Alternator warning light - Cessna 150

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Matthew Rogers

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How does the Alternator warning light in a Cessna 150F work? I am putting in a new radio (with AP blessing) and found out that the wire going from my Alternator warning light (just a simple red light, not anything special) was cut off just past the firewall. So it has been inactive all this time.

But now I can't figure out how it actually works. The light should go on when the Alternator is not producing power, but how do you make a light bulb light up when there is no power? The plane is old enough that there are no complicated circuits to make this happen, so is there some sort of solenoid switch that is held open when the field wires are energized and it closes when the field is cut, thus completing the circuit and activating the light?

Anyone else have a small Cessna with a single Alternator warning light and know how it is supposed to be hooked up?
 
Thanks, that was a great link and makes sense so far.
 
That light, in a car, will illuminate if the alternator fails. The alternator has an "S" terminal that feeds the "S" terminal on the regulator to turn it on when you start the car, and if the alternator fails, the "S" signal dies and the regulator turns off and the light goes on.

But in the typical lightplane it doesn't. The regulator is turned on by the ALT switch on the panel, or in your older 150s, by the master switch. If it's a split master you can turn the mastger on and the ALT side off and the light should illuminate, but if the alternator fails--worn-out field brushes (most common), broken wiring, broken drive belt or drive coupling, or if the alternator falls right out of the airplane--that light won't come on as long as the ALT switch is still on and the regulator still works. Later aircraft replaced the regulators with alternator control units (ACUs) that worked the light properly.

The light is connected as shown in the diagram on that site posted earlier. Look and see if the "I" terminal on the regulator has anything connected to it.

A burned-out light in an airplane just means a burned-out light. In some cars, that light was part of the regulator switch-on circuitry and if it burned out the alternator was dead because the regulator wouldn't start up.
 
I hooked up the missing “I” terminal today and worked out a few things.

First is that the light was originally wired to get its 12v power from the nearby fuel gauge instead of the Alt field fuse power wire. Will this actually affect the operation of the light?

Second, as described by Dan, the light does not come on when the master switch is on, but the engine (and alternator) is not on. The bulb does actually flash for a tiny split second when the master is turned off as the regulator shuts down and there are a few electrons left in the wire to light the bulb. So does this light only actually show that the regulator has failed, but does not actually tell if the charging system or alternator has failed? I don’t have a ammeter, voltage, or any other gauge that shows the charging system output. What I recently added was a cigarette lighter usb charger with a voltage readout, but it is not prefect because of the type of connection.
 
You'll need to find a wiring diagram for your year 150 to determine if the light is wired properly, or in the case of an stc'd ACU look at the install manual. IIRC, some Cessna's took power from the instrument panel light breaker. It wouldn't get power from the field circuit unit in any case.

Generally speaking here's how it works. The low volt light comes on when battery voltage drops below a certain threshold. This happens most often at idle when the alt output can't keep up and systems are drawing the battery down below that threshold, the ACU (alternator control unit or regulator) senses that and grounds the 12v light circuit turning the light on. The ACU doesn't care where voltage is coming from, battery or alt. If the battery is fully charged, the system may not drop below the threshold necessary to turn the light on with engine off. The flash you're seeing may be a momentary grounding of the light circuit as the ACU shuts off with the master. Inner workings of the ACU are a bit of a black box to me as I've never laid eyes on a real schematic for one.
 
Chip Sylverne said:
You'll need to find a wiring diagram for your year 150 to determine if the light is wired properly, or in the case of an stc'd ACU look at the install manual. IIRC, some Cessna's took power from the instrument panel light breaker. It wouldn't get power from the field circuit unit in any case.

Generally speaking here's how it works. The low volt light comes on when battery voltage drops below a certain threshold. This happens most often at idle when the alt output can't keep up and systems are drawing the battery down below that threshold, the ACU (alternator control unit or regulator) senses that and grounds the 12v light circuit turning the light on. The ACU doesn't care where voltage is coming from, battery or alt. If the battery is fully charged, the system may not drop below the threshold necessary to turn the light on with engine off. The flash you're seeing may be a momentary grounding of the light circuit as the ACU shuts off with the master. Inner workings of the ACU are a bit of a black box to me as I've never laid eyes on a real schematic for one.
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The light in his 150 won't light up when the voltage falls off unless he still has the airplane's original generator. Different animal, that.
 
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