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.

[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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