Alternator Circuit Weirdness ... Options

[SirAndy]

This one has me stumped (sp?) ...

3.6L '95 993 Engine with stock 993 alternator. stock 993 gauge with stock 993 alternator warning light.

when i start the engine, the alternator light stays on until i spin the engine above 2500 rpm. (IMG:style_emoticons/default/WTF.gif)

if i just let it idle, the alternator light will never turn off. i measured the current on the battery and confirmed that the alternator is *not* charging when the light in the gauge is on.

now, here is where it get's really strange. once i rev the engine past 2500 rpm, the light goes off and *stays* off. (IMG:style_emoticons/default/screwy.gif)

the alternator charges just fine after that, even when the engine dropps back to idle.
it seems like the alternator needs a initial "jolt" to start working properly.

anybody ever seen anything like this?
(IMG:style_emoticons/default/idea.gif) Andy

[Spoke]

First question: is this new behaviour or has it been like this all the time?

The alternator circuit for the 914 is a bootstrap circuit. In other words, the alternator powers itself. The alternator provides the voltage to the armature (spinning part of the alternator) to provide magnetize the armature so when it spins, the alternator produces voltage. No voltage to the armature means no voltage output of the alternator.

Without an outside helper to get some voltage to the armature, the alternator will not turn on and produce voltage. That helper is the idiot light.

Not sure about the 911 setup so this example pertains to at least the 914.

There are 2 sets of diodes in the alternator. One set powers the car and one set powers just the voltage regulator and the armature. If the armature is not powered then it is not magnetized and no voltage is developed by the alternator.

When first started, the armature has no power and the only power to it and the VR is through the dashboard idiot light. The current through the idiot light provides just enough current to get some magnetization going in the armature thus it produces some voltage. Not quite 12V but some voltage and the idiot light stays on.

Rev the engine and the alternator spins faster and the little magnetization in the armature produces more voltage until the output is 12V or higher. At this point, the second set of diodes provides enough voltage for the VR and the armature to fully turn on and the idiot light goes out and stays out.

This bootstrapping should happen at a real low RPM so you don't have to rev the engine to get the circuit going. Maybe one of your diodes is bad?

I've seen in my 71 914 that if the car is started and the engine runs real slow (like 200 RPM), the idiot light stays on. Blip the throttle and the light goes out and stays out.

Spoke