75/76 Alternator Light, Trying to understand an ignition feedback issue. Options

[McMark]

I thought all 914s had a diode on the charging light, but looking at the 74/75/76 diagrams I don't see one. A certain green car doesn't shut off when you turn the key off, but disconnecting the charge light wire kills it.

[SirAndy]

I thought all 914s had a diode on the charging light, but looking at the 74/75/76 diagrams I don't see one. A certain green car doesn't shut off when you turn the key off, but disconnecting the charge light wire kills it.

I recall seeing cars without the diode but i don't remember what year they were.
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[ejm]

Early cars had a diode behind the dash for the brake warning light but I don't think there's ever been one for the alternator light. A bad diode in the alternator can cause this along with the charging problem mentioned in another thread. Pull the 3 wire alternator plug off the relay board and see if that kills the engine.

[SirAndy]

Early cars had a diode behind the dash for the brake warning light

Ah yes, you are correct. That diode is behind the gauge ...
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[mgphoto]

Behind the dash.

Anyone have a photo it's location?
Thanks,
Mike

[jcd914]

I am avoiding writing a convoluted e-mail at work right now, so this is longer that necessary. (IMG:style_emoticons/default/smile.gif)

If you had a diode in the charging light circuit the light could work as it should.

Normally you have current flow from the ignition through the charge light through the alternator fields (excitation charge) and to ground when the key is turned on.
When the engine starts and the alternator starts charging the voltage at the charge light connector on the alternator (blue wire) matches the voltage in the output of the alternator which matches the voltage of the battery and current quits flowing and the light goes out.

This indicates normal charging, light out while engine runs.

If the alternator output drops and voltage drops below battery voltage, current again flows from the ignition switch through the light and the charge light glows.

This indicates charging problems, in this case low alternator output.

If the alternator starts over charging, regulator failure or inter short causing over charge. Voltage at the alternator output and the charge light connection (blue wire) goes up higher than battery voltage and current flows from the alternator through the charge light and back to the battery (sort of) through the ignition switch. And again the charge light will glow.

This indicates charging problems, in this case over charging.

If there were a diode in the charge light circuit current could only flow one way and the light could only indicate under charge or over charge not both.

Jim

[914Sixer]

None on 74-76 models.

[McMark]

I've solved the issue on Zach's car by working around it, but I'd still like to understand why it wasn't working like it should.

Basically, the alt-charge indicator light was feeding back into the fuse #9 circuit and keeping the car running.

Here were the symptoms:
1. If you turn the key off, the gauges would go dead (no more tach rpm, etc). So the ignition switch was working correctly.

2. Once the car was key-off-running you could kill it by stepping on the brakes or putting the car in reverse. This would kill the car because of the brake or reverse lights coming on. I suspect these bigger bulbs would demand more power than the feedback loop could supply, dropping the voltage in the feedback loop to the point where the ECU dropped out.

3. Once the car was key-off-running, you could also kill it by pulling the alt-charge light wire (blue) from the 14p connector at the relay board or by pulling the three prong alternator connector.

4. With the car fixed as it is now, the key works like it's supposed to, but I noticed the alt-charge light comes on when you turn off the key and stays on while the engine winds down -- going out once the alternator slows down below its effective RPM. I think this is the power that used to be feeding back.

I can't compare because I don't have any other TypeIV 914s with a stock alternator in the shop right now. So can anyone confirm if your alt-charge light illuminates after the key is turned off? Please include what year your car is.

[Spoke]

Basically, the alt-charge indicator light was feeding back into the fuse #9 circuit and keeping the car running.

Yes, this absolutely can happen.

The secondary charging circuit of the alternator connects to the primary (battery) side through the GEN light. The secondary side drives the voltage regulator and regulates the alternator output.

The primary side of the GEN light gets switched power through fuse #9 and back to the ignition switch.

When the ignition switch is turned off, the vehicle's electrical system is now powered by the alternator secondary through the GEN light. Theory is the GEN light is so low wattage (ie., high resistance), that the GEN light current cannot power the car's electrical system and the electrical systems power off and the engine stops.

If the GEN light is too high power, it will have lower resistance and may be able to power the car's electrical system while dropping just a few volts and lighting gently.

So my guess (assumption: vehicle electrical system wired as per factory manual) is GEN light too high power.

[SirAndy]

So my guess (assumption: vehicle electrical system wired as per factory manual) is GEN light too high power.

Isn't that one supposed to be a special bulb? Maybe he replaced it with something different?
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[ejm]

the alt-charge light comes on when you turn off the key and stays on while the engine winds down -- going out once the alternator slows down below its effective RPM.

Just checked..my '74 does this... pretty sure they all do

[McMark]

So my guess (assumption: vehicle electrical system wired as per factory manual) is GEN light too high power.

Isn't that one supposed to be a special bulb? Maybe he replaced it with something different?
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It's got a 1.2W bulb in there (correct for late cars AFAIK), and earlier I tried this with a 2W bulb (correct for early cars AFAIK) and no difference.

[McMark]

Theory is the GEN light is so low wattage (ie., high resistance), that the GEN light current cannot power the car's electrical system and the electrical systems power off and the engine stops.

If the GEN light is too high power, it will have lower resistance and may be able to power the car's electrical system while dropping just a few volts and lighting gently.

So my guess (assumption: vehicle electrical system wired as per factory manual) is GEN light too high power.

I'm not sure I understand what your mean by 'too high power'.
Tried it with a 1.2W bulb and a 2W bulb and both acted the same.
Can some failure in the alternator cause a larger than normal amount of wattage to be supplied to GEN light wire?

[Dave_Darling]

What would happen if you put a resistor in the circuit in series with the light? I wonder if the bootstrap functionality would be affected?

--DD

[ejm]

3. Once the car was key-off-running, you could also kill it by ..... or by pulling the three prong alternator connector.

Looking at the diagram that means there is a bad diode in the circled array