Check the length of the brushes on the voltage.... (archive)

...regulator. Disconnect the battery. Remove two screws attaching the regulator to the back of the alternator. If either brush extends no more than 5mm from the holder, the regulator needs to be replaced. New brushes extend 12mm. Bosch # 30 062 ($25) seems to work just as well as #30 097 ($70). I bought # 062 from BMP when I owned a 1984 318i (BMP recommends it for almost everything). It`s been on my M3 for 44K miles. Nobody`s ever been able to explain to me why a $70 Bosch regulator is better than a $25 Bosch regulator.
The issue with having to rev the engine to get the regulator to switch over and start charging probably has to do with the larger M3 pulley. The larger pulley keeps the alternator from spinning too fast at 7000+ RPM. Keeps the alternator from exploding.
John
88 ///M3 Cinnabar
But why does the battery take a charge after the lights go out? Or, why does the alternator start charging after the light go off?
It is my understanding that the car does not charge until the RPM hit a certain minimum at any given time. If you are idling at a traffic light I thought the alternator is not charging at that time.
I am going to hit the garage and run some tests to see what I find.
Eric
That's not true. The alternator will charge your car at idle but after startup you have to press the accelerator before it starts charging. This is true with the M3 only. All E30's from 86-on have this resistor in parallel also, which allows the alternator to charge even if the bulb is burned out. I think this is unrelated to what we are talking about since our issue is unique to the M3.

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