Alternator failure warning in Piper PA28 aircraft

Some time ago I came across this Air Accident Investigation Branch report concerning a PA32R that landed gear-up following an alternator failure. The report noted:

It was apparent that the ALT caption, which illuminates whenever there is no output from the alternator, mounted between two other warning lights on the instrument panel, was very dim in comparison to these lights…

and later:

The ‘zero error’ on the ammeter, together with the dim ALT light, would probably have made it difficult for the pilot to appreciate that a problem existed with the electrical system, until the relatively rapid onset of failure of the electrically powered services.

Piper alternator warning light

Piper alternator warning light (picture Crown Copyright, reproduced here under the Open Government licence)

I’ve seen a dim ALT light on some PA28s too, which have a similar annunciator panel as the one in the subject aircraft (see picture). Is there a common issue in these aircraft? I think so, and I’m going to explain why. Please note that some time in the late 198os or the 1990s, Piper introduced an alternator failure relay and changed the circuitry. The following only applies to PA28s that pre-date this change.

1. When the ALT lamp fails, what is the correct replacement part?

According to the POH for my aircraft, the bulbs are pilot replaceable. Unfortunately, it doesn’t say what the correct replacement part is. For that, you need to look in the parts list – an expensive document that few pilots have. If you fly a PA28, ask your engineer to look up the annunciator bulbs in the parts list for your specific model and serial number. He’ll probably tell you that the VAC and OIL bulbs have a part number ending in -330, but the ALT bulb part number ends in -328. [If the parts list prescribes -330 bulbs for all 3 lamps, you’re flying a more recent PA28 and you can stop reading this.]

What is the difference between the -328 and -330 parts? Simple: the -330 part is a 14 volt bulb, but the -328 part is a 6 volt bulb.

Unfortunately, most pilots and some engineers don’t realise that the ALT bulb should be a 6V part. So, when they find it isn’t working, they replace the blown bulb with a regular 14V one. You can hardly expect a 14V bulb to light up fully if the circuit was designed for a 6V bulb, can you? This is a common reason for the ALT lamp being dim.

So, you’ve established that your aircraft requires a 6V ALT lamp, and you’ve found that you actually have a 14V bulb in that position. Voila, you just swap the bulb out for the correct 6V one and the problem is solved, right? Unfortunately, things are not necessarily that simple. Let’s look at why Piper specified a 6V bulb in the ALT position, and why these bulbs tend to blow quite often. For that, we’ll need to look at the circuit diagram.

PA28RT-201 early models annunciator wiring diagram (click to enlarge)

At the top of the diagram, one side of each lamp is connected to the +14V bus through the fuse. The other sides of the OIL and VAC lamps are grounded by pressure-operated switches that sense low oil pressure and low vacuum respectively. However, the other side of the ALT lamp is connected through a diode to the alternator output and to ground through a 50 ohm power resistor. When the alternator is operating, it puts around 15V across the resistor, so the lamp is out. When the alternator is not operating, the lamp sees the battery voltage (12V or so, since it is not charging) through the diode and 50 ohm resistor. The bulb is rated 6V 1W, so its resistance should be 36 ohms when hot. Therefore, a little under half of the 12V will appear across the bulb, and the balance across the diode and resistor. That’s why you need a 6V bulb in this position, not a 12V one. But let’s look a little further…

2. What happens when you press the Lamp Test button?

One side of the lamp test button is grounded. The other side is connected to 3 isolating diodes, which in turn connect to each of the 3 lamps. So when you press the button, one side of each lamp is grounded via a diode. Since the other sides are connected to the bus, all 3 lamps light up. Simple. Except… how many volts does this put across the ALT lamp???!

If you do have the correct 6V bulb in the ALT position, you’ll notice that it lights up rather more brightly than the VAC and OIL lamps when you press Lamp Test. That’s not surprising, since you are putting 12V or so across a 6V bulb. Unfortunately, if you use the Lamp Test button a lot, then the ALT bulb is likely to fail prematurely, because it is only designed to work on 6 volts. I’ve also known the diode to fail.

What can be done to avoid this happening? A simple solution would be to put a suitable resistor in series with the lamp test diode for the ALT lamp. Unfortunately, things are never that simple in aviation because of the need for approval from the relevant authorities. So I suggest one of the following:

  1. Get yourself a proper Low Voltage warning indicator so that you don’t need the ALT lamp to work. UK registered PA28s are required by the CAA to have a bright red flashing Low Voltage light anyway – much more attention-getting than the ALT lamp, even when it is working properly! [I wish I had an attention-getting vacuum failure indicator too.] If you have a JPI engine monitor fitted, it will alert you to low bus voltage as well as to engine temperatures outside limits.
  2. The other solution is simple: don’t use the Lamp Test button unnecessarily! If you turn on the battery master before starting the engine, all 3 lamps should light anyway – so there is no need to do a separate test. [It’s not quite the same in a Turbo Arrow, because the Lamp Test button also tests the overboost light.]

Before I finish, here is another possible cause of dim ALT lamps.

3. Is the 50 ohm resistor in good condition?

The 50 ohm resistor sees the full alternator voltage (before it passes through the power diode to the battery), so it has to dissipate a little over 4 watts continuously in normal use. That calls for a power resistor thermally attached to a heat sink, such as the aluminium sheet that the other annunciator components are attached to. The only detailed drawing of the annunciator panel assembly that I have seen in a Piper maintenance manual showed this type of arrangement.

Unfortunately, it appears that in some aircraft, Piper took a shortcut. I have seen a bundle of 5 two-watt (or so) 220 ohm resistors connected in parallel, used instead of a power resistor. All the resistors were charred and two of them had gone open-circuit. In this arrangement, each open-circuit resistor will reduce the current getting to the ALT lamp (and hence the brightness) when the alternator is not operating.

That’s enough on this topic. Fly safely, and be alert to alternator failure!

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2 Responses to Alternator failure warning in Piper PA28 aircraft

  1. I have a 1979 Warrior II. The airplane has been very reliable with the exception of the ALT annunciator circuit. It has failed several times in the 5 years I have owned the aircraft. I have had several diode failures in the past and the light is currently not working, probably due to an open resistor. The lamp lights when the test button is pressed, but fails to light when the master switch is on and the engine is not running. My wiring diagram states that the resistor you reference is a 5 (five) ohm 6.5 watt one. Using ohms law, 50 ohms makes more sense. The current flow with a 50 ohms resister would be about 240 milliamps , which sounds about right for a small indicator bulb. A 5 ohms resister would pass about 2.4 amps, which sounds too high for the 6.5 watt resistor to handle. I have the official Piper maintenance manual. Is this an error in my manual?
    I considered replacing these dim bulbs with brighter LEDs which would make the annunciator more effective, but the thought of trying to get a 337 approved stopped me cold.

    • davidcrocker says:

      The only wiring diagram I have seen for the annunciator circuit that gives the value of that resistor shows it as 50 ohms, unspecified wattage. My own aircraft originally had five 220 ohm resistors in parallel, which gives 44 ohms. I had it replaced by a power resistor thermally attached to the aluminium frame, as shown in the sketch in one of the manuals. The value was 47 ohms AFAIR, which works well (reasonably lamp brightness when the alternator is turned off). One of the diodes had failed, so they were all replaced with new glass-passivated ones at the same time.

      Calculation suggests that under conditions of alternator failure, 27 ohms would be about ideal (the lamp rating is 6V 0.2A and the resistor needs to drop about 5.4V). However, the lower the value of the resistor, the more power it will dissipate. I guess 50 ohms was a reasonable compromise between lamp brightness and power dissipation.

      5 ohms is definitely wrong. It would dissipate about 45 watts when the alternator is running.

      Yes, it would be nice to use LEDs instead, also to use a transistor or IC to drive the alternator failure LED to avoid the need for that power resistor.

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