Alternator fail light; what does it mean?

Please don't laugh at these 2 questions. There will be more to this after I can get a failure analysis completed on my alternator.

What does an alternator fail light actually tell you? does the light come on instantly?

Thank you for your feedback.

You need to know what it's wired to tell you, to know what it's telling you. In a car, it just means 'malfunction'; it can be low voltage or over voltage.

In the plane, use your voltmeter. 14v-14.8v: probably charging ok. 13.8-14v: questionable. <13.8v: not charging.

Ammeter: worthless, unless you know exactly how it's wired *and* how to interpret the numbers.

I can't find out specifically what it would mean on a Plane Power alternator wire per their instructions. http://www.planepower.aero/pdf/AL12-EI60_C.pdf

AX-O said:

I can't find out specifically what it would mean on a Plane Power alternator wire per their instructions. http://www.planepower.aero/pdf/AL12-EI60_C.pdf

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Looking a that diagram, if your seeing the light, it tells me either the field is not getting energized and the alternator is not producing power, or your getting a false indication. As shown, that light would be getting 12+ from the bus all the time. When the alt is not producing power, it makes the other side of the light a ground so the light illuminates. When the alternator is making power, it sends +12v to the other side of the light through a diode, when you put +12v on both sides of a light, it stays off. Is your battery still charging with the light on? or is the alt totally dead and the light is on? A large voltage drop between the (battery) and the alt output stud can light the light as well, perhaps a frayed or high resistance connection between the alt output stud and the battery?

The diagram shows typical automotive style wiring of an alternator 'idiot light'. Google will show hundreds of diagrams. Unfortunately, most either show a dumbed down 'hookup' drawing like what you have, or an incomplete schematic that omits the circuit wiring on the 'other side' of the idiot light.

Functions:

Bus powered up, engine not running (dead alternator): should illuminate.

Bus powered up, engine running (alternator working normally): should go out.

Bus powered up, engine running (alternator defect causing overvoltage): should illuminate. (BUT: I wouldn't depend on this for OV indication; other failures in the alternator could cripple that function.)

Note that you must use a 'regular' light bulb in the resistance range spec'd by the alternator mfgr for it to work properly; you can't substitute an LED & have the circuit work correctly.

Use it for a "hot mag" buzzer

I use a DPDT locking toggle switche for each ignition. On the side not used by the ignition I connect a simple 12vdc buzzer. The switch, when in the ignition on position, provides 12vdc to the buzzer. The buzzer ground is connected to the alternator light connection. This way when the engine is off and the ignition is in the on position, the buzzer will sound. Once the engine is started the buzzer is silent.

This has proven to be a good safety step up.

Carl

I installed a Plane Power set up and substituted an LED in place of the supplied incandescent warning lamp. I put a drop down resistor on the positive lead of the LED per it's manufacturer's recommendation. Seems to work fine with one exception. Instead of being completely off in good alternator mode, the LED glows so dimly its not noticeable. Even at night it's not very detectable.

Since an an LED lamp is a diode polarity matters. Polarity doesn't matter on a regular incandescent bulb. Wrong polarity on an LED lamp means it won't illuminate. But in the desctibed scenario what does positive voltage on both pins of an LED lamp do? Should I be concerned about the slight glow?

Jim

@Alex - Wow! I'm glad you asked this question. I have dual electronic ignitions and I "assumed" if the indicator comes on I need to get on the ground before the battery decays (~30 mins)

Now I'm going to have to look at the wiring diagram and understand it.

Thanks for asking the question!

Basically, the way it works is that one end of the bulb is tied to the 12V bus and the other end is tied to the alternator. When the alternator is not working, the current flows from the 12V bus to the alternator, and so the bulb glows. That's why an alternator light makes for a good warning light if you leave your master switch on.
Once the alternator is working properly, current can no longer flow from the bus to through the light, as there is 12V at both ends. Hence, the light goes out.

It's best to use a simple incandescent bulb but you can make an LED work.

Vic

All,
Thanks for the feedback. I contacted the Plane Power folks regarding the light bulb and this is what they said.

"It needs to be an 100ma 14 volt incandescent bulb. You can buy them through A/C Spruce P/N 08-14616"

Well there is problem number 1 on my side. I used an LED. I don't remember that being on any of the docs that I read. I must have missed that part somehow if it were in the Do Not Do section.

That being said. I had an alternator failure at approx. 150 hrs of flight time. Alt fail light did not come on (now I know why). The problem is still TBD. I have to send the alternator in. I have a good guess on what it is but I will wait until they check it and report the failure back.

The only way I noticed the failure was due to the amount of amps that were on the negative side (-10 amps in that configuration). I must of caught it early enough as the battery was not showing much drop in voltage.

I have been thinking about using this lead but wasn't sure how to connect it via the G3X, now I think that I can connect it as a discrete high to the EIS and when it is powered at 12 volts the warning will be off and when it is not powered as the alternator is either not on line or faulty it will show a warning.
Am I on the right lines?

Ian Coates said:

I have been thinking about using this lead but wasn't sure how to connect it via the G3X, now I think that I can connect it as a discrete high to the EIS and when it is powered at 12 volts the warning will be off and when it is not powered as the alternator is either not on line or faulty it will show a warning.
Am I on the right lines?

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You can do it - perhaps by adding a dropping resistor to simulate the current draw of the lightbulb - but why? The best indicaor of alternator health is output voltage. Just set your EFIS low voltage alarm at 13.5vdc or so.

Carl

Ian Coates said:

I have been thinking about using this lead but wasn't sure how to connect it via the G3X, now I think that I can connect it as a discrete high to the EIS and when it is powered at 12 volts the warning will be off and when it is not powered as the alternator is either not on line or faulty it will show a warning.
Am I on the right lines?

Click to expand...

I have it hooked up directly to a G3X discrete input as active low. It seems to work: when the alternator switch is off / alternator not producing power (thus the wire is grounded,) a CAS displays "ALT OFFLINE." When the alternator is switched on, the CAS indication goes away. No need for the +12v side of the circuit.

I wired the light per plane powers instructions. The light is located above the master power switch. I have a two stage master power switch, first notch is batt only, second notch is batt/alt combined. When the switch is in the first notch, (batt only position and no active alternator field), the red light is illuminated; indicating that the alternator is not working.