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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 10 Jan 2001 16:48:08 Share Post by Email
In Reply to: I know this sounds stupid, jeffla, Wed, 10 Jan 2001 14:11:17
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Whacking the alternator with something heavy will help if the brushes hang up, but that isn't the reason for needing a few thousand RPM.

The alternator needs a magnetic field to create current/voltage. When you first turn on the car, current flows out of the battery, through the battery light, and into the field coils in the alternator, creating a weak magnetic field. Since current flows, the battery lights glows. When the alternator starts spinning, it creates its own voltage. This voltage increases with increasing RPM. At some point the output of the alternator is enough that the voltage regulator starts supplying the current to the field coils, and the current through the battery light stops - and the light goes out. The voltage regulator 'regulates' the output of the alternator by deciding how much current flows throught the field coils.

If the brushes are tired and/or the commutator dirty, it's going to take more RPM for the alternator to 'bootstrap' up and take over from the battery. That's why it takes 3000 RPM on Terry's car for the light to go out. Replace the brushes, and if the commutator's in good shape, idle speed will be high enough to click over to the voltage regulator.

I've seen some alternators that just want a little blip to 1500-2000 rpm to turn off the battery light. If the alternator is otherwise providing good output, it's simple enough to do.

hope this helps.
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