spent a lot of money this last few months to keep the plane flying and threre is no money left to have a new autopilot installed, i would like to try to troubleshoot my autopilot myself, i am a pilot and an electronic engeneer and have been reading a lot about this system before doing nothing, i would like to ask your opinion for my system behavior, my aoutopilot engages, period, it does nothing more, not in roll mode, or heading or nav, on ground test, i turn it on, on roll mode i move the rool knob right and left and the yoke remains still, it does engage because i can feel the yoke to be hard to move, on a flight test if i engage it, it will try to hold the yoke on the position it was before engaging and then gently turn left, it does not respond to roll, heading or nav either, any suggestions where to look first, i have downloaded the manual and printed it so i can take it to the hangar when working on the AP, Regards.

    • davidcrocker says:

      Preferably, swap the amplifier for a known good unit to see if the problem is in the amplifier. You can also check the servo motor for continuity, and the associated connector (the receptacles on those round blue connectors lose their grip on the pins after a while).

      If you suspect a problem with the amplifier, then I suggest you start by checking the output transistors. You can isolate them by removing the amplifier PCB and unsoldering one end of each 68 ohm resistor. If any of the transistors are blown, it’s a good idea to replace all 4 in that channel. You can replace them with 2N3055. Also check the associated 68 ohm resistors and replace any that have gone significantly out of tolerance.

      When you put the autopilot in pitch hold mode, can you make the trim indicator move over its full range by adjusting the pitch wheel and/or the pitch angle of the aircraft? If so, then that suggests the fault on the pitch channel is in the output stage, or the motor or associated wiring.

      • Daniel Tortolani says:

        Hello David, thank you for your advise, I will check all components you mentioned, my AP does not hold altitude, just heading, I spent I few hours yesterday studying the manual schematics and I would like to replace the original oscillator for a new Cristal type one, the doubts I’m having is regarding the DG, part of the oscillator (the Coil) is inside the DG, I am planning to disconnect this to cables from the panel and the connectors, but I can’t find the schematics for the DG and I don’t know if disconnecting the coil will affect the signal feed to the console, do you know if someone has made this modification before, or if the coil is a stand alone component inside the DG, I’m working in the design, if it does work I will publish this upgrade so anyone interested can do the same, the way a see it, the clock signal from the oscillator is very important for the correct operation of the AP, the old tank oscillator are not reliable and usually they need to calibrated often, they are sensible to temperature and by doing this mod the plug connectors and wiring won’t be an issue anymore as the complete oscillator will remain inside the console, let me know what you think, regards.

  74. davidcrocker says:

    OK, I guess you really did mean it was based on the Century II. I thought that the name “Altimatic” implied control in the vertical axis as well and must therefore be based on the Century III. I’ve not heard of anyone having problems with the oscillator in these autopilots, or needing to calibrate them. The exact frequency is not critical, because the autopilot works on differential signal amplitude between 2 coils in the AI, and between 2 coils in the DG. Perhaps you are thinking of even older Century models that used RF, rather than the 5kHz used by the Century II and III ?

    • Daniel Tortolani says:

      David I’M sorry I made a mistake, I have an autocontrol IIIB wich is the same as the century IIB, I will use a function generator to emulate the oscillator so I can check the rest of the circuit, at the end i’m still motivaded to simplify the oscillator with new modern components but I am still evaluating the problems I could have with the DG an AI signals if I modify the original design, anyway I will work this weekend on it and let you know the results, regards

  75. davidcrocker says:

    I’ve never known the oscillator to be at fault. If you want to dabble with your autopilot, my advice is to restrict yourself to replacing faulty or old components with more modern equivalents, and not to change the way it works. You might introduce failure modes that the original unit doesn’t have. Bendix-King (now Honeywell) produced an autopilot adapter to allow the DG to be replaced by a KI-525 HSI, but they didn’t replace the oscillator – they just simulated the inductance of the DG electronically.

  76. Joel Keesecker says:


    I have a Century 1 installed in a Cessna 180, 1975 vintage. My IA & I are having trouble re-winding the bridle on the servo after removing wings for a major rebuild. Do you know if the rigging would be in the service manual or the install manual and the part number for the manual that would explain the process?

  77. Vicente Collazo-Davila says:

    Hi David,

    My previous problems were resolved when our avionics shop cleaned up our servos and replaced a driver and power transistor on the pitch side of the IC1515-1. The high breakout voltages on the pitch and roll servos were causing us to blow pitch power transistors, and on the roll side the gentle 3 to 4 degree rocking. Everything worked great for about 4 months, but now the plane is abruptly pitching down when the pitch mode is engaged. It’s not a trim problem since it even happens when the electric trim CB is pulled. It initially works OK, but then it fails. I notice that when the dive occurs, the trim indicator on the control panel is suddenly showing a max descent request without any input from the pitch wheel. When this happens I disengage pitch and notice that the pitch wheel has no effect on the trim indicator. Based on this, I believe the problem is in the control panel and not in the amplifier or servos. Am I correct in this belief, and what could be causing this problem? Thanks in advance, and thanks for your past help.

    Take care,
    Vicente Collazo-Davila

  78. davidcrocker says:

    Check the connections between the AI and the amplifier. In particular, check the round blue connector on the back of the AI, trace the cable from it a few inches until you reach an inline plug & socket (where the main loom connects to the glideslope loom), and check that connector. Over time the receptacles weaken to the point where they don’t grip the pins tightly, resulting in a bad connection. I had a similar problem to yours, and the problem was that second connector. When I undid the locking ring on that connector, the plug and socket fell apart from each other. As a temporary measure you can tighten up the socket receptacles using a needle, and for a more permanent fix you can replace the socket.

  79. stefank007 says:

    I have spice models of a Century-III autopilot, which have been very useful to me for troubleshooting and might be to others. If you wanted to post them, I’ll send them to you.
    They are 25 kBytes zipped.

  80. Martin Marinaro says:

    Hi: I have a Mitchell Auto Flite wing leveler system in my PA28-180, and the good folks at Auto Pilot Central just overhauled the NavTracker and the technician there said it is capable of tracking a GPS signal as well as a VOR. Does anyone have experience with setting up the NavTracker to receive data from a GPS? I have a panel mounted AvMap EKPIV GPS that will interface with an autopilot and transmits a NMEA signal. Any ideas on this appreciated.

  81. Jeffrey Boccaccio says:


    I have a CenturyIIB. The roll control works fine however in heading mode it will only go right and not left. I can take the bug and move it left and it will try to bring it back but never passes the left side of where the bug is at 12:00.

    Checked all connectors and PCB fingers.

    The oscillator nulls perfect when you hit the 360 mark on the DG. It just will not go left of the 360 mark.

    Any ideas?

    • stefank007 says:

      Jeff, if manual roll control can move the ailerons as to roll left but the heading bug can’t I would suspect the E-I coil in the DG. This could be confirmed by borrowing another DG. Such a test can be done on the ground, engines stopped. A bad transistor in the amplifier front end might also do it. I suspect the coil though; they go flaky quite often.

  82. Jeffrey Boccaccio says:

    Thanks for the heads up. I will give it a try and let you know.

    Yea, the roll works fine.

    • Jeffrey Boccaccio says:

      I was able to find a head unit to try since it is so easy to take in and out.

      That did it. My left hand control returned. I put my unit back in and it was still bad. So I need to either find another one our fix this one.

      I will at least try to repair this one. Will let you know what I find

  83. Ted says:

    I have a Century 2000 installed in my Lake Amphibian. The autopilot works fine in all axis for about 15 minutes and then the AP light starts flashing and the unit cuts out. Sometimes if I pull the breaker, wait ten minutes, and then re-insert it I can get it to work for another 15 minutes. Sometimes it won’t work again at all for that flight.

    Any ideas?


    • Dwight says:

      For years, I had a similar problem (and others) with my Century 31 A/P. The computer made several trips to the manufacturer over the years, and the unit would work for a short while after being returned, About 2 years ago, I sent the computer to ‘Autopilots Central’ in Tulsa, OK. No problems with the unit since they returned it. I have no idea what they did – they say they have more parts for these computers than Century, and a very experienced tech. You might give them a call and discuss your issues.

      Dwight – Mooney M20J

      • Ted says:

        Thanks Dwight. I will give that a try. What sort of turnaround and price did you end up with?


      • Dwight says:

        I called them to ‘get on their schedule’ – and to review symptoms. As I recall it was 3-4 weeks turn around. They said my unit would ‘go in the que’ as soon as it was received. The charge was time and materials I believe, and seemed fair at the time. It now seems like a real bargain since an intermittent problem for well over a decade – maybe two decades ( including at least 9 trips to Century, and multiple ‘fixes’ at the authorized avionics shop) is no longer there.

  84. rick says:

    Dave, i have a altimatic IIIB-1 ALL WORKS WELL EXCEPT when i try to decent to capture alttitude it dives at 1000fpm even if i turn trim wheel full up.eny ajustment to the rate at witch it will climb or decent

    • davidcrocker says:

      Does it work properly in pitch mode – can you get it to fly level with the pitch wheel approximately centred, and to climb or descend by adjusting the pitch wheel? If so, then the problem is probably in the altitude capsule or the wiring between it and the rest of the system. Check out wiring first, in particular look for any round blue connectors that are very loose once you disengage the locking ring. If the wiring seems OK, check that the static vents and tubing to the altitude capsule are not blocked, then swap the capsule for a known good unit. There is another comment here from someone who managed to fix his altitude capsule, it might contain some useful info for you.

  85. rick says:

    The trim wheel function works fine, i can use trim wheel to climb and desend.The altitude function works fine it will hold altitude perfectly when i go to desend turnning the altitude knob it will start to go down at to high of rate say 1000fpm i then try to ajust rate of decent with trim wheel to slow the rate but does not respond.

  86. davidcrocker says:

    I’ve never disassambled a unit with direct altitude adjustment, however on the units without, if you remove the front panel of the control console it reveals 6 heilcal potentiometers. One of them adjusts the maximum descent pitch.

  87. Tod Dickey says:

    Hi David,
    I recently had some panel work done, so I am wondering if I am dealing with a cut wire on my Century III. Heading mode works fine, but it does not respond to any VOR/ILS/GPS deviations. It will not track any of these. If I turn the course selector to the right or left however, it will initiate a turn in those directions, regardless whether the CDI needle is right or left of course. The nav signal originates from a Garmin 530 and goes through a King KI525A HSI.

    On another issue, when the aircraft is trimmed for level flight using the CIII Pitch Trim mode, engaging Altitude Hold cause the aircraft to dive down about 100 feet before it holds that altitude.
    I have gotten used to just engaging it 100 feet higher than the altitude I want to hold, but I am curious what could be happening.

  88. davidcrocker says:

    It sounds as though the CDI signal from the Garmin is not getting through to the radio coupler, or that a fault has developed in the radio coupler. I would start by checking the radio coupler connector. The altitude hold issue can be fixed by adjusting one of the six helical potentiometers (from memory, I think it is the second one from the right) underneath the front panel of the autopilot control console.

  89. Jeroen van Brakel says:

    Having a Century III in my 78 Piper Lance.
    system always worked fine after you helped me out with a pitch problem
    Now suddenly the HDG mode gives problems.
    After engaging the HDG mode shortly after take off, the switch engages fine but it is not following the heading bug. The roll is working if only roll is engaged. Tried numerous times engaging the HDG, but nothing happens when the heading bug is rotated.
    Then after about 10 – 15 minutes in flight and I engage HDG mode again, it works and keeps on working until I land the plane and switch it off.
    I have encountered this problem on my last 3 flights. Sometimes it follows the heading bug only on one side for a while, until its fully functional again.

    Hope you can help me out here



    • davidcrocker says:

      I suggest you check the wiring between the DG and the autopilot, in particular the connector on the back of the DG. If the fault isn’t there, then it probably lies in the amplifier, especially if it is a 1D395.

  90. jeroen van brakel says:

    Thanks for the fast reply.
    I checked the wiring already and also the connector on the DG.
    What could be faulty in the amplifier, and why would the fault not be permanent.


    • davidcrocker says:

      If the amp is a 1D395, then the PCB doesn’t have plated-through holes. Instead, it used rivets to join traces on the upper and lower surfaces. These rivets don’t always make good connections with the traces, and can be temperature-sensitive. A dodgy connection can usually be repaired by soldering the rivet to the trace. You can also get dry joints on these boards. Best bet may be to have the amplifier overhauled by Autopilots Central. Preferably, try a known good amplifier first, in case the problem lies elsewhere.

    • davidcrocker says:

      PS – is the DG connector a firm or a loose fit, once you have disengaged the locking ring? If it is a loose fit, that might be the problem.

  91. jeroen van brakel says:

    David, forgot to ask
    Could a 78 Piper with the Piper Altimatic IIIC have the old style 1D395 amplifier.
    I have not looked at the part number of the amp yet.


  92. davidcrocker says:

    The Altimatic IIICs that I have seen use the 1C515-1 amplifier. For testing purposes, the different amplifiers are more or less interchangeable as long as they are set to the correct voltage – but do the test at a safe altitude and at your own risk, since the amplifier will have been set up for a different aircraft, and substituting a different amplifier type is unlikely to be approved.

  93. jeroen van brakel says:


    Can’t remember if it was a loose fit. I will check again next week.
    Thanks for the help, and will return to you when I have some more info.
    Have a good weekend

  94. John says:

    David, My 1974 Arrow with a Century IIB still works. Recently had the roll servo overhauled. First time in 39 years! Now, the wings oscillate left/right/left/right. Before the OH, they did not rock. Any suggestions? Are there any potentiometers to adjust?

    • John says:

      Oops, I meant PiperAutocontrol IIB.

      • davidcrocker says:

        I’m not familiar with the Century IIB, but from the schematic, I don’t think there are any pots relevant to your problem. One possible cause of wing rocking is excessive stiffness or friction in the controls or the servo.

      • Scott Grant says:

        I have a 1969 Arrow with an Autocontrol III (Century IIB), and I had a wing rock problem for years until I sent the control/amplifier to Autopiot Central in Tulsa. Turns out there were lots of components out of tolerance and neede to be replaced. They repaired it and the wing rock went away. My controller does not have a threshold adjustment. I tried everything else…..bridal cable, servo OH, etc, but the old components in the controller were the problem. $600 for the fix. Good luck. Thanks.

      • Make sure the reinstall of the servo is checked again, slack bridal cables or incorrect cable tensions can cause wing rock, and I would be far more suspicious of a control or mechanical cause rather than the actual AP or servo if the servo has been correctly overhauled.

        Also run a ground check as per the AFM for the Autopilot, if not rocking on the ground, it’s likely a bridal cable or control issue only reproducible in the air with controls loaded.

      • davidcrocker says:

        I just found a relevant section in the service manual (68S40):

        “Wing rock – this is a condition where the aircraft tends to oscillate or rock back and forth in smooth air. In the Century IIB, wing rock can normally be corrected with the roll threshold adjustment. See Section IV, paragraphs 6.3 for adjustment procedure. Wing rock may also be caused by a loose bridle cable on the servo or incorrectly adjusted roll signal filter (1B440).”

        I suggest you check the bridle cable tension as per Ian Wilson’s advice before you try adjusting the roll threshold.

      • John says:

        My autopilot is a Piper Autocontrol III. It oscillates right/left wing rock. New O/H servo. I removed the faceplate and found 3 adjustment screws. Will these adj screws reduce the wing rock or is the bridle cable too tight?

  95. Hi Dave,

    First of all, this is a great blog, thanks for sharing your expertise. I have an original Autocontrol III (Century II) in my Cherokee 6 -1973.(Amplifier 1C385, Roll servo 1D363-1-183R)
    It worked fine and was even coupled to the GNS430, but suddenly stopped working. The clutch still engages, but the roll servo will not move. I put a multimeter at the end of the cable (where it connects to the servo) and I can see the voltage changing as I change the Roll left and right in roll mode (HDG off). I checked the wring and it seems fine and connectors too. What are the voltages I should expect between the pins? Since everything is original, it can either be the servo or the amplifier, which seems very common on your blog. I am a Mechanical engineer, and my basic electronics go back close to the plane’s age ( 🙂 ). I have the service manual, but I cannot find any reference to the voltage other than 3V are needed to start the servo.
    I would like to do a basic troubleshoot because we have no avionics shop at our airport.

  96. Thanks Dave.

    I was another victim of the Amphenol connectors.

    There was a huge drop and the changes I was seeing were not enough to engage the servo. I then measured the voltages at the amplifier connector and got 0 to +11 in two of the pins depending on the knob position. I cleaned all contacts from there to the servo with an electronic contact cleaner and reattached them firmly and that was enough to get the same signal at the servo end.

    Just came back from a taxi test and it all seems to work great. I’ll test it in flight tomorrow to ensure the 30degree banks are still good.

    Double thanks for checking your blog during the weekend!


  97. davidcrocker says:

    Glad you got it working. You will probably need to do more than clean the pins to make it reliable. Is the connector a loose fit once the locking ring is disengaged? If so, try tightening up the socket receptacles by using a needle to push to two parts back towards each other. Or replace the socket.

  98. It was quite firm actually, but I did used a needle anyway to compress them a little after cleaning, like you said in the post.

    I’m doing a 3 1/2 hour trip tomorrow. I’ll let you know how it went. But in the flight test it worked like a charm.

    Thanks again,

  99. Andy says:

    I have a Altimatic IIIb in a Piper Navajo and everything is working great other than the altitude set is off by a couple of hundred feet from what is set and I can’t get it to capture the glideslope. Any ideas?

    • Andy says:

      I have a Century III autopilot manual available if that helps.

    • Vicente Collazo-Davila says:

      Hi Andy, I too have a Navajo with the same autopilot. Forgive me if I’m stating something you already know. The altitude disk serves 2 purposes; it not only shows the selected altitude, but is also used to calibrate for different altimeter settings. You should level off at the desired altitude based on your primary altimeter and then rotate the disk to agree. Once calibrated you are good until there is a significant altimeter setting change. It is possible to run out of calibration range since the disk can only move about 1500 feet. For example in our airplane once the altimeter setting goes above 30.10 the disk is cranked up as far as it will go, and then we just have to turn the altitude set knob 100 feet low for every .1″ of pressure above 30.10. (eg if altimeter is 30.30 then for 9500′ we use the altitude knob to set ~ 9300′ then fine tune it by setting the knob a bit higher or lower as needed.) You can adjust the calibration range, but it is a bit of a PITA, and to do it right it has to be done by an avionics shop with the proper equipment.

      Now, to your second issue: Assuming your glide slope coupler is working you should be able to capture a GS, but the AP is very demanding that you follow the proper procedure. You have to be in altitude hold on the AP panel, the radio coupler has to be in Loc Normal, and you have to be at least 60% below the GS for at least 20 seconds. I give myself at least one minute with the above conditions and I also press to test the green GS capture light about a minute before i intercept the GS. Also airspeed seems critical. Try to be fully configured and between 120 and 145 mph at capture. The green light and capture usually come on about 3 to 5 seconds after the GS indicator centers. Once you’ve captured and started down the GS don’t let AS get below 120MPH. The stall protection is set at about 110MPH and if you slow to that poing, the AP will fly you at 110 and drift below the GS.

      • Andy says:

        Thanks for the assistance. Great information. I found out that our plane has no glideslope coupler installed, thus no glideslope. I am procuring one now and will have it installed shortly. I also ordered the maintenance manual and that has been a great help as well and will help with the harness for the glideslope coupler. Thanks!

  100. Nick says:

    Hello David,
    This is the first time I’ve ever written to a blog. I apologize if my entry ends up in the wrong area. I have a 1966 Twin Comanche with the original Altimatic 2 autopilot. The autopilot worked fine up until a few years ago when the altitude hold started becoming intermittent. For a while, it would work on really cold days now it doesn’t work at all. When you turn on the autopilot, it tracks fine and keeps the wings level. Before you engage the altitude hold, there is a knob you use to dial in the altitude until the yellow up and down lights go out. Only the up yellow light comes on but it goes out when you dial in just a little below the current altitude (more on that later). When you engage altitude hold, the trim crank immediately starts spinning clockwise in the trim-up direction (the plane has electric trim). The plane holds altitude for a few seconds and then the nose rises sharply. When I look behind the autopilot control panel, I can see a twisted pair of small black wires that are disconnected. One of them ran to the yellow down light socket (which explains why it doesn’t come on) but then there’s a 2” piece of disconnected wire that I can’t see where it was connected to. Do you have any idea where it is supposed to be attached or could you please tell me where I might be able to get a wiring diagram for the altitude hold system? Thank you.

    • Vicente Collazo-Davila says:

      I got my maintenance manual for the Altimatic III from Essco at a reasonable price. I would assume they would also sell the Altimatic II manual.

    • Ronnie Hughes says:


      Warning: This is a long-winded reply and will contain spelling and gramar errors.

      I have an Altimatic II (or as you said Altimatic 2) autopilot in my 1964 Aztec. When I bought the airplane in 1999 the autopilot was marked as INOP and the breaker tied off. The former owner refused to even attempt to operate the autopilot. Maintenance shops I talked to said repair of the unit was not practical. So I decided to buy a service manual and at least understand a bit about the autopilot system before writing it off. I called a Piper parts distributor and purchased the service manual from them. It is Piper part number 753 798. Appendix 1 is the section you really want to review for later model Altimatic II systems. The system wiring diagram is there along with a theory of operations, and a great section on the radio coupler including schematics. There is also a section on the automatic trim systems. What is not there are schematics for the autopilot amplifier. I’ve yet to find a schematic for this component, but I have one for an early version of the amplifier that is close.

      As I began to look at mine, I first was confused by Part 1 of the service manual. The attitude gyro and heading gyro described there were no longer in my airplane and had been swapped with newer, 3″ gyros. On closer examination I was pleased to find that the AI (model 52D55) did have the RF pick-offs for the pitch and bank circuits of the amplifier (model 1D293). The DG (model 52D52) had the 5KHz sensor circuit needed to work the radio coupler (model 1C292) and that the radio coupler was interfaced to the radios and to the amplifier. I also discovered that the pitch servo and the bank servo had been disconnected from the flight controls. Long story short is that once the servos were re-connected and the system was setup and adjusted via the service manual, the wing-lever / bank portion of the autopilot worked ok.

      The pitch axis had an issue with severe nose down pitching motion when engaged and caused the electric trim to run to the limit. My UP and DOWN yellow pitch indicators did not work. The problem was burned out lamps and bad connections in the lamp sockets. Once that was solved it was easy to see if the autopilot would command nose down, level, or nose up if engaged. A careful set-up of all of the components in the pitch circuit was carried out according to the service manual and the nose down pitch issue remained. About this time I started looking for spares on EBay, and also acquired a complete of complete systems which had been pulled out of other airplanes. One of the amplifiers from a spare system worked well and solved the nose down pitch issues. FYI, it is a great aid to have a complete system set up on the work bench to play with and to test components with. I was also luckily enough to find an old Mitchel test box for the system.

      The system is completely different from the Altimatic III and later systems. Here is a quick Theory of Operations description:

      The system uses tuned RF circuits running at approximately 7.5 to 8MHz for both the pitch and bank sections. The bank circuit is brought to resonance by adjusting the bank trimmer on the amplifier with the AI upright and level and with the wing leveler trim knob centered (which is another variable capacitor. There is an aluminum and iron vane in the AI which changes the inductance of the bank circuit when the airplane banks left or right. This causes the RF frequency to increase or decrease depending on which way the airplane banks. The amplifier senses this off resonant condition and develops a DC output from a frequency discriminator. This DC level is amplified by a couple of power transistor stages to drive the servo the proper direction. There is a follow-up capacitor mechanically coupled to the servo shaft which brings the circuit back into resonance and causes the servo to stop moving once the controls have been moved far enough. As the airplane responds and the wings return to level, the process is reversed to center the ailerons.

      The radio coupler is an analog computer which takes heading information from the DG and adds in any NAV / LOC error if in NAV / LOC mode, and produces an error signal which is DC coupled into the bank circuit of the amplifier. This causes the amplifier to drive the servo just as if the AI had indicated a bank.

      The pitch circuit is composed of another RF oscillator the frequency of which is determined by the pitch vanes in the AI, the pitch trim control (a variable capacitor) and the pitch centering control on the amplifier (another variable capacitor). In addition, there is an additional capacitor in the attitude hold unit, a bellows assembly which varies the capacitance as the static pressure changes and the bellows expands or contracts. When the Altitude Hold control knob is pulled out, this capacitance is in the circuit. When the Altitude Hold control knob is pushed in, a fixed 10pF cap is switched into the circuit in place of the variable cap in the altitude hold bellows assembly. There is also a trimmer cap in the pitch trim assemble which can be adjusted through the front of the pitch trim unit to adjust for differences when switching between pitch mode and altitude mode. The pitch servo has a follow-up capacitor like the bank servo. The yellow UP and DOWN pitch indicator lights (with series diodes) are directly connected across the wiring driving the pitch servo motor. These show if the amplifier is driving (holding) the pitch servo away from neutral position indicating an out of trim condition. If you didn’t have electric trim, you would adjust the elevator trim until both lights are extinguished.

      With the electric automatic trim system installed, when the pitch side of the autopilot is mechanically engaged, a micro switch in the pitch servo disconnects the manual electric trim switch and powers on the pitch trim sensor and pitch trim servo motor. The sensor detects either tension in the up or down elevator control cables or currents in the pitch trim motor and runs the pitch trim motor up or down until the cable tension or motor currents are reduced to minimum.

      Based on your description of the pitch-up problem, your problem could be with the automatic pitch trim system or with the autopilot pitch circuit.

      To trouble shoot, I would first get both the UP and DOWN trim indicator lamps working. I found that white LEDs could be mounted into the yellow lens assemblies and then hard wired (soldered) into the circuit which eliminated issues with loose sockets and burned out lamps.

      Second, I would pull the electric trim breaker and check out the pitch operation of the autopilot in pitch mode (PITCH CONT knob pushed in) and in altitude mode (PITCH CONT knob pulled out).

      If on the ground, you can fully engage the pitch servo and the elevator control should follow. If in the air, I would do this test without engaging the pitch servo completely. Since getting both lights outs also depends on the position of the elevator control (the follow-up capacitor in the pitch servo) your actual pitch attitude will affect this as well.

      Start in pitch mode and rotate the PITCH CONT knob back and forward and watch the reaction of the pitch indicators. If everything is working properly, around the center of the control range both indicators should be out. If you turn the knob clockwise, the UP indicator should come on. Turn it back counter-clockwise passed center and the DOWN indicator should come on. That shows if the amplifier outputs are working and capable of driving the servo in both directions.

      In Altitude mode, you can do a similar test by rotating the altitude selector knob and watching the indicators. With the selected altitude well below your actual altitude, the UP indicator should be on. As pass through an altitude that matches your actual altitude, both indicators should go out. With both indicators off, you should be able to make them flicker on and off as you vary the selected altitude back and forth and as you vary the pitch trim control up and down. Continue increasing the selected altitude to well above your actual altitude and the DOWN indicator should come on and stay on.

      The dial which indicates the selected altitude is designed to slide left and right with the CAL knob allowing for adjustment for changes in barometric pressure.

      If all this works as described with you manually trimming the airplane, the autopilot system is working and the problem is with the automatic trim systems.

      If you can’t get the indicators to behave as indicated above, some simple things you can try are to unplug the RF cables from the AI and make sure they are clean and re-tension the barrel tabs to make sure the connector is held in place firmly when plugged by in. I’ve also seen intermittent connections where the RF cable bundle plugs into the amplifier. If that doesn’t get it going, you have a more serious issue such as an amplifier failure, broken RF cable, bad AI indicator, etc.

      If the issue is the automatic trim system, then you will need to determine which system you have. There are three different types of automatic pitch trim sensors and pitch trim servos shown in the service manual.

      Final comments. Be aware that this system was designed around 1960 to 1961 by Mitchel Industries. It is absolutely and completely obsolete. As an example, all of the transistors are germanium, not silicon, and all the power transistors in the amplifier have custom Mitchel Industries part numbers on them. Mitchel has not been helpful in providing any technical information and has not supported this system with parts for a long, long time. If you have a transistor failure for example, you have to find a spare amp that works or that you can rob parts from. They occasionally show up on EBay. As of a few years ago, there is at least one shop which still overhauls the RF style Attitude Indicators. For the most part, this is not a practical system to maintain any longer, but playing with mine has provided many hours of entertainment and learning. And once I got it working, is has worked great and flies the airplane very smoothly.

      Give me an email address if you have any question to discuss one-on-one.

      Ronnie Hughes

  101. Vicente Collazo-Davila says:

    Hi David,

    Thanks for your help in the past. New problem. Yesterday one of my partners had been in cruise for over an hour in smooth air. The CB for the Altimatic IIIB popped open and the AP died. He waited a minute for cooling then reset with another trip so left it out and hand flew rest of the way home. First question is could the CB fail in such a mode that it opens with normal current draw? If it is a short, what would be the best procedure to try to isolate the location? I think using a VOM in resistance mode to check between the load side of the CB and ground would check for a short on the way to the control panel. Also I think that by disconnecting the harness at the amplifier I could check for shorts on the wiring to the servos (by the way, what would be a normal resistance reading between the motor and common leads?). I can’t figure out how to check for a short inside the control/input panel since you need current for the roll rocker/on switch to remain engaged.

  102. davidcrocker says:

    I would check the power transistors in the amplifier first. They often go short circuit, and this causes the CB to trip. If replacing one power transistor, replace all 4 on that side. A short to ground in a cable between the amplifier and a servo is another possibility.

  103. Randy Knuteson says:


    Great troubleshooting article on the Century autopilots!
    Do you have access to Service Bulletins and SILs and ADs related to the Edo-Aire Century Autopilots that may have been issued between 1973 and 1986? Or could you refer me to a service center that would have copies of these publications?

    • davidcrocker says:

      Unfortunately I do not have a such a list, nor have I come across any SBs pertaining to these autopilots.

    • silentflier says:

      I have managed to obtain most of the ex CSE a/pilot & instrument manuals. If you can be a bit more specific on which autopilot I’ll see what I can find for you.

    • silentflier says:

      A quick search through the mound of manuals has revealed a Century “Service Letters & Service Bulletins Manual” – No Edo/Century part number. It’s not listed on the Century web site but it might possibly be a main dealer only item. The issue I have is Nov. 2004. Any SBs etc. issued after this appears to be on their web site so It’s possible they have discontinued it. I’d be happy to search for specific items but the manual is almost an inch and a half thick so it’s a bit much for me to photocopy complete.

  104. Vicente Collazo-Davila says:

    Hi David.

    What are the differences between the Altimatic IIIB and IIIC. I notice they both use the IC 515-1 amplifier so can you use the amp from one on the other? Thanks in advance for your help.

    • davidcrocker says:

      The only difference I am aware of is that the trim amplifier in the IIIC is driven from the output of the main amplifier pitch channel, whereas I believe the IIIB (like earlier models) uses a separate mechanical trim sensor that detects differential tension in the elevator control cables. However, one amplifier I came across had a sticker on it saying “This amplifier is set up for Altimatic IIIC”, so perhaps there is some difference in the amplifier setup as well.

      • Kristin says:

        I have converted an Altimatic IIIB to a Century III, which is the same as an Altimatic IIIC. This was in a twin Comanche. I had to change out the amplifier which was a 1D395, to a 1C515-1, the control head, the altitude sensing unit, and the pitch servo. The trim system, roll servo, and wiring harness were the same.

  105. Bill Dino says:


    I just found this site and have been reading with great interest. We have a Baron with a CIIIc and it keeps blowing one of the roll servo power transistors (one of the four on the case). I noticed you wrote earlier that …. “Also measure the 2 power resistors attached to the transistors in case they are exhibiting greatly increased resistance, which could make the transistors overheat.” It seem that if these resistors increase in resistance it would only result in less base drive an less of a chance of overheating. What am I missing??? ( both our 68 ohm resistors are much higher, one is almost 90 ohms and has signs of overheating)

    Also, I’ve been told that the shield going to the servos is “hot” and that if it shorts to ground it can cause this problem. However my view of the schematics seems to show that the shield goes to aircraft ground and in any case only affects the clutch, not the servos. Comments?

    Thanks very much, this is a great site.

    – Bill

    • davidcrocker says:

      The power dissipated in a transistor is the product of the voltage between the collector and emitter, and the collector current (plus a contribution from the base current, which is generally small). When the autopilot is commanding full power from the servo, the upper transistor on one side of the H-bridge output stage will be turned fully on. The lower transistor on the opposite side is turned on by the current flowing through the 68 ohm resistor. If there is enough current, the transistor will be put into saturation (i.e. turned on fully) and have a very low voltage drop, so the power it dissipates will be low. If the resistor value is too high, then the transistor will not be turned on fully, have a higher voltage drop, dissipate more power, and so get hot.

      I suggest you replace those resistors. However, if the transistors are failing frequently, then it is likely that there is another cause, such as a short from motor output or motor common to ground.

      If an output transistor fails, always replace all 4 on that side, because one shorted transistor will overload the others.

      The schematics I have don’t show what the shield of the servo cable is connected to, however there is a ground wire in the cable and I would expect the shield to be connected to that. On the other hand, the ‘motor common’ wire is ‘hot’ and connected to lots of other parts of the autopilot system, so a short from there to ground is a real possibility. However, if there was a dead short between motor common and ground, then the servo would only run in one direction, even before any of the transistors fail.

      • Bill says:

        Dave, Thanks very much for the fast reply. I am also concerned that we may not necessarily have a short to ground in one of the motor leads but simply a failing servo motor perhaps with a short in the windings due to age. The Roll servo is positioned in an area that has accumulated a lot of gunk over the last 40 years (its a 1973 Baron)

        Do you know what the correct DC resistance is for the servo motor windings when open circuit (not connected to the A/P computer)? I don’t know if it matters but this is a 28 Volt System

        Thanks again,

        – Bill

      • davidcrocker says:

        Sorry Bill, I don’t have that information. If the pitch servo is the same part number, you could compare the resistances. I would expect 28 volt servo motors to have 4 times the resistance of 14V motors.

      • Vicente Collazo says:

        Hi David,
        I recently made a discovery that I wish to share with you and others here. The 2N3055 that we can get today is not truly the equivalent of the original ones. It has the same max voltage and gain, but is not as tolerant of high temperatures. Apparently they shrunk the base region to increase the frequency response, but this resulted more susceptibility to heat damage. After my last failure, I was able to buy some old, but unused 2N3055HOs. I’ll let you know how those work out. If you can’t find the HOs then I understand that the 2N3055Hs are the next best thing.

      • davidcrocker says:

        Thanks for sharing that observation. I’ve looked up the specifications of two 2N3055, one 2N3055G, one 2N3055AG and two 2N3055H transistors from several manufacturers. One of the 2N3055s (Multicomp) had max junction temperature of 150C. All the other transistors (including the plain 2N3055 from ST Microelectronics) had 200C. So it appears to depend on the manufacturer.

  106. Ian McKelvey says:

    You seem to be the most knowledgeable in regards to the Altimatic auto pilots.I have a question for you. I have an Altimatic IIIc with a King 525A HSI. The auto pilot works well, but seems to be a bit lazy when intercepting a lateral course. When I turn the heading bug the A/C turns smoothly with about a 15* bank and rolls out smoothly and tracks the bug dead on. But when intercepting a VOR radial or localizer it has a lazy shallower bank and tends to fly through the course. Also, when finally set up on the localizer and all the conditions are met (1.Localizer freq tuned in., 2. all rockers switches “on”, 3. 20 seconds time, 4.LOC/NORM selected.) The auto couples the G/S and pitches the nose over but once again it does this kind of late and lazily the G/S needles stays halfway deflected and it never tries to correct the too high attitude. Finally, every time I push the “ALT” rocker on. It always captures the altitude 75′ high. Would that have any issue with the fact we are high on the G/S? I do have a service manual and have read about how to correct the Altitude high problem with the “E” potentiometer. Please advise me if you have any suggestions. I’d truly would appreciate it, as I am out in Hawaii and we don’t have anyone out here who’s knowledgeable with these old auto pilots.
    Thank you,

    • davidcrocker says:

      You should be able to correct the altitude hold error and glideslope error by adjusting the second helical pot from the right under the faceplate of the control console. There will be an autopilot adapter between the KI525A and the autopilot, AFAIR the usual one is King KA57. The maintenance manual for that unit shows two potentiometers R108 and R109. One of them sets the gain in heading mode, the other sets the gain in omni/nav/loc mode – looks like you may need to adjust that one.

  107. Ian McKelvey says:

    Hi David,
    First let me start by saying thank you for the advice! I went out today and adjusted the “E” pot and got the altitude spot on! Shot the ILS and she nailed it right down to the DH! I landed then took a look under the dash and for the KA-57 and to my surprise I found it! But it turned out to be a KA-52. I opened it up and found the 2 potentiometers and put an ohm meter on them both. Both of them were the same .0080. We changed the course potentiometer down to .0050 and took it back up for another try on the ILS. Still the same. Very shallow bank angle when the localizer needle came alive and I flew through the course. She was trying to bank towards the needle but very shallow. It would have eventually captured it but I had to turn the LOC/Norm mode off and back to HDG (heading bug) as the auto is very aggressive in this mode and I quickly got the plane on the localizer. Oddly enough..once I got the plane on the localizer I switched back to LOC/NORM and after 20 seconds she coupled the G/S spot on started a descent and stayed on the localizer. Although…Every time the plane navigated down the localizer she stayed to the left side of the course (1.5 dots on the HSI) as if the center is off by 1.5 dots. I did see in the service manual that we can recenter the OMNI cousre…by holding the red button on the back of the 1C388 and tuning the “B” pot screw. Am I right by saying this? Will this screw up the heading bug? As she is spot on and tracks perfectly in the HDG mode. Finally…any other advise on stiffing up the bank angle in OMNI LOC/NORM mode? I appreciate the time you have given this.

  108. davidcrocker says:

    Yes, you need to adjust the B pot to get the autopilot to track a VOR or LOC, before you concern yourself with the details of the intercept. This can be done even without holding the red button, although holding the red button should make it easier by making the autopilot respond faster. When you have got it holding the needle in the centre, try intercepting from one side, and then the other.

  109. doughughbanks says:

    Hello David I have a Rockwell Commander 114 Century III w/ GS coupling and Century I backup. After level flight and trim and engaged alt, when PTT to the coms the trim meter moves up and the aircraft pitches up 1000 fpm climb until up release and goes back to level flight. I’ve moved coax to an external antenna thinking rf is getting into my system somehow what to do next?

    • davidcrocker says:

      How is it that you have both a Century III and a Century I installed?

      It sounds to me that either RF is getting into the autopilot, or possibly that there is a poor ground connection shared between some part of the autopilot and the comm unit, so that when the comm unit is transmitting, the supply voltage to some part(s) of the autopilot is reduced.

      • doughughbanks says:

        The aircraft came from the factory with the Century I installed when the aircraft was 5 years old the Century III was added and theirs another relay box that allows only one unit can drive the roll servo. Their are 2 autopilot master switches and when the Century III master is turned on the right/left and clutch of the Century I is interupted. I found a letter from Century about R.F.I. Interference and I will have to go throught and check all grounds of the cables.

  110. silentflier says:

    Been a while since I was on the flight line (workshops are so much warmer, and drier, and nearer the kettle!) so excuse me if I’m talking rubbish in this case but, for what it’s worth, I have seen a similar problem where there was a problem with the yoke cabling (cable too tight, rubbing removed insulation) and the PTT was shorting to the manual trim switch/AP Disconnect… Just a thought.

  111. Daryl purpera says:

    David, my 67 Arrow has the Autocontrol III. I’ve had it overhauled by Autopilot Central but I still get lots of wing wobbling in light turbulence, works great in smooth air. Noticed the bridle cable on roll servo has slack on trailing side of spool when rotating. Is this normal? Where do I find cable tension specs?

  112. joshua657 says:

    Good afternoon David, I have a Piper Navajo Chieftain with an Altimatic IIIC installed. The autopilot initially had two issues. 1) when steering the plane with the heading bug it would immediately roll into a 60 degree bank. 2) When the autopilot was engaged and the altitude hold was engaged the autopilot would work perfect. If you disengaged the altitude hold mode then reengaged it, the aircraft would porpoise up and down 300+ feet. The avionics shop found the artificial horizon needed overhauling as well as the HSI (NSD360). The altitude control module as well as the autopilot console were sent to MidContinent for bench checking and repair. The altitude control module and the console both bench checked good. The artificial horizon and the HSI were both beyond repair. We reinstalled the autopilot components as well as an overhauled HSI and artificial horizon. On the test flight everything worked perfect. The banking was nice and soft and the autopilot would hold altitude after the altitude hold mode had been cycled. On the next trip, during the run up while performing the autopilot ground checks. As soon as the pitch mode was enabled the trim wheel rolled all the way back to the stop and the pitch servo wouldn’t engage. When the avionics master is turned on you can hear the pitch servo motor continuously running. I removed the cover of the servo and it is running in the nose down direction. The autopilot amp was sent to Duncan for bench testing. They found several issues with the pitch aspect of the amp. They sent us a loaner amp while ours was being fixed. Upon installation of the loaner amp, the autopilot did the exact same thing. No pitch mode at all. The pitch servo motor runs continuously. The servo checks out fine. Pin A on P-6 at the pitch servo does not have voltage. Is there anything else we should be checking? Thank you for your help!

    • davidcrocker says:

      Since the AI and autopilot amplifier have been checked, it’s probably an issue with the wiring between the AI and the amplifier. Check the round blue socket that mates with the pins on the back of the AI. The socket receptacles become loose over time. If the socket is not a firm fit over the pins, replace the socket. If the aircraft has a GS coupler installed, also follow the cable attached to that socket for a few inches until you find the round blue plug and socket that connects the main loom to the glideslope loom. Again, if the socket is not a firm fit with the plug after you disengage the locking ring, replace the socket.

      Also see paragraph 7 in the original blog entry to see how you can meter the wiring from the amplifier to the pitch coil of the AI.

      • Don G says:

        Hello David,
        I have a Century IIB in my Piper Turbo Arrow III and I must say I have been very fortunate that it has been trouble free over the 13 years I have owned the aircraft, until now. We had to replace the AI with a new SigmaTec unit and the problem began. The AI was beginning to show the typical signs of a slow death however, the autopilot continued to function normally. After installation of the new AI I found that with the AP on the roll control functioned perfectly but when in heading mode I could move the heading bug on the DG in any direction to no effect, likewise with any of the other modes, VOR, Omni etc. There did seem to be an almost in-perceptible drift to the right however, again wherever the bug was placed. Given this occurred in the last 24 hours we have not begun serious trouble shooting yet but thought I would bounce it off of you for a few suggestions that might narrow our actions.
        Thank You,
        Don G
        Scottsdale, AZ

      • davidcrocker says:

        Perhaps the wiring between the DG and the autopilot was disturbed when the new AI was fitted?

  113. Jeroen van brakel says:

    Checked the connector on the back of the DG, and when I loosen the ring, the connector feels very loose. It actually just falls of the DG pins.I have tried to pinch the little female connector in the connector, but that does not make a lot of difference.
    Any solution



  114. Robin says:

    Hi David,

    I have a Piper altimatic IIIC installed in my piper PA-34-200T, in the roll mode everything is fworking perfectly. But when i want to engage ALT or PITCH, the autopilot just moves the control wheel in a nose up direction. I can not make it go in a nose Down direction ( in PITCH mode). It just goes automatic in a nose up direction.

    My avionics shop has already send up that Altitude hold unit, the amplifier and the horizon for a bench test but they didn’t found anything wrong. I have already measured the continuity between the ALt hold unit and the console.

    Also they have replaced the amolifier and the altitude hold unit with a known working unit. Do you have any suggestions, what else I can do to fix the problem??

  115. Bill says:

    Hi Dave, this is a great service you’re offering, thanks for taking the time to help all of us. I have a question on the trim servo in a beech baron. We’re having problems (doesn’t work at all) and I want to send the amplifier out for repair (servo checks out ok.) It looks like the servo amplifier and servo are mounted on the same base. In other words, it looks like my mechanic will have to remove the entire assembly (amplifier and servo motor and associated rigging) in order for us to get the servo amplifier out. Is there a way to only remove the amplifier and leave the servo in place?

    BTW, we have the single button type of trim where you hold the button down and the trim servo engages to automatically remove the pitch pressure from the yoke. It’s not a ‘trim forward’ or ‘trim reverse’ type of switch.

    Thanks – Bill

  116. David says:

    Hello David,
    I appreciate your web page. I have a PA 28 which uses a Mitchell 1C359-1 tracker. The problem is that when engaged the Aircraft pitches fully to the left slowly when potentiometer cantered, but this turn is exaggerated when potentiometer is rotated fully anticlockwise. There is no RH response. Looking for advice and a CCT diagram can you help


    • davidcrocker says:

      Sorry David, I have no experience with the 1C359. Does it have a connection to the AI, like the later versions have? If so, then I suggest you check those connections, particularly if they use the same round blue connectors.

      • Bill says:

        Hi, do you know the type and value of the potentiometer used in the Autoflite wing leveler trim? It connects to the 1C359-1 accelerator module and on the back says 18A18 and 1376614. Would it possibly be a 10K linear?

    • Patrick Morris says:

      Mine is doing the same thing, did you ever figure it out?

  117. Stephen Gansky says:

    Hi David

    Great blog! How does one change a bridal cable on a Century servo? Is it that 050 Allen screw in the edge of the pulley? I’m letting it soak with PB blaster because I can’t seem to budge the set screw.

    PS. Someone was looking for 2N3116 transistors ( in an old post ). I have plenty!

  118. frankf33a says:

    David after working my F33A air conditioner the altitude will not work. Climbs or descends without reason.

  119. frankf33a says:

    By the way it is a CIII autopilot!

  120. Dusty Carnahan says:

    Fabulous blog !!! … I have a Century IIIAK557 installation in my Cessna 310D … works great but recently my pitch trim down went out and that was followed several flights later with trim up going out … autopilot altitude hold works fine so the assumption is there is a problem in the trim amplifier (1C709) … before pulling it I want to check the trim servo to ensure it works properly … what is the best means to check the trim servo ? … thanks in advance …

  121. Maureen Beintema says:

    any one have a STC for Century III auto pilot for a Comanche 400?

  122. craig moodie says:

    I just bought a 78 Lance turbo II. I think it has a altimatic III. No power to the unit at all. Wont engage any of the rocker switched. Haven’t looked at it much. Just got it yesterday. Any obvious culprit? Fuse Breaker connector?

  123. Vern Eliason 1973 Aztec "E" model says:

    I have entered this topic in the past and have now done more to this auto trim system than should be allow’d. I have an Altimatic 3 b 1 in my 73 Aztec which has had auto trim issues for the past 3 years. We removed the trim sensor which was under the belly p-an and sent it to Barry Sparks for an overhaul. Had it installed by Des Moines flying service and after re-tempering some of the plugs attached to the trim sensor, it was declared ready to go. Took off and in 10 minutes of flight it died. We then decided that the plugs must be the answer so the plugs were cut out of the system, and hard wired. Took off and in 10 minutes it stopped working. After reading Dave Crockers response I decided to change the trim amplifer, so I found one at Muncie Aviation and flew the bird down to have it installed, but was told that the trim amplifer that was in my plane had checked out to be ok. So the technition cut out the hard wired trim sensor and examined it and re-connected it and replaced the sensor. Did a test flight and it performed flawless. Paid the bill and got into the airplane to head for home and was all happy that it was working when 10 minutes into the flight it died. We needed the plane for a planned trip to Florida, so on the way I decided to do some more testing. I engaged the autopilot and turned the crank a few turns to put the plane in an out of trim position. The autopilot was holding the plane at altitude. I pulled the breaker on the electric trim and left it for 10 minutes. Then I pushed the breaker back in and “walla” the crank spun around 4 times and put the plane back in trim. Then I tried it the other direction and did the same thing. Pulled the breaker on the electric trim, put the plane out of trim and let the autopilot hold altitude. Waited 15 minutes and popped the breaker back in and sure enough, the crank spun again in the other direction and put the plane back in proper trim. So what ever is cooling off is coming to life for a few minutes after it has had no power for 15 minutes. I need someone to tell me now that the trim amplifer is the culprit?? Please advise.

    • davidcrocker says:

      Intermittent faults can be difficult to pin down. It’s certainly possible that the fault lies in the trim amplifier. One way to test that would be to borrow a known working trim amplifier if you can find one, and test fly with it.

      • Vern Eliason 1973 Aztec "E" model says:

        I agree, that gets expensive in that the servo and the amplifer are on the same mounting. That is going to be my next step. If I can negotiate with muncie in that my directive origionally was to change the amplifer, but was talked out of it.

    • Bill says:

      The trim amplifer circuit board is very prone to “cold” solder joints. It can test fine on the ground but then fail intermittently in the air due to the additional vibration, etc. The solder can look fine on inspection. As onerous as it might be, you might consider asking a tech to resolder all the connections on the PC board (ALL of them, transistors, wires, etc.) The board is not that large; it would not take more than 15-30 minutes to complete this task. This assumes that your A/P as a whole is working correctly and it is only the auto-trim that is out of whack. (Also, check the connections on your auto-trim button on the yoke.)

    • Vicente Collazo-Davila says:

      Vern it certainly sounds like your problem is in the autotrim amplifier. It’s made up of a bunch of discrete components, about a dozen transistors and associated resistors and capacitors. Odds are a component, most likely a transistor, is overheating. If you can’t find a substitute amp you could try to figure a way to cool it by using a small fan or dry ice to see if that extends the time before failure. Be aware that there are at least 4 kinds of auto trim amps depending on such factors as whether you have the autotrim that only works with the AP on or the kind that allows you to autotrim by just depressing a yoke mounted button when the AP is off nand you need to trim out control pressure.

  124. Larry Miles says:

    I have appreciated reading the comments here. I have a problem that has driven me crazy for several years. I have an ’75 A36 with a Century 1, coupled with an STEC 55 for altitude hold. The altitude hold always works. Thank you STEC. The Century 1 works on some flights but not other flights. It typically does not work on the first flight of the day, especially if it is cold or damp. Later in the day, typically on the return flight, it will work perfectly. The failure mode is that it will not work in either mode: the VOR track mode or as a wing-leveler. Instead, it enters into a slow turn and, if not disengaged, would send me into a death spiral. I have no confidence in any particular flight that I can rely on the autopilot. I’ve taken it in several times a year to my shop, they do a few things, and nothing ever seems to ever get fixed; the failure occurs again within a few flights. Thoughts?

    • davidcrocker says:

      I’ve not worked on a Century I, but given that symptom, I would check the connections between the autopilot and the AI or turn coordinator, especially if it uses the same round blue connectors as the Century III.

  125. larrysb says:

    Thanks David for all the great info on the Century autopilots. I’ve got a Seneca II, with Altimatic IIIc. I did an avionics upgrade and we went through the A/P system, restoring all the wiring to very much what is in the Piper installation drawings (Piper was very helpful in providing the factory installation dwgs). It had been the victim of many avionics techs over the years who’d done a lot of strange things. We also replaced the blue AMP connectors with the new, solid pin designs. So, I’ve actually seen it work more or less, including the G/S coupler. I need a pitch servo and the trim servo is weak.

    My remaining issue is an intermittent that I think is in the computer (amplifier). On cold days, engine running on the ground, I can command a roll left with the knob, but no right roll even with knob at full. At best, it makes little jerks to the right. However, on a very warm day (or sometime after it’s been on for a while), it will literally work nearly perfectly for hours, keeping very solid wings level and tracking headings and the gps/nav signals. It will even trigger and properly lock the G/S.

    Thoughts? I’m thinking the amp, but I guess there could be a bad connector or even the horizon gyro.

    • davidcrocker says:

      I agree, I think the most most likely cause is a bad joint or poor connection in the amplifier. I would start by looking at the connections between the circuit board and the output transistors, and at the 68 ohm resistors connected to the output transistors. After that, the circuit board.

  126. BMARTIN says:

    Dave, thanks for being such a dedicated information source to our pilot community. I have a Commander 114 with a century III. All ground tests from the service manual seem to work good. But in the air… Even if I only engage roll or just roll and heading… The auto pilot will start oscillations to the the left and then over correct back to the right with bank angles significantly more than standard rate… It just recently started this issue and on very rare occasion will perform normal. I am planning to evaluate all connections this weekend but thought I would seek your advice in advance. Thanks again.

    • davidcrocker says:

      The first things I would check are the connector on the back of the AI, and (if your aircraft has a glideslope coupler) the connector a few inches from it that connects the main loom to the glideslope loom. The receptacles in the sockets of those round blue connectors lose their grip over time and fail to make good contact with the pins. Lack of roll feedback from the AI to the amplifier might well cause that symptom.

  127. Ted says:

    Excellent information on these old autopilots. I’ve spent most of my flying career with Century IIIs of some form or another.

    I currently have an issue with the Century III in a Cessna 310N. The pitch neutral requires the pitch trim wheel to be significantly nose down (with the neutral point on the wheel below the bottom of the opening on the main control panel), but it does otherwise function. When the altitude hold is engaged, it seems to oscillate significantly +/- about 200 ft, my guess being because it’s trying to go towards what it thinks is a neutral pitch position. Attitude indicator was recently overhauled, but it did this before replacement. Plane does not have a GS intercept box nor electric/automatic trim.

    When my Aztec with an Altimatic IIIB did the same, it was solved with the computer getting repaired, but it seems that reading on here perhaps adjusting the potentiometers would be a first step. The AP didn’t used to do this when I first started flying the plane. Interested in your thoughts.

    • davidcrocker says:

      The oscillation problem could be as simple as a loose bridle cable or sticky controls. It’s not unusual for the pitch trim wheel to be somewhat off-centre for neutral pitch – in any case, the required pitch for level flight (and hence the position of the wheel) depends on the airspeed.

  128. Peter says:

    Dave Thanks for your indefatigable work.
    I have a 1975 B55 Baron with a Century III. It has been working well, but three flights ago, it began significantly over-shooting on heading changes. The last two flights it got significantly worse, changing either the heading bug or the roll control on the AP both lead to very steep bank (I disabled the AP at 60degrees). It was worse rolling to the right than to the left. I have not yet tried adjusting the Rt. Bank & Lt. Bank Pots behind the faceplate, but I doubt they would be helpful since the problem came on rather suddenly and seems to be bad in both directions — I wouldn’t expect both pots to develop bad spots at the same time. I will try the pots, but any other suggestions?

  129. davidcrocker says:

    It sounds to me like either an amplifier fault or a wiring fault, probably in the wiring from the AI back to the amplifier. Does the autopilot work correctly in roll mode? If not, check the blue connector on the back of the AI, making sure that the receptacles grip the pins tightly. Also check the same on the connector that joins the glideslope loom to the main loom, which is usually just a few inches away from the AI.

  130. Bill Weber says:

    Hi David, I have a Century IV in my P-Baron that has a pitch pump in Alt Hold mode only. All other modes work perfectly. When switched into Alt Hold mode, the yoke starts to pump back and forth making Alt Hold unusable. I just spent 4 very costly days back at Century in TX and THEY COULD NOT FIX IT! They say all components are perfect and went thru all the wiring and connectors.

    Could you help with this one? I am very upset with them and now need some miracle help. I have a video of it in flight if that could help… If you had time, we could take the details offline and post the results if anything is found? Im not sure who to turn to now. Thanks for your great input!


    • Dwight says:

      I had problems with the AP computer (Century 31) in my Mooney. Many trips to the manufacturer, ‘couldn’t find a problem’ – or did something resulting in a temporary fix. Sent the computer to AutoPilots Central in Tulsa, OK. Now over 2 years with no problems. Note – I also had pitch & trim servos serviced during the ‘troublesome years’ as well, but apparently the main problems were in the computer connections & flex wiring.
      Dwight Wilcox M20J

  131. Humphrey Penney says:

    Hi David, Do you know what the trim servo ( 1C469-6-674) clutch torque setting should be, and to how best measure it in situ. Thanks in advance

  132. Steve Von Gruben says:

    David, I am having trouble with my Altimatic III in my 67 Twin Comanche again, this time in the pitch axis. The AP powers up and works fine in roll axis. The pitch also engages but does not follow the pitch wheel except in full nose down command after a slight delay. Nose up command does not seem to work. Needles to say, the altitude hold does not work. Can you suggest a starting point? Also where can I get a wiring diagram for the installation?
    Thank you for your help.

    Steve Von Gruben

    • davidcrocker says:

      Steve, are you saying that (1) the pitch channel does nothing unless you select full nose down, or (2) after a small delay it always pitches nose down whatever the pitch wheel setting? If (1), after the aircraft has pitched down, if you then move the pitch control wheel to neutral, does the autopilot stop applying the nose-down elevator?

      • Steve Von Gruben says:

        The pitch channel does nothing unless pitch wheel is selected full nose down and then after a few seconds the airplane pitches over. I don’t have the courage to select full up and wait to see if it goes the other way as the airplane is in a substantial dive. I have tried it in the hangar and don’t think it ever reversed direction, however I may have not waited long enough. I was concerned with having the yoke in the full down travel and having the servo holding it so tightly.

  133. Steve Von Gruben says:

    Hello David, as a follow on to my last post, the pitch wheel does not command a pitch up input, only nose down as described before. I tried it with engines running on the ground and waited 20 seconds but nothing moved.

  134. davidcrocker says:

    Hi Steve, given the age of the aircraft, you’ve probably got the IC395 amplifier, and that’s probably where the fault lies. It may just have blown some of the output transistors. If you can, borrow a known good amplifier and substitute that. Another possibility is that one of the blue connectors (2 if you have the glideslope option fitted) between the AI and the amplifier is losing its grip, but in my (limited) experience this tends to cause a pitch up or down almost regardless of the pitch wheel setting.

    For the wiring diagram, see my earlier reply dated July 11 2012.

  135. Michael Olson says:

    Many valid points here. In the old days when I worked at the Beech Aircraft facility in Salina Kansas where the model 60 Duke was built. One of the problems I saw a lot was a porpoiseing factor when in ALT HOLD mode. This is the Century 4 system. I used an extender board to slide in the AP computer, plug the AH card into the extender, and adjust it with a dual sweep oscilloscope. I would also have the mechs check primary cable tension, then I would check bridle cables. This always fixed the problem. Also, to set up Yaw Damper at the factory if it is kicking I/8 or 1/4 ball on the Turn and Slip indicator, I would desolder the YD pot located on the YD Amplifier, remove the pot from the YD Amplifier, solder long wires to the wires in the YD AMP, and run them all the way to the instrument panel. There I could resolder the YD adjust pot and tape it to the IP. The pilot could then set the pot, torque seal the pot, then I would reinstall the pot back in to the YD Amplifier.

    • Steve carter says:

      Is it possible to remove the YD pot and reposition it permanently on the IP ? Different POT for that? Where can I find drawings on the YD?

      • Michael Olson says:

        Hi Steve,
        Thank you for the question. As to the permanent relocation of the YD adjust pot, I do not see a problem. Since these systems are so old, there would be no problems with “unauthorized modification of vendor systems” or warranty concerns. Now as far as ata Chapter 22 – Autopilot, I do not see any safety of flight concerns as long as the extension wires are routed safely away from any primary or bridle cables. I would route them right through the hole where the pot was located from the YD Amp. If you have to go through a pressure feedthrough/pressure bulkhead, just use normal shop common sense in routing through so you create no pressure leaks. Once in the cabin, the best way would be under the floorboards with an existing wire harness routing, then up the side and to your new location. Just have a valid A & P and maybe someone in a flight safety position sign off on what you do. Probably no vendor reps left. I see no reason why you could not keep the existing pot that is on the YD Amp as long as it maintains its electronic integrity. Drawings…..Wow. Finding an internal on the YD Amp itself might prove difficult. I will look through my old drawings and notes from those days. Not sure what I have left from Edo-Aire. I also have experience on century 21 systems. As a flight department avionics tech, Beech sent me to KCS55A KFC 200 – KFC 250 auto pilot school back when King had a store on Rogers Road in Olathe Kansas. For that one, the flight director had to be removed from the aircraft and placed on a rate table to set bank angles and GA, etc. Please let me know if this was helpful or if you have any other questions

      • Steve Carter says:

        Thanks Mike!


        Sent from my iPad


    • Steve Carter says:


      We have installed an Stec yaw damper in place of the Century and have found the output voltage from the C2000 computer to the roll and pitch solenoids to be 24 Volts.instead of 14 volts. I called Century about this and they so far have no answer. We were attempting to hook up the auto feature of the Stec when we discovered this. It is pin 34 from CD200 that is giving us 24 volts? Any ideas out there about this?

      • Frank Hall says:

        You could try a King KA39 Voltage converter, the output from 14Volt side Pin “A” is 1.5A continuous or 5A intermittent.
        If you’re hard up for one, I can supply a used one. It uses “Dime” transistors which are easy to swap if they go down

  136. Dusty Carnahan says:

    First, fabulous blog !!! … I have a Cessna 310 D with a Century III with GS which I have been fortunate that has been relatively trouble free for the last eleven years … on the last flight with altitude hold engaged and heading mode the plane suddenly went into a left bank – I immediately disconnected the autopilot … once things were settled back down I engage roll only and the plane immediately rolled left … on the ground engines running when roll is engaged the control wheel goes hard left … since this failure occured suddenly I assumed the problem was one of the connectors in the system was an issue … I checked every connector and all appear to be tight and connected … any suggestions ??? …

    • Vicente Collazo-Davila says:

      Hi Dusty, Sounds like one or more of the power transistors for the roll servo has shorted out. These are on one side of your amplifier/computer. There are 4 on one side for pitch and 4 on the other side for roll. They’re 2N3055 transistors.

    • davidcrocker says:

      Have you checked the connector where the main loom is joined to the glideslope loom? To find it, follow the cable from the AI.

      If one of the power transistors has shorted out, replace all 4 on that side.

  137. Dusty Carnahan says:

    I will check the main loom to GS loom first (easiest) and then the power transistors … thanks, I’ll report back what I find …

    • Dusty Carnahan says:

      All the connectors were tight … finally had the opportunity to wiggle intot he nose wheel well and pull the amplifier/computer … using the diode checker of the multi-meter it was quickly obvious that the transistors were the issue and one was completely shorted out B>E and B>C … as these are pretty expensive transistors (80 cents a piece 😉 I opted to replace all eight (roll and pitch) … did a ground test and the roll and pitch work … I will print out the directions on doing the adjustments with the six pots on the controller and head intot he air with a safety pilot to get the system trimmed up … thanks again for the guidance … very much appreciated !!!

  138. Fred Jedairplane says:

    Our CenturyIII Auto-pilot on our P68B is acting up again . after our annual inspection.
    The Pitch-trim channel will not function, or it will not trim up or down.
    On the console trim indicator, it shows that it is giving the command for up/down, but the trim wheel does not follow or move at all.
    The auto-pilot’s pitch command for the elevator tries to hold, push/pull until the forces are too big.
    The electrical trim on the control yoke functions and the trim wheel moves correctly.
    The CBs are all in and I cycled them to check for contact errors.
    I suspect it is the relay/amp or curcuit to the trim motor, but I have no schematic to locate and follow up my troubleshooting.

    Could you please advise me as how to proceed, and if possible send me a schematic of the pitch control curcuit?
    Thank you for your assistance on this matter.

  139. J Melendez says:

    I need a good Avionics Freelance Tech not too far from Miami, Fl to help resolve some installation issues. Can anyone recommend someone. Thanks I am an A&P/ IA If he/or she can help over the phone I can actually do the work and pay them for there help.

  140. jeroen van brakel says:


    You helped me before, so hope you can help me again.
    Altimatic IIIC in a 78 piper lance
    I had an intermittent problems with the auto pilot not following the heading bug. I soldered a new blue plug on the wire going into the DG. This solved the problem.
    Now after a few months, the same problem occurred again, but its definite now and not intermittent.
    If I engage the roll and heading, you feel the autopilot engaging and the steering gets stiff. moving the headingbug does not do anything.
    When I step on the rudder, the autopilot reacts by leveling the wing immediately, but not the direction.
    What can I test to find the cause. Is it possible to measure something with a scope or volt meter.



  141. mike salemi says:

    I have a century 3 (1979) installed in a Bellanca viking. The system works pretty well but I am concerned when the master is on (engine not running) the relay in the back of the autopilot continues to cycle every 1-5 seconds. If I turn the electric trim off it seems to cycle considerably less. Some times the unit will not cycle on but this infrequent. Any thoughts?
    Best regards
    Mike Salemi

  142. Lloyd Webb says:

    Really enjoyed reading your article and responses, wealth of information. I have a Century III autopilot in a 78 Mooney that I purchased in late November. At the time of purchase the autopilot was none functional, still is. However the seller indicated that the testing had indicated the problem was in the amplifier. The was one amplifier installed in the aircraft with a spare. I had a local avionics shop test both. One tested OK and the other one they repaired. Needless to say, the amplifier did not fix the problem. When the Autopilot is engaged the autopilot light comes on. However there is no evidence of stearing when the unit is on the ground or in flight. The only noticeable thing the happens in flight with the autopilot engaged is a turn to the left, no matter whether the HSI heading bug is possitioned left or right of current heading. Have any ideas of what the problem could be?


    • davidcrocker says:

      Hi Lloyd, does the autopilot work correctly in roll mode? You should be able to test this on the ground, as long as the AI is level.

      • Lloyd Webb says:


        No it does not. Doesn’t seem to respond to roll or heading bug. Just seems to cause a left turn tendency when engaged.

        Thanks, Lloyd

        Sent from my iPhone


      • Michael Olson says:

        Just a shot here but what do the V bars do in Flight Director mode only when selecting your desired lateral or vertical mode?
        Michael Olson

      • Lloyd Webb says:

        Michael thanks for the reply. My entire Century III is all old school, including the HSI which is a NSD 360. Unless I miss understand, and I very well could, (new to HSIs), I do not have a flight director.


  143. davidcrocker says:

    If it always banks left when roll mode is engaged regardless of the setting of the roll knob, and the amplifier has been tested, then the problem probably lies in the round blue connectors that connect the AI to the amplifier. The sockets lose their grip on the pins over time. There is one on the back of the AI, and if you have the glideslope coupler option installed, there is another one that you can find by tracing the wire from the AI for a few inches.

    • Lloyd Webb says:

      Thanks Dave. I will take a look at the connectors. Do the connectors usually require replacement or cleaning and reseating of the connector lock? Its really good to have you out there on the blog. I really appreciaite it.


      • davidcrocker says:

        The problem is the receptacles losing their grip on the pins. After you release the locking mechanism, you may well find that there is no longer any friction holding the plug and socket together. Replacing the sockets is recommended. See the main article for where to get them from. As a temporary repair, you can squeeze the receptacles together, by inserting a needle into the socket on the outside of the receptacles.

      • Lloyd Webb says:

        I will try the needle trick to see it that will get the autopilot active and order new receptacles. Thanks! Sent from my iPhone


  144. Lou McAbee says:

    I have a 1973 V35B Bonanza and have a problem with the trim circuit on a Century III autopilot.
    Initially the trim was intermittent and eventually it began working for about 6 months. Then it began to trim backwards. The avionics shop determined that the trim amplifier was at fault. After two used replacement trim amplifiers, the problem continues. All connections that were on the trim amplifier and the servo has been checked. The autopilot also trims backwards, and caused a aboupt pitch up while cruising with altitude hold engaged. At this point the avionics shop has ran out of ideas. I sure would appreciate any help you or other could give me.

    Thanks, Lou McAbee -at, or 815-621-6222

    • davidcrocker says:

      Hi Lou, unless the version you have is an Altimatic IIIc or the Century equivalent model number, there is a mechanical trim sensor under the floor that senses differential tension in the elevator cables. Has this sensor been checked?

  145. Mr. David, I have a Turbo Arrow III with Altimatic IIIb-1
    Right now I bought an Aspen PFD 1000 pro plus a Garnin 650,
    My question is about the possibility to install a digital autopilot Avidyne DFC90 instead of the original one ?
    What about servos ? is it legal ?
    Thanks in advance for your answer

    Lino from Buenos Aires, Argentina

  146. Mr. David,
    One more question; which Century is an altimatic IIIb-1 ?
    Thanks and BR

  147. Frank Hall says:

    Any idea where to source a servo motor Mitchell part number 59A19 from Autocontrol. I have mine working OK in Tripacer but would like a standby in case the motor goes crook.

  148. Jose Monroy says:

    I have a Century 31 on my 1982 Mooney. The problem is that in-flight there is no pitch control in ATT or ALT mode. Heading/roll works perfectly. Pressing the pitch rocker switch UP or DN has no effect. The yoke moves slightly back and fore causing pitch oscillations. On the ground test the rocker switch works perfectly moving the elevator and the trim UP and DN. I sent the computer and the pitch servo to Auto Pilot Central in Tulsa but they could not simulate the problem but did cleaning of the ribbon connectors, replaced two transistors and calibrated the servo. I re-installed the unit back and still have the same problem.

    Any hints on this problem will be appreciated

    • Dwight says:


      I have an identical set up – ’82 M20J w/ Century 31 AP. As I recall, there are two servos – Pitch & Trim. Turn OFF the Elec. Trim and see if there is a difference. I have had a problem where the Pitch servo ‘has a mind of it’s own”, and will cause a pitch up when the elec. trim is off. Since you have sent the computer to AP Central, I suggest that your symptoms point to a wiring issue either at the computer frame connector, or at one of the servos. Also, an earlier comment on this blog mentiond a bad connector between the AP Computer and the A/H – I’d start there (my ET said that this connector ‘fell apart when I touched it’).
      Dwight Wilcox – ’82J
      PS – you might also post your symptoms on ” ” and see if you can get more inputs.

  149. Bill says:

    Hi, can you explain the operation of the basic wing leveler in the original AutoFlite system? We’ve yet to test it, but the gyro amplifier spins up with the master power on. I loosened the nuts that hold this board in place and tilted it slightly to emulate in flight motion and the roll servo appears to only move left and never right. Do you think there is an issue with the roll servo turning right, or do you think there could be an issue with the gyro amplifier?

    Thank you.

    • davidcrocker says:

      Sorry, I’m not familiar with the Autoflite system. However, my guess would be that there are 4 power transistors in the gyro amplifier driving the roll servo, and that one or more of them has failed. If so, then I suggest replacing all four.

  150. davidcrocker says:

    If there are 4 of them, probably in TO3 cases, then almost certainly yes.

    • Bill says:

      Yes, that seems to match up. I’ll try to replace them and see if that helps.

      Thanks again.

    • Bill says:

      Hi, I replaced this 4 power transistors and still have the same issue.

      The aircraft is on the ground. I turn it on, it spins up it engages and I hear it lock the yoke. I then manual move the gyro accelerator module to emulate in flight motion and it _only_ ever turns left. It never turns right. There is a small circular module beside the gyro accelerator with a red cap on it. I’m not sure what it does, do you know?

      I would really like to get this wing leveler operational. Does anyone have any ideas, or do you know where I can send it to get looked at??

      Thank you.

      • Cory says:

        Just found this blog. If you’re still trying to figure out your Autoflite, let me know as I’ve got one that I tinkered with and made functional

      • zn105 says:

        Cory can you contact me about how you got your working? I have spares and want to figure mine out.

  151. Rick Dozier says:

    Hi. Rick here. Great Blog full of info. Thanks for the service your providing.
    My C-III in My Super Viking without auto trim has a simple push switch on top of the plots Left yolk handle that does nothing when pushed. The A/P continues to work. I checked the switch with an Ohm meter and it is working as advertised. am seeing 12 volts out when depressed.
    I hear a clicking back on the Pitch servo when the button is pressed, but I don’t hear anything from the area of the roll servo, when the button is depressed, so it is sending a signal. How does this disengage system work? What could I try to get it working properly again?
    I’d like to try and fix it first before I send it to the avionics shop. Thanks

    • Kristin says:

      If it is an A/P disconnect switch, the rockers on the control head will click off. Are you sure that it as the A/P disconnect? Are you sure you don’t have an automatic trim system? Does the A/P adjust the trim when it is flying the plane? I have never seen a Century III that had any lights to tell the pilot to trim.

      I am not sure about Vikings, but in versions of Century electric trim on Piper’s, they have a set up where you push the button on the yoke and hold it when the aircraft is out of trim. The system then will trim the pressure off. What the button does is to momentary turn on the pitch part of the autopilot and then the pitch trim sensor will detect whether you are holding forward or aft pressure on the yoke and will trim off the pressure for you.

      • Rick says:

        Thanks so much for that info. I never knew what it did. I just assumed it was a disconnect. Ill go out and try it again and see what happens. Hopefully it was just a loose nut bhind the yolk!

    • Rick Dozier says:

      OK I figured out that this button runs the Auto trim for Pitch. After adjusting the switch that senses the cable tensions, I got it working,(somewhat). On the ground with the chain removed, the servo runs in both directions when the switch is moved manually. Now what it does is, the servo will adjust in flight in the nose up pitch just fine, but in nose down the trim locks up the trim system instead of turning. I can feel the trim servo engage and it locks the trim from moving in either direction, until I release the button, So I know the servo is getting power. It engages the servo, but it won’t turn in the nose down direction. Manually the trim works fine and easy in both directions.
      Why would that be happening?

  152. CB Sharp says:

    Great reading!! I have just purchased a Comanche 260b with a freshly rebuilt Century III that worked great for the 1000 mile flight home. (rebuilt because the Alt hold had issues). Once home I had a complete new interior installed in the plane, including headliner, and new paint. 1st flight after installing the new interior etc, the trim indicator on the Century III constantly indicates up when the AP is engaged, and will not center. Heading works great, but when alt hold and/or pitch is engaged it continually starts climbing. Appears to be something stuck. Any suggestions?

    • davidcrocker says:

      Likely causes of that fault are blown power transistors in the amplifier, and loose contacts in the round blue connectors on the back of the AI and where the glideslope loom connects to the main loom.

      • CB Sharp says:

        Many thanks for the suggestion! Will certainly check that out today!… Best to purchase replacement power transistors direct from Century or ?? Never been in the amplifier. It was supposedly just replaced but … Do the the transistors plug in or soldered?

      • davidcrocker says:

        The transistors are soldered in. There are 4 on each side. One side is for the pitch channel, the other for the roll channel. If you find a failed transistor, replace all 4 on that side. Type 2N3055 is a suitable replacement.

  153. CB Sharp says:

    The local tech, who admittedly is not a Century III expert, said that If the pitch drive transistors are blown in the amp, he thought the pitch servo would not drive at all. Is he correct? More details, and your suggestions are certainly appreciated!! … The trim indicator on the AP panel immediately displays a slightly ‘up’ indication when the AP is turned on (any mode), In Roll or Heading mode the AP seems to work perfectly, when switched to Alt or Pitch mode, or when the auto trim button is pushed on the yoke, AP starts trimming ‘up’, Trim indicator on the AP panel never levels. When the Trim wheel on AP panel is turned to full ‘down’ trimming slows slightly, but continues up… Where does the trim indicator on the AP panel get it’s indication information?? Could this be the culprit, or does this additional information produce the same diagnosis as above? …With the new interior and headliner just installed, could something have been damaged? Thank you again for your thoughts!

    • davidcrocker says:

      If one of the power transistors fails short circuit then you will get permanent pitch motor drive. However if the rate of trimming is affected by the pitch wheel then a more likely cause is a bad connection between the AI and the amplifier. Perhaps the wiring was disturbed during the refit. You can check the wiring by measuring the resistance of the AI through the cable as I described in the original blog entry. I don’t have access to the schematic right now but AFAIR the pitch indicator shows the output of the amplifier or something similar.

  154. Avner Shacham says:

    Hi David,
    I am impress with your knowlege and may be you can help me.
    I have a PA-31-310 with Altimatic 5 when I try to engage the autopilot
    the light is on when I put my finger ond then off.
    all the other buttons are on when I push it. but nothing is working.
    The flight director is not working .

  155. Steve Carter says:


    I have replaced my century Yaw damper Amplifier(IC 753-100) with an Stec 0121 Amplifier to save weight in the tail. I still have the century servo installed. The wire harness has been rung out and all is correct but the amplifier is not making the servo move fast enough to correct in flight conditions.Full deflections of the pot right and left takes a lot of time to respond. Is the century servo different from the Stec servo? Is this set up not compatible or what am I missing here?

    Thanks as always Dave

  156. Jack Donoven says:

    This is a great source of info. Thanks for all your advice here. I have a 1978 piper Lance with a CIII AP. When the AP is engaged, in any mode, it will roll hard over one way or the other. I’ve found that you can control the direction of the roll with the heading bug or the roll knob, but it goes hard over in the direction you give it. It appears to not be getting any feedback to tell it to stop the roll. This was an intermittent problem for a while but now is present all the time. Any ideas?
    Thanks, Jack

    • davidcrocker says:

      It should be getting feedback from the AI. I suggest you check the usual 2 round blue connectors: the one on the back of the AI, and the one a few cm away from the AI that connects the main loom to the glideslope loom.

      • David Viewing says:

        Hello David and thanks for this most informative blog.

        My Warrior has a Century IIB unit. Sometime during the regular moving of cables to restore operation (probably loose pins in the DI connector) the edge connector at the back of the console unit became broken. (one of the pins broken off). Do you know of a source for this connector? Any information gratefully received!

    • Gary Hall says:

      Hi Jack, Hard roll over is usually a faulty roll signal from the Attitude indicator.

      Best regards


  157. Rick Dozier says:

    HI Dave. The last question never got a response. “Is the trim amplifier the box just below the servo? With the blue plug going into it?” or is it elsewhere?

    • davidcrocker says:

      Rick, the location of the trim amplifier depends on the aircraft. The positions of the various units for some aircraft (but not the more recent ones that the Century III was fitted to) are shown in manual 68S54.

  158. davidcrocker says:

    David Viewing, I’ve never worked on a Century II series autopilot, so I don’t know about that edge connector. You may be able to find the manufacturer and part number marked on the old one. Failing that, measure the pitch and the number of pins, then search component distributor websites for something similar. Or try Autopilots Central.

  159. Kobus says:

    Hi David
    I have a Century IIIc autopilot in my Seneca II. Have just installed a Garmin 500 and Garmin 430W.
    It flies the hold and approach perfectly with ons problem. The rate of turn is too slow. It turn 280 degrees in 2min. How can I turn the rate up? The AI has been serviced to ensure that the output to the AP is strong enough.

    • davidcrocker says:

      Remove the console faceplate. This gives access to a row of 6 helical potentiometers. I don’t have access to the manual right now, but counting from the left, AFAIR the 1st one sets the maximum left bank angle and the 3rd one sets the maximum right bank angle. Check the manual for the full in-flight setup procedure, and take a safety pilot with you.

  160. Curt Castagna says:

    I have a Century II, heading only, in my 67 c182, connected to an Aspen and G430W. When I activate the heading switch the contacts seem to release and then the airplane wanders of course. If I press the heading toggle in with some firmness , it reconnects. Any ideas?

  161. Tom Ludwig says:

    This blog is great!
    I have a Century IIIc with 1C515-1 amplifier in my Comanche 250. Cap#68 on the pc board self-destructed and spewed its contents all over the board. It’s a 47uf 20v tantalum cap. Can it be replaced with an aluminum cap or should I stick with the tantalum? Would C68 take out any other components? I’m worried about T2 transformer 25B34 which sits next to the cap. Is there an equivalent for T2?
    Thanks, Tom

    • davidcrocker says:

      Yes those tantalum caps can be replaced with modern aluminium ones. I believe some maintenance shops replace them all as a matter of course. But check the transformer winding for continuity first, because I have known a failed cap take out the transformer.

      • Tom Ludwig says:

        Thanks for the quick reply. The transformers T1 and T2 check out ok. However, the four 68 ohm 2w resistors on the heat sinks read 115 ohms or more. Will replace them as well. Are metal oxide resistors an acceptable substitute?

      • davidcrocker says:

        Yes those resistors often increase in value. You can use 2W metal oxide or metal film resistors.

      • Lloyd Webb says:


        I understand from previous blogs that the 30 pin amplifier plug looking in to it is numbered, left side A-S and right side 1-15. What letters are skipped on the left side in the numbering scheme?
        I know I, O, and Q are often skipped, but there must be one other letter. Thanks!

      • Tom says:

        No “G” as I recall. I recently made up a new 17 foot harness from the Amplifier to the Console. The Service Manual has the schematic. I bought new connectors since I didn’t want to cut the old harness. The schematic shows that the shields are not tied together. However, the old harness had the shields connected. An examination of the PC board in the Amplifier shows the shields are joined through the board anyway. Be careful that the servo common leads are not grounded or touch any of the shields. You should check the condition of the Amplifier internals. I found that C-68 self-destructed, and the 68 ohm resistors on the heat sinks were way over 100 ohms, one was 198 ohms. I’m sure Dave can verify this.

  162. Cyril Gibb says:

    I have a Century 2000 autopilot. It’s got 7 pots under the front panel cover.

    I know the first 3 are for roll and I believe the 2nd 3 are for pitch. What is the 7th for?

  163. Doug Crociati says:

    I have a 1970 Aztec with an Altimatic IIIB Ap, It has been working intermittantly, pitch portion is always good but I keep losing the Roll functions. I believe it is in the amplifier it currently has a 1C515 Mitchel amplifier installed, I want to replace it, however all I seem to find is the 1C515-1 which I can buy New surplus, is that amplifier interchangeable?, also would that be a plug and play installation ? Thank you in advance for you insights

    Doug Crociati

    • davidcrocker says:

      Hi Doug, I can only advise on what will work technically, not what is allowed by the regulations. I have never seen a 1C515, however the 1C515-1 should be a plug-and-play replacement for it because according to the manual it is essentially the same design built on 1 board rather than 2. In fact, the 1C515-1 is electrically and physically compatible with the older 1C395 amplifier too, even though the circuitry is more advanced.

      If the main board of the 1C515 uses same the gripple-board technology as the 1C395 (i.e. rivets instead of plated-through holes on the PCB), then I would certainly advise replacing it by a 1C515-1. However, the problem you describe could also be caused by one of the small round blue connectors losing its grip, in particular the connector on the back of the AI, or the connector that joins the main loom with the glideslope loom (if you have the glideslope coupler installed).

  164. Doug Crociati says:


    I have a 1970 Aztec with an altimatic IIIB. I have been having problems with it being that all roll functions were working intermittently and pitch functions working. now even the pitch portion has failed. The AP
    Will not function in any capacity. I was toying with the idea of buying a new amplifier, does this sound like the root of the problem. Thanks in advance for your insights


    • davidcrocker says:

      Intermittent faults are typically caused either by poor joints on a 1D395 amplifier board, or by the round blue connectors losing their grip. Your Altimatic IIIB may be recent enough to have the more reliable 1C515-1 amplifier, so I suggest you check which sort it is. Before you buy a new amplifier, try measuring the resistance of the AI pitch coil via the loom, as I described in paragraph 7 in my blog entry, and if it’s much more than 80 ohms then sort out the connectors. If the symptom is that the autopilot is completely dead, that suggests lack of power to the amplifier. If it produces movement in one direction only, that suggests faulty output transistors, which are easily replaced.

  165. Tom says:

    Have ’61 Comanche 250 with CIII and 1C515-1 amplifier. Replaced the blown Cap#68 with alum 47uf 35v cap. This cap has a higher voltage limit and seems to be suitable replacement. Also replaced the 8 pitch and roll transistors with ON 2N3055G and the 4-68 ohm resistors. Haven’t yet replaced the rest of the 47uf tantalum caps in the amp. Set up system on bench with attitude and directional gyros, servos, vacuum and power. The roll servo seems to work correctly and I can center it with the roll knob on the console. However, the pitch servo motor runs continuously even if pitch or alt hold buttons are off. Only the motor runs, the clutch won’t engage unless the pitch or alt hold buttons are on. With pitch or alt hold on, the pitch clutch engages and the motor will stop after a few seconds. Is this normal? Should the pitch motor be off until the pitch or alt hold are on?
    Thanks for your help.
    Tom in NJ USA

    • davidcrocker says:

      It’s been a while since I had one of these on the bench, however I think this behaviour is normal. There is no signal to the amplifier to tell it not to activate the outputs, so it will always produce pitch and roll outputs when it is powered. If you adjust the position of the AI and pitch wheel, you should be able to get the pitch motor to stop.

  166. Ney Feijo says:

    David, I have an Arrow III with Century II autopilot. It worked fine except for the heading. We checked it in the bench, everything fine. Cables also OK. We found the problem in the Mitchell 52D54 directional gyro. We never found a manual for the DG. We opened it and it has a few capacitors and three coils. One of the the coils was open. We remade it and everything is fine now. I can send the diagram of the electronics if you or anybody wants. Congratulations from Brazil on your generous colaboration with DIY people flying with these old APs.

  167. Sam Hart says:

    David, We have a PA28 R200 Arrow with an old Edoaire roll control Century 2 autopilot. When brought to us the autopilot would operate intermittently. And after some operation would then not move the alerions at all. We removed the servo cover and observed that there is a clutch that is not engaging. The motor itself is responding to roll control inputs. How can we verify if the clutch coil is defective? Thank you for any input.

    • davidcrocker says:

      Have you checked the clutch coil for continuity with a multimeter?

      • Sam Hart says:

        David, thanks again for this blog. Looks like our problem turned out to be some sort of loose connection on the control head. Could not find anything wrong with the survo so went looking behind the panel. Started working again after pushing on all the wires. Found one of the screws on the multi pin connector loose. Not sure if this was it, but it’s working now. We will post again if we find anything. Thanks again.

  168. selbyjr says:

    Could not figure how to start new post, so just added in here, sorry. We have a PIper Lance with Altimatic IIIc. I have little prior autopilot experience. We have had a number of issues as mentioned above but one I haven’t heard and maybe it is user error but it is disconcerting. This has happened twice, very similar setting.
    Flying an approach (IMC) different airports, one RNAV, one ILS, not trying to use autopilot for approach, just on Heading and using heading bug for intercept localizer. Both cases heading bug worked fine entire rest of flight.
    On final intercept with heading of about 300, was told by ATC turn right 020, report established. I quickly turned heading bug to 020, made radio call, and then noticed that plane was turning left. Each time I quickly disconnected AP, probably within 5 sec, and hand flew from there in but it was very disconcerting to say the least.
    Any explanation/thoughts? Thanks.

  169. Bill says:

    Dave, I have what might be a silly question. We have a Baron with a Century IIIc A/P. The G/S coupler seems to work fine but the G/S intercept light is very dim, almost un-noticable. The Baron has a 24 V system. Is the lamp a 24 V lamp or 14 V lamp? The push to test feature of the lamp works fine and the light is bright. The indicator assembly (and lamp) was recently replaced but I’m not sure when the problem started.

    Any help would be appreciated.

  170. Mike says:

    I have an Edo Aire Century IIIc that when engaged in course and altitude hold intermittently runs the trim full nose down until the AP kicks off. It is in a V35B that had a straight tail installed in 1987.

    I have observed the auto trim adjust back and forth and then run nose down until I turn off the autopilot.

    Thanks for your help in advance,


    • davidcrocker says:

      It sounds to me like an intermittent connection between the AI and the amplifier. First check the round blue connector on the back of the AI, and check whether the pins are gripping well. If you have a glideslope coupler installed, follow the wire from that connector for a few cm until you come to a pair of blue connectors that connect the main loom to the glideslope loom, and check that the pins are gripping well on that connector too. I had a similar problem to yours and that’s where the problem was. The connector is behind the instrument panel so it gets quite warm, and over the years the receptacles in the socket lose their grip on the pins.

      The problem may also be caused by a fault in the amplifier, especially if it is a 1C395, or in the wiring to the altitude capsule.

  171. Mike says:

    Thanks for the reply Dave.

    I have not been able to get to it yet but when I do I’ll let you know what I find.


  172. David May says:

    Dear All

    What a fantastic group! – I am so pleased to find you all this evening and this exchange of knowledge really lifts my spirits! – This sort of forum is the best of the internet so thank you for your help in advance!

    I have an Autoflite 11 in a PA28 -181 built in 1977

    This Auto Pilot was recently only working in one mode which was with a heading on the DI bug – the other modes were not working but this did not worry me. A heading on the DI was all I needed (The Autoflite 11 was never designed to maintain altitude but keeping the wings level and a heading was all I needed and really helped during long flights).

    Today it stopped working on the bug. If you turn the Autoflite 11 “on” it rolled slowly to the left and it did not matter where the bug was.

    If you take switch it “off” , and you operate the roll knob, you can make it roll left and right – this indicates the motors which make the ailerons operate actually work.

    I am an ex radio amateur and can usually fix old analogue bits of equipment – any help would be gratefully accepted.

    Thanks – David

  173. David May says:

    I have just looked at my panel and it actually says “Piper Autocontrol 111B” – I got the model number from the original Handbook so not sure if it has been changed or this is the Piper name for the Autopilot? Anyway, it is definitely a “Piper Autocontrol 111B”

  174. David May says:

    After reading this forum in depth I decided that it might be a “lead or plug issue” or an electrolytic capacitor had gone “pop” somewhere.

    I put my head under the panel and noticed that a round plug looked a bit loose and on a slight angle (work had gone on under the panel on an altimeter during my recent “Annual” so I am guessing the plug was knocked by mistake).

    Anyway, I pulled the plugs off, sprayed “switch cleaner” on the pins and pushed the plugs back in. Then all started to work again. I just took the aircraft for a test flight and all OK so thank you all – this forum got me thinking and gave me the confidence to give things a prod!

    By the way, each side of the Rocker Switches there are two semi circular holes which look a bit “rough” like somebody drilled them out as a repair. I would like to post a photo but can’t work out how to on this forum. These holes have always been a mystery any suggestions?

    Thanks again – David

  175. Lloyd says:

    David, it’s been a few months since I last sent in a question. Your responses are always helpful to everyone on the blog. I did get my Century III Autopilot working after finding a broken wire between the servo (C) and amplifier (E). Now it has went on the blink again. The servo clutch is not engaging. I get approximately 10 volts at the servo’s A ( roll selnoid) terminal. I’m also getting approx. 9v to the servo’s C (roll motor) terminal. If I rember correctly, I getting around 12 volts to the amplifer N terminal. This is all on battery, aircraft engine not running. I have severa questions. (1) Does the amplifier supply the roll servo’s selnoid voltage that engages the clutch which should be 12 volts? (2) Does this indicate a problem with the amplifier. I noticed with the aircraft on battery that the servo motor is running but the ailerons are not moving because the clutch is not engaged. Should I even have a voltage at the servo’s C terminlal without the aircraft engine running and instrument vacuum active? I did test the servo independently with a smaller 12 volt battery. With that battery the servo clutch enguages and the servo moved the flaps fine. Any idea what the problem might be?

    • davidcrocker says:

      The amplifier does not supply the voltage to the roll solenoid, it comes from the relay in the console. The solenoid should operate the roll clutch whenever the autopilot is engaged. The amplifier schematic shows that terminal N has no connection inside the amplifier. The autopilot is dumb and doesn’t know whether you have vacuum at the AI or not, so the roll motor will still be activated regardless.

      • Lloyd says:

        Since the V+ for the servo clutch does not come from the amplifier and the is only 10v to the servo clutch and the clutch is not engaging, what do you think the problem might be? Does the clutch take a full 12 volts to engage?

        Thanks again.

    • Frank Hall says:

      I have a number of Autopilot manuals for disposal and hope to find good homes for them. Do you have any objection to my posting the details here?

  176. davidcrocker says:

    I’m not sure how much voltage the clutch needs to engage because I generally used to test the AP in my aircraft with the engine running so as to get the AI steady. The roll solenoid is connected to terminal M on the amplifier according to the wiring diagram, not terminal N. I suggest you remove the edge connector from the amplifier, then use a multimeter to check the resistance of the roll solenoid and associated wiring, between pins M and R. Compare it with the resistance of the pitch solenoid between pins 4 and 6. The problem may be a worn-out roll solenoid connector – AFAIR it is one of the round blue ones. See if it feels loose when you unlock it.

    • Silentflier says:

      If I remember my numbers correctly it’s 11.8V maximum for clutch pull in on a 14v System. Apart from a low supply voltage the usual problem for slow pull-ins is dirt build up on the moving arm of the solenoid.

  177. Lloyd says:

    Dave, thanks again. Will check the resistance between the roll and pitch servo solenoid pins. Yes, I knew from the diagram that pin M on the amplifier went to the roll selnoid. Pin N, I understand to be the +A (battery voltage) from the console which seems to be Ok. I do indicate close to battery voltage on the Amplifiers pin N. The voltage from the amplifier to the roll selnoid appears to be about 2 volts low. I suspect this is why the roll clutch is not engaging. If I directly connect 12v and ground to the A and B pins at the servo, the clutch engagesring. I have wiring diagrams (Illistrated block diagram 69D672-16 and the Century III with G/S coupler wiring diagram) that seems to indicate that the bus voltage goes from the breaker through the relay box 1A526 then through the console then amplifier and finally to the roll solenoid. I don’t have the wiring diagram for the 1A526. It is not addressed in the Century III wiring diagram, only in the illustrated block diagram which has slowed the troubleshooting down a bit.

    • davidcrocker says:

      2V does sound a lot to be losing, so it’s probably caused by a bad connection somewhere, unless there is a diode in the relay box. Use your multimeter to find out where the 2V is being lost

    • Dr J Schwartz says:

      You solved my alternator warning light problem–it was the 50 ohms resistor and I put in a 6v bulb–THANKS✈️👍

      Sent from my iPad


    • Dr J Schwartz says:

      Everything working however in the air the alt light comes on–alternator is working–when on the ground light goes off–6volt bulb-new 50 ohm resister replaced-can you help

      Sent from my iPhone


  178. K Huffine says:

    I have a Baron and the electric trim switch seems intermittent…Auto elec trim works fine….Could it be the switch on the yoke or is it something else…All else is ok and GS coupler works too….Ken

  179. david, fine website here. i have a bonanza f33a that is equipped with a Century III autopilot. The autopilot recently began to turn left when HDG was engaged on the console. it has the optional 1C388-2 radio coupler. It is a controlled left turn, on others words, it doesnt exceed the maximum bank angle.

    ground checks: roll mode. everything works flawlessly. Engage heading mode, and the yoke immediately goes left. moving the roll knob on the console has no effect on the when the heading mode is engaged. aircraft is equipped with a edo HSI (NSD-360). id like to troubleshoot the HSI/DG but not exactly sure where to start.

    at this point i feel like its the HSI, wiring, or the console. The amplifier seems to be behaving normally, as it sends the proper signals to the servo in ROLL mode.

    id like to bypass the radio coupler to get that out of the discussion.

    any help would be greatly appreciated. i do have access to the Service manual 68S54, oscilloscopes, and meters.

    • davidcrocker says:

      It could well be a problem with the signal from the HSI or a problem in the radio coupler. I suggest you check all the round blue connectors in that wiring. However, it could also be a problem with the amplifier (especially if it is a 1D395), because the DG input is separate from the roll input. The fact that the turn is controlled indicates that the feed from the AI and related circuits in the amplifier are OK.

      The radio coupler is not just a switch, it contains a fair amount of electronics. So I don’t think bypassing it is an easy option.

  180. SIlentflier says:

    Bypassing the radio coupler is not possible. With the HSI type DG’s (NSD’s, KCS55 etc.) half of the excitation oscillator (which is usually in the ‘standard’ DG) is in the coupler.

    One remote possibility is that it could be the coupler itself, The -2 (and -3) radio couplers have a relay to reverse the polarity of the HDG excitation when in LOC Reverse mode. (Other couplers just reverse the radio left/right input) If this relay or its drive circuit has failed it could make the HDG input to the A/P appear 180 degrees out so as soon as you select HDG the A/P would be trying to fly to the reciprocal of the HDG bug. If you get a chance to fly just put the HDG bug at the 6 o’clock position then engage HDG. If it still rolls it’s not this.

    • John Landwerlen. says:

      Done. CD33 out of the coupler to the amp had a broken wire inside the backshell. One of the excitation wires. Customer is ecstatic. 3 other shops tried to sell them new autopilots. Maybe someday…..just not today. Thanks for the helpful advise.

  181. Pierre Sirois says:

    Excellent blog David. I am impressed by the depth of your knowledge of the Altimatic IIIB-1 like the one I have on my PA24. You know when to say: I don’t know. This is unusual amongst the avionic technicians. I would give a lot to have you look at my autopilot. I shall give my tech the address of your blog.

  182. John Loughmiller says:

    Hello David:

    Having a problem with a Seneca II installation of an IIIC

    Can you advise what the function is for the 4 pin Amphenol connector (CD-40) on the rear of the IIIC console? The Century III service manual shows the CD-20 connector but does not show the CD-40 connector for the IIIC and the Piper IIIC installation schematic is not a true schematic but rather at the block diagram level without any detail on individual functions.

    It appears that individual wires from CD-40 go to CD-37 – the relay box, CD-39, the trim amplifier, CD-42, which appears to be the pitch trim on/off switch and CD-26 about which I cannot find any information.

    The underlying reason for this post is intermittently the autopilot will either not power up at all or will power up and then drop off line (all modes) while in flight. Naturally, I am suspicious of the round, 4 pin Amphenol connector that is used at CD-40.

    It appears from the Century manual that power enters the console at pin 1 of CD-20 and if the roll rocker is depressed to the on position that rocker should be electromagnetically held in the on position and power applied to the roll servo amplifier which would enable the manual roll knob function. Instead, it is as if there is no power whatsoever present at the console when the problem occurs hence the question about CD-40 since the round Amphenol connectors are known problem children.

    In a few days, I will troubleshoot the unit and verify that power is present from the CB panel all the way to pin 1 on CD-20.

    In case there is power available to CD-20-1, I want to move on to CD-40 and have a look there for bad pins (but I cannot determine if a bad CD-40 connector could be the cause of my problem since I cannot find a suitable schematic in either the Century manual nor the Piper installation schematic.)

    Any thoughts?

    Thank you,


    • davidcrocker says:

      I’m sorry, I can’t remember the function of that connector. I suggest you unlatch the locking ring of the round connector and see how much force it then takes to pull it off. If it falls off with little or no force, replace the socket, or in the short term tighten it up by using a needle to squeeze the two prongs of the metal socket pin closer together.

    • Alan Hadwin says:

      CD40 switches the supply for the automatic trim when you select PITCH mode.
      CAUTION. The connections vary depending on what trim system and which Trim Amplifier you have. The following is what I have found looking at the various trim systems. Hope it helps.
      Trim supply A+ is connected to CD40 pin C (Wire CD42). With no pitch functions selected this is connected to pin D which always appears to go the the relay box (Wire CD37). When you select PITCH a relay in the controller switches the A+ from pin D to pin B. This seems to be the supply line to the Pitch Trim Amplifier (CD39). If you have a 1C709 Trim Amp this usually connects to the CD45 connector pin B
      I can’t find a reference to CD26 but some of the trim systems do have two feeds from the trim master switch, one to CD40 and one direct to the relay box. Possibly (just possibly) the trim A+ is fed to CD40 pin C and a second wire from this pin goes on to the relay box. That may just be it IF you have that trim system.

      I don’t think this is related to your drop out problem. The autopilot A+ feeds in direct to the ROLL switch and the first thing it does when you press the switch is energise the hold on coil so if you have power and ground it should hold in – even if nothing else happens. I would suggest you look outside the box for this. Circuit Breakers have been known to get tempremental and give no or intermittent connections even though the breaker is still in. Also the A/P disconnect wiring. I have seen the flex wire, that carries the A/P disconnect and electric trim signals from the yoke, fray where it emerges from the bottom end of the yoke ‘shaft’ and short to the shaft and/or each other. This caused some really wierd fault indications. Good luck, intermittent faults are always a swine…

  183. Franklin Canosa says:

    Hello David. Again, thanks for your commitment to this blog. Volumes of great information.

    I have a Century IIB on my Piper Turbo Arrow III. Amazingly it worked pretty well in all it modes. Roll function was normal, however, I typically used it in ‘heading bug’ mode or VOR track mode. It even worked in ILS mode.

    After a major overhaul, (new engine, engine mount, etc.) I have the following issue (please note this happened right after it come out of the shop):

    The amplifier unit on switch will latch ‘on’ but I will get no friction in the steering yoke. The roll command does not engage, nor does any of the lateral guidance functions work.

    I know the servo motor works because I also have a Century I that also still works (there is a toggle switch to designate what A/P we want to use) and as soon as I toggle it from the Century IIB to the Century I, the servo clutch grabs and I can control roll function with the Century I.

    So my thoughts run to a loose Amphanol connector (who knows what the mechanics were touching under there, the plane was in the shop for 3 months). Barring a loose connector, I would probably think it was a bad transistor (I have a 1C385 amplifier with 4 Motorola 48S43 transistors on the bottom which I presume control the roll function).

    How do I check the transistors prior to replacing them? Is the replacement for the transistors a 2N3055?

    What should I attack first the loose connectors or the transistors?

    Many thanks.

    • davidcrocker says:

      If neither roll not pitch works, then most likely the amplifier is not getting its 14V supply due to a wiring problem. So check the connectors and the wiring. If pitch still works, try replacing the transistors. The replacement is 2N3055. When replacing them, also check the resistors soldered to them because they tend to go high resistance. They are 68 ohm 2W.

      • Franklin Canosa says:

        Hello David, thanks for the advise. You were spot on as it was the Amphanol connector at the relay box. I tightened them up and the Century iib worked like a charm. I have been using it for about 8 months now and just recently the a/p circuit breaker tripped. I reset it and it tripped again so I just left it alone. When I got back on the ground, I reset the breaker once more and it did not trip, however, the roll command only would initiate a left turn (I could not get it to go right). I tested it in heading mode as well and the same thing (it would only turn right). I suspected the power transistors and replaced them all. I tested the 68 ohm, 2watt resistors and they tested out at 71 ohm and 70 ohm respectively so I just left them alone. When I put the control amplifier back in it still was exhibiting the same issues. Do you think it could be the transformer? What components should I test next and what is the best way to test them. Thanks in advance.

      • davidcrocker says:

        I suggest you check the output driver transistors Q9 and Q10 next, the type is 2N3116 I think. I have only come across one blown transformer, and I believe it failed because a tantalum capacitor that was driven from it through a diode failed short circuit.

      • Franklin Canosa says:

        Ok I raced home and check the Q9 and Q10 transistor to the best of my ability and they seemed ok to me (no shorts). What voltage should I get to the roll servo output? Anything to try next ? Thanks for all the help.

  184. Gary Hall says:

    Anybody have a schematic for a Century Flap Trim Amplifier P/N 1C744. I’ve a problem with the wiring hook up to an Electric trim amplifier 1C709. Many thanks in advance, for info I’m an Avionics engineer with lots of schematics and manuals to hand except this one! Happy to share any info, I’m based in the North of England and have the Century test breakout boxes. Great site David!!
    Cheers Gary

    • Silentflier says:

      Hope you don’t mind me coming in here David…

      I have a copy of the Century Command and Automatic Trim Systems Manual which gives the installation interconnect but, unfortunately, not the internals of the 1C744. It’s an older issue but might be some help… The manual lists at least two different 1C744 installations (on my first quick flick through of the 57 trim installations listed!) It appears that it’s just the pinouts of the CD133 that change, looks like there could be some internal configuration changes dependant on A/C type…If a copy of what I’ve got would help just let me know.


      • Gary Hall says:

        That’s great Alan, anything referencing the Flap Amp 1C744 would be great, as I need to build a new interconnect cable between 1C709 Electric trim Amp. It’s for a Vulcanair (Partenvia) PN68 if that helps.



      • Silentflier says:

        OK I thimk I’ve found it. The Century STC for a P28B lists autopilot kit AK511 which uses trim system type 29. If you drop me an email I’ll reply with what I’ve got. Use my tag name above at tesco and then dot net.

  185. Patrick Morris says:

    I have an Autoflite in my 1967 PA28-180 that wants to go left all the time. Changing the rheostat will speed up or slow down the speed but it continues to move the controls left. I have checked continuity for the wires from the rheostat to the amplifier and replaced the transistors with new 2n3055. The servo will work in both directions when tested.

    • Silentflier says:

      Is it an Autoflite I (with just a switch and knob on the panel) or Autoflite II (controls on the T&S) ?

      • zn105 says:

        Autoflite I with just the rheostat and on off switch on the panel. Just the wing leveler with the gyro amplifier unit under the rear seat.

    • Gary Hall says:

      Hi Patrick, usually with Century A/P’s this fault is the Roll signal from the AI. I’d measure the resistance of the pins for continuity. From memory C-D are the roll pickoffs on connector CD18 and should be about 7 ohms. Excitation lines are A-D. I’d personally leave the trim pots alone till you get movement in both directions. Hope this helps.


  186. Cory says:

    I’ve got an Autoflite I and have tinkered with it quite a bit. There’s another trim centering pot that IIRC is more coarse-adjustment than the trim one on the panel. It’s accessible from a hole in the gyro box mounted under the rear seat. If that pot can’t “center” the drift, then it’s most probable that some other component is bad…. hopefully not the gyro and/or inductor rate-sensing pickups on it.

  187. Cory says:

    Sounds like there’s something more serious wrong with it then. Possibly some caps that have gone bad. Try swapping those first to new. If there aren’t any bad components and the gyro pickups are centered, the pots should have plenty of control authority to zero out any L/R bias in the dither.

    If the pickup inductors and/or adjustment cams on the gimbal springs haven’t been fiddled with I highly recommend NOT fiddling with them, even though it’s very tempting since they’re right there. That can definitely center the drift by fiddling with them, but adjusting them messes with other important parameters which will affect the operation and stability (rate-of-turn gain, Left rate gain vs. Right rate gain, etc). If they were centered once upon a time and haven’t moved, the circuit should be able null out any drift if all of its parts are non-broken.

  188. silentflier says:

    I would suggest your first check is to see if the gyro’s running. If its original (or thereabouts) to ’67 then its probably got a magnetic switch starter which clicks faster and faster as the rotor runs up
    until it goes quiet. The same thing happens on run down as the rotor stops. You should hear it when you apply power without any engine noise. You will probably also be able to hear some gyro ‘whine’ if it’s running (when was the last time the bearings were replaced?) A non running gyro does still give an output (which would be constant of course as the gyro is not precessing) and that is often offset from the running null. At the age these are now there will be question marks over the condition of the capacitors in the amp but other than these and the power transistors I have found sticky rotor bearings and dirty starter switches stopping the gyro running up to be the most common problems.

    • zn105 says:

      I can hear the gyro and sounds to be running fine.

      • Silentflier says:

        You’ll have to excuse me asking ‘stupid’ questions but sometimes ‘snags’ are so simple it’s easy to miss things which everyone would say is obvious but no one actually asks the question…. So, do you have the tracker option fitted and if you do is the trimming knob (the one by the on/off switch) pulled out? (out is manual trim & basic stabilizer, in is stabilizing & NAV tracking) If knob is out or no tracking option is fitted it does sound as though the gyro pick-off or amplifier has failed I’m afraid. Have you got a local serviceman old enough to remember these? The Piper Autoflite manual (753 720) or edo/mitchell/ century 1C359 stabilizer manual (68S48) both have circuit diagrams and interconnects. I’d look for common signal line components first seeing as it’s not running hard one way or the other or the excitation oscillator (around Q7/Q8) may have a problem. Let me know if you have any problems with circuit diagrams…

  189. Gary Hall says:

    Good call Alan, Gyro pick up or amp sounds the most likely.

  190. Lloyd says:

    I’m tracing the wiring from the radio coupler (1c388-2) to the amplifier (1c515-3) to confirm the connections from the HSI-360A to the amplifier. The input to the coupler pig tail indicates the following from the HSI.21-A, 22-B, 19-D, 20-Fand SGRN-H which are HDG Signal, CRS Signal, Roll Excite, Roll Excite, and Shield respectively. The connections shown at the end from the HSI/DG which does not show the radio coupler interconnections shown from the HSI/DG to the AMP as A-A, B-D, C-B, D-L, E-J, F-C, H-F which from the HSI/DG as DG signal, Roll Common, Roll Excite, DG Excite, DG Excite, Roll Excite, and Ground respectively. The CD-33 output receptacle of the radio coupler has 6 connectors. Do you know if the connectors of the input CD33 pigtail when the radio coupler when is in the heading mode is straight through? That is A goes to A of the CD-33 output to the amplifier. Additionally, connections D-L and E-J indicate a DG excitation signal. There is not a DG excite signal shown between the HSI 360 and the radio coupler. In this case is this signal common to another one of the DG signals where there is are cross connections at the amplifier? I currently do not indicate continuity from the CD33 output plug from A of the radio coupler. And, if I remember correctly, pins A and D at the amplifier is jumpered. Pin D at the amp is the common roll signal which doesn’t seem to make sense. I’m hoping that you, or one of the other bloggers may know the pin in/outs between the coupler and the amp and the answer to the DG excite signal so I can finish verifying the connection between the coupler and the amp.

    • Silentflier says:

      OK, Let’s see if this will help:-

      First off I think you have a typo or two. In line 3. HSI 20 goes to pin E not F and lower down the CD33 output (i.e. to Amp) connector has 7 ways, not 6.

      Your point to points are OK apart from the last bit, ref Amp pins A and D. If you have the 30D207-2 harness some of the drawings do appear as though pins A and D are linked. You have to look really closely at the line to A and you’ll see the connection to pin D is actually the shield from the A wire. The circle and dashes indicating the shield do get blurred in the print and end up looking like a connection ‘dot’ to the actual wire. So no, A and D are not linked at the amp.

      The CD33 connections on the Radio Coupler are not straight through for the -2 type although input (pigtail) A is connected to output (7 Way) pin A when HDG is selected but via an inline 29k resistor (I think it’s 29k, my print is not that clear) that matches the NSD o/p to the autopilot i/p

      The DG Excitation signals are not needed by the HSI but they do have to go to the Radio Coupler. When this autopilot was first designed the DG excitation oscillator was half in the Amp and half in the original, simple type DG’s (the coil used for the HDG signal pick-off formed part of the oscillator, and the signal is still used for phase sensing in the amp). With any other sort of DG or HSI that coil is missing so Century have installed components in the Radio Coupler to simulate it. (One reason why you have different types of Radio Coupler for different HDG systems)

      If you have no continuity from CD33 (7 way) pin A to the Amp Pin A then you have a problem. You will get no HDG or CRS steering signals to the A/P

      Hope this helps

  191. Lloyd says:

    Thanks for the input reply. Yes I did have a typo in line 3. 20 to E was the correct connection in the circuit drawing. Understand A and D are not jumpered at the Amp. Your reply on the Radio coupler really helped clarify understanding the coupler function for the DG excite(s).


  192. Pierre Sirois says:

    I need a replacement for my trim switch Piper P/N 587-862. This is for my Altimatic IIIB-1 autopilot. Does anyone know where I can get one (used or not)? Is it the same as the Century 1C670?

    • zn105 says:

      I upgraded my Autoflite I and have some parts if anyone needs them. Gyro/amplifier with plate and relay, rheostat, dash plate, servo and a few plugs.

      • Steve gansky says:

        They are available but you won’t like the price of around $2,000. Google the part number. Look at the controller ads and also try Preferred Airparts

    • silentflier says:

      If you can give me the Century AK number for the autopilot kit or the aircraft type & model details I’ll see if I can identify the correct Century P/No. for the switch. Might be cheaper to get it direct from them rather than Piper..

  193. apynes says:

    !976 V35B, Garmin 530W, SL30, CIII AP, NSD360Slaved HSI, Radio Coupler, second VOR head (106?)
    Will track HDG fine. Alt and heading.
    Cannot figure out how to get it to track to Garmin. It is coupled and was demonstrated at purchase of plane
    Obvious connection to look for? operator error?

  194. apynes says:

    Also has the Nav1/Nav2 switch

  195. Silentflier says:

    Not very familiar with this type of install but wil try to help.
    I think we might need a little more information to help isolate possibilities here. A couple of simple checks will help narrow things down a little.
    Firstly is it just the 530 that it won’t follow or will it not follow the SL30 either? and:-
    Does the HSI deviation bar move at all? In either Nav switch position with either 530 or SL30 selected?

    I believe the interface for this installation should be analogue. If so the A/P is looking for the L/R deviation signals. If the bar is not moving there is nothing for the A/P to follow. If the bar moves with either or both inputs check the Radio Coupler selection switch position is correct for the input you are using for starters.

    • apynes says:

      Have been able to get the AP to fly the magenta line on the 530 using LOC position. Takes a little while for it to start and previously when it did not capture immediately I had moved the selector to other modes to try and see movement. It seems it may just need a few more seconds…to activate. In NAV it seems to be only flying in the general direction. Next is to try on an approach sequence to see if I have a different experience. It will follow heading very well in HDG mode.

  196. apynes says:

    I had a chance to test again.
    I find it will actually track the Garmin 530W but does not track very precisely varying 5-8 degrees it seems at times. SL30 is the same. I also noticed that when switching from a heading mode to the Garmin and switching from Vloc to GPS it takes a couple of tries before it will change to the GPS heading..and then sloppy.
    HDG mode is spot on heading selected though. I plan on going back up this week to try some more.
    I tried Nav, OMNI and LOC…if I am in LOC and following GPS and then switch to OMNI it reverts to course….will need to try more and document combinations.

    • Silentflier says:

      Silly question that you probably have already thought of but I always ask them because sometimes they slip through the gaps…

      Does that Garmin have a ‘Select Course Width’ option? If it has check that it’s not been set to anything silly like 5 miles or more. With a wide Course Width set it would not really worry about correcting until you were near the edge of the band hence wandering and low level inputs to correct… Narrower course width would ‘tighten up’ the response, too narrow and you’d be overshooting all the time.

    • apynes says:

      Latest update. Resumed my instrument training and the first day the instructor and I went to test the CIII autopilot. First up was an ILS and it captured the localizer and followed the glide slope perfectly.had auto pilot all set on including altitude, then moved the selector to Loc Norm. Course was aligned too.

      Tried a GPS approach in Nav mode next…no bueno….I think it will require Loc norm too…will test it Saturday.

  197. Jon says:

    I have an 1969 Aztec “B” with an Altimatic III B. It all works great. But I noticed the altitude hold will not hold at high altitudes on warm days. Above 12000 with OAT 55 F it did not know what to do. Down to 8000 worked fine, up to 11000 (different MEA) no hold, back down fine???
    Any high altitude thoughts?
    Thanks Jon

  198. A1Topgun says:

    Searching for answers and found this excellent autopilot blog. I’ve noticed in reading the 495 comments that most are for Century III autopilots. I have a Century 41 in an 82 Piper Arrow IV. I would first like to know if I am in the “right place”, before I get too detailed with my problem. It’s a long story starting with the annual last January.

  199. Stan Owen says:

    Century IIB in Turbo Arrow 3 goes into steep turn rate when heading bug or 430W is requesting turns of greater than about 60 degrees. I just removed my NSD360 and all radios and installed Aspen Pro, 430W, and KX155 etc. Autopilot worked perfectly before and does now too except now when I turn the heading bug more than about 60 degrees the autopilot will put the plane into a steep turn. Before it limited the turn rate to 30 degrees. Avionics shop did put in relays to connect the autopilot to the Aspen Pro. I did a basic ground check of my artificial horizon as per a previous post to see if it was outputting to the A/P. Rotating it while having the A/P roll control on does instantly effect aileron/autopilot control. Ideas?

  200. Stan Owen says:

    I have subsequently adjusted the maximum bank angle pots on the Century IIB and turns are now 22 degrees with the heading bug.

  201. Phill says:

    I’ve been overhauling my Century II on PA28-181 Archer 1979. There’s a metal box, (about 4x6x1.5 if I remember correctly) behind the sidewall by the pilot’s left knee, which connects the Attitude Indicator to the autopilot console. Any idea what this box is? It doesn’t seem to be mentioned anywhere in the Century II/III maintenance manual. All it says on it, at least on the exposed side, is Mfg. 1979, which implies it came from the Piper factory. I’d look it up in the Piper Maintenance manual, but I thought I’d ask.

  202. Phill says:

    By the way, anyone looking for it, the Century II, III Service Manual can be downloaded free here:

  203. Matt Langenbach says:

    Hi all. Great blog! I have a Century III AP in my bonanza. I have had all the components (excluding servos) benched and tagged, so I know they are working. I have 2 major issues:
    1. when I engage altitude hold, the trim wheel starts pitching the nose up. The first time I engaged this, the plane held altitude fine, but when when I disengaged, the nose of my plane pitched up violently.
    2. In heading mode (NAV), the plane tries to trach the HSI but the corrections are way overblown and I go right through the course then the plans kinda wildly swings back toward course, then goes through it then the game repeats.
    Other than scrapping the entire system, does anything obvious stick out to you

    Thank you for a great site.

    Matt Langenbach

    • davidcrocker says:

      #1 sounds like a problem with the altitude capsule or the connections to it.

      For #2, what is the maximum bank angle that the autopilot commands? If it’s more than 20 degrees, adjust the console trimpots to reduce it. It could also be that the gain trimpot in the radio coupler needs adjusting to reduce the gain.

      • Matt langenbach says:

        Thank you for such a fast response. Maximum bank is probably 15 – 20 degrees but I will check it and have my avionics guy do the other things you suggested. I will let you know what happens.



        Thank you so much

  204. Greg Schwartzkopf says:

    Wow, what great info on this post.. Thank you!!

    I have a 1979 Seneca II with the Alitmatic IIIc. Whenever the pitch trim is used, either in Altitude Hold mode or just manual pitch mode, it seems that the unit applies too much ‘nose up’ trim, resulting in the pitch servo having to apply significant ‘nose down’ control pressure. Of course, what this results in is a dramatic nose up situation when the autopilot is disconnected. Over time you get used to it and apply down force before disconnect and them trim up to adjust, however due to another problem (guessing amplifier– it’s in the shop now) where the autopilot started to trip a breaker, this happened without warning and caused me a bit of a scare. Everything works fine otherwise– alt hold is good, maintains pitch in manual mode, flies the glideslope without issues, etc… It just seems like it’s getting a poor reading on pitch position and/or the amount of force on the controls.

    I’m certain that we’ll get the tripping breaker fixed (BTW, like another user posted, it happens after about 15-45 seconds even in manual roll mode only, which you indicated points to common or roll circuit problem) but I also want to fix this out of center trim issue as well.

    Any idea where to start? Any advice would be appreciated.. Thank you!!

    • davidcrocker says:

      Perhaps the pitch trim amplifier is faulty? That’s a separate module from the main autopilot amplifier.

      • gregs007 says:

        Well in between my last post and now I did a LOT of reading and also obtained the Altimatic IIIc service manual for my airplane. According to it, there is a centering adjustment potentiometer in the pitch trim amplifier that likely would have corrected this problem– you do need the test harness in order to perform this diagnostic/adjustment. However, the story goes deeper than that so I thought I’d relate it for anyone else out there who might be having a similar problem.

        It started with my autopilot circuit breaker popping when I was on a climbout a couple of weeks ago– I had it in manual pitch mode on a 5 degree cruise climb tracking the heading bug. The breaker popped and because of the off center pitch trim, the nose popped up violently. Suffice to say it caught my attention. After my heart stopped racing, I quickly figured out what happened and then waited until I was at altitude to run the autopilot through a few tests. No matter what I did the breaker would pop after about 45 seconds.

        Unfortunately I had to hand-fly for another 2 hours to my destination, and then the same all the way back. The weather was VFR, but it was turbulent, and I had forgotten how a simple thing like a long flight in turbulent weather could be so exhausting when hand flying the whole time. Anyway, I digress.

        I have a really good avionics shop here at home base (Bragg Avionics at KCRG) and I knew that they’d be able help me. (Shortly after I first bought my plane, they helped me figure out a dried up flux sensor and misbehaving glide slope coupler). Turns out when they removed the roll servo from the equation, the breaker wouldn’t pop anymore. So a reasonable person would assume that with age (none of the autopilot components had been overhauled or even repaired since 1979– pretty darn impressive) that the roll servo needed some TLC to get the startup voltage and current draw under control. For good measure however, the folks at Bragg thankfully decided to also send out the 515 computer/amplifier and control head to Century to have diagnostics run. Turns out that all the transistors were leaking (none shorted) and there were a few other things wrong that they repaired and brought back to spec, in addition to repairing the roll servo and getting the startup voltage down to less than 1v.

        After plugging it all back in, guess what? Breaker popped again!!!! Nooooooo! So the problem wasn’t solved. So finally the guys at Bragg went back in and cleaned/tightened up all of the ghastly blue molex plug pins and the problem went away.

        So I was feeling pretty bad about spending the money to get my autopilot serviced when the likely problem was just the roll servo connector being corroded and loose. That was until I took it for a test flight and realized that my autopilot, while previously functional, was performing nowhere near its capability. No more wing-waggle, no more out-of-pitch-trim-center-up-nose-pop on autopilot disconnect, no more overshooting the final approach course intercept. Ran it through its paces– altitude hold, flying the bug, tracking the omni, flying the ILS and glide slope. Perfect in all areas.

        Century only charged about $800 for the comp/amp and another $600 for the servo repair and control head diagnostics (still haven’t gotten the labor bill from Bragg :/) but it probably flies as good as it did when it was first equipped at the factory 37 years ago.

        Moral of the story? Repair or replace those blue connectors. Also, if your autopilot is almost 40 years old, it probably needs to be serviced. Find a good A&P to remove the components and have them sent over to Century if you’re experiencing steadily degrading performance of the AP. It’s very likely that some of the aging components in your system have failed. It is so well engineered that it turns out that most of the critical components can be failing and the thing still can manage to fly an approach down to minimums safely.

        I continue to remain impressed with the ingenuity and engineering of these aging aircraft systems. I look forward to many more hours of hands off-flight in my Seneca II.

  205. David Hello I need to get an MS number for a century IV part # 65s67 they are proud of that part
    in mineral wells any help is appreciated

  206. Lucas Trombeta says:

    Hello David, how are you? I hope fine.
    I am from Brazil and i have a century 3 in my arrow.
    My amplifier conector brokes and i will change to db25 conector. The board has pin up and down, in service manual i saw that up is marked by numbers and down by letters but i did not find wich pin is wich letter and wich pin is wich number. Can you help me?

    • silentflier says:

      You can replace the connector see posts below (12/14 November) for connector details.
      For information pin numbers are:-
      PCB Track Side top to bottom = A thro’ S. Note that G, I, O and Q are not used
      Component side, top to bottom = 1 to 15

  207. Lawrence Miles says:

    Dave, I have a 75 A36 with a Century turn n bank indicator Autopilot with an STEC 30 ALT for altitude. When it’s hot, everything works fine. When it’s cold, the Century will wander, not hold a heading, and enter into a steep turn. Any ideas why this happens when it’s cold? Thanks for your insight. My shop just can’t get this fixed.

  208. Silentflier says:

    Intermittent faults like this can be a real swine to find but, as it seems to be temperature dependent, it sounds like you possibly have a dry joint somewhere in the roll circuit on the PCB. (When the unit is cold the contraction on the lead is just enough to cause a bad joint. Warm it up, the component lead expands and connects again.) It’s a bit tedious but I’ve found that the quickest way to check this is to carefully reflow any solder joints on the PCB that look at all suspicious – especially any that seem to have a thin, dark ring around the component lead between the lead and the solder to the land that look like old flux deposits. (a 5X magnifier will help) Be very careful not to make any solder bridges between tracks. It’s not clever but it’s certainly faster than spending hours trying to identify exactly which joint is bad.

    Or:- Get your shop to use a can of freezer spray while they are testing the amp and spray a small area of the PCB at a time and see what happens. At ambient shop temperature the unit will probably work fine but once it’s cold the fault should reappear so then they should know where to start looking for the problem.

  209. David says:

    One problem people should be aware of is the conductors in the gray multi-conductor cables rotting away. I have found this in two aircraft, one a retrofit done probably in the late 1970’s and one in a 79 Arrow. I forget exactly which cables were at fault, I think the cables going to the DG or the roll filter perhaps. The copper inside the insulation turns to powder for a short distance. I take care of 7 or 8 a/c with Century II or III autopilots and “reworking” the female contacts in the blue circular Amphenol connectors is SOP. As others have mentioned the connectors are still available, just without the Amphenol name. Replacing the is worthwhile in some cases.
    Now a question for others; Where might be a good source for obtaining 1 new or used contact for the edge connector at the back of a Century II amplifier? It is similar to the common contacts Bendix/King use with the KT76A, KX155. etc., but a little narrower. Avionics shops I have checked with either don’t have any or don’t want to be bothered.

    • Frank says:

      Maybe you already found this out but the KT76A has commonality with the Trig slide-in replacement and the contact pins for that come with the new Trig, often never used because the existing rear end plug is still good and doesn’t need replacement of the contacts.

  210. David K. says:

    One problem people should be aware of is the conductors in the gray multi-conductor cables rotting away. I have found this in two aircraft, one a retrofit done probably in the late 1970’s and one a 79 Arrow. I forget exactly which cables were at fault, I think the cables going to the DG or the roll filter perhaps. I take care of 7 or 8 a/c with Century II or III autopilots and “reworking” the female contacts in the blue circular Amphenol connectors is SOP. As others have mentioned they are still available, just without the Amphenol name. Replacing the connectors is worthwhile in some cases.
    Now a question for others; Where might be a good source for obtaining 1 new or used contact for the edge connector at the back of a Century II amplifier? It is similar to the contacts Bendix/King use with the KT76A, KX155. etc., but a little narrower. Avionics shops I have checked with either don’t have any or don’t want to be bothered.

    • Silentflier says:

      I think the contact pitch on the Century edge connectors is slightly smaller then those on the King radios so they probably won’t work.
      If it’s the Century II-B (I think the C II is the same) it’s a 2x 15 way connector, the same as on the 1C395/515 series amplifiers (I think it’s a 3.96 mm pitch). The problem is finding one. I’ve just had to replace the one on my test set harness and the only one I could find was from Hong Kong on e-bay and they were absolute rubbish – plastic body melting as soon as a soldering iron went anywhere near, pins shearing and contact numbers and letters on the card (slot) side completely different to those on the wiring side!!!.
      I suppose one option would be to source one that has been recovered with the harness if someone has updated their aircraft’s A/P or from a scrapped A/C. The original Edo/Century/Piper connectors are quite sturdy (not like the round ones!). I’ve had one on a bench rig for over 5 years with no connectivity problems so I should think one recovered from an A/C should be OK so long as the body is not cracked.
      I wish you luck with your search. If you do find a supplier of genuine Amp ones or a quality alternative I’d really appreciate you letting me know…

    • silentflier says:

      Hi David, Further to my earlier reply see the addditional comments and information below regarding this connector. Seems like we were not the only ones looking…

  211. Gary Hall says:

    Hello Again Alan, What’s the likely fix for a Piper III (similar to the Century 2B without the rocker switches) autopilot not operating in either roll direction. I’ve had the console on my century test rig and whilst the clutch engages, the servo motor doesn’t work. The Servo and associated wiring is ok, so just looking at likely causes in the console. I’m going to look at the power transistors P/N 2N3055,first do you know if these can be tested in situ?

  212. Gary Hall says:

    Thanks Alan, power transistors are ok, for some reason it started working, and I managed to rig it successfully…..However gone back to it a day later and dead as a door nail again, servo engage is ok just no drive to on the roll/HDG.
    Ref the 15 pin molex it’s part number 480110-5, looks like they’re still available. I use Digikey or Mouser for the obscure stuff, but showing on Amazon for £37. If I come across any from old installs I’ll let you know.



    • David Viewing says:

      This connector was made by AMP, not Molex. It’s still available from TE Connectivity which is AMP’s brand these days. The inserts are at although I can’t confirm this is the exact same part as used originally, it does fit in the housing.

      • silentflier says:

        Gary/David, Thanks for the info on connectors. Not sure if I was reading the Mouser catalogue correctly but I think it said: currently Nil stock, min order Qty 1,100 (!!!) for the connector body… I will investigate further to get rid of my ‘Hong Kong Special’

        Gary, your CII, So much for me writing a saga about checking test points etc. now you know the electronics do work – just not all the time!
        As it goes completely dead I’m going to suggest looking in the DG excitation oscillator area around T1 and Q15, 16,19, C20 etc. Without this running nothing will work except the solenoid. Suggestions:- Dry joints on component leads, wires from T1, broken cores on top of board and only held in place by flux residue sticking the wire insulation to the board (Yup have seen that!) C20 (Blue tantalum cap If I remember correctly) can dry up and ‘pop’ sometimes only showing a hairline crack around the periphery as a sign that it’s gone – all the usual fun stuff really. You are looking for a 5khz signal if it’s running.

        If you get a chance try GENTLY flexing the pcb when it’s not working. If there is a dry joint this will often cause it to ‘make’ and the unit spring into life – all you have to do then is find out where it is (or run an iron over all the joints which is sometimes quicker!)
        Have fun…

      • Gary Hall says:

        Genius Alan!!
        Dry joint around Q1 Transformer, not sure which one, but all working. I’d had a guess at the oscillator circuitry, but just needed the extra prod, to have another go, so thanks for leading me to the fix. Sorry about the reference to “Molex” David, Avionic Engineers tend to refer to anything with an F Type crimp as “Molex” even if it’s made by another manufacturer.
        Great site!!!



  213. David Viewing says:

    Hi Alan. Unfortunately the 480110-5 is not available as a sample from TE Connect. However, they do show it as being available from ONLINE COMPONENTS (24 as of 2016/11/13). These are blue not black as in the original but should be genuine. However you do run into min costs ($100) and shipping charges. I might order a couple to pick up when I’m in the US next (January). Let me know if you’d like one (guess you’re in UK?) approx $50.

    • silentflier says:

      I’d like a couple if possible please. I’ll need the pins as well as mine are currently all solder tags…. Yes, I’m in the UK. If you’d like to contact me direct use my tag name above at tesco d0t net (I hate those sneaky mail mining bots!!!)

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