I was driving my car up a steep hill with the AC blasting and my stereo system ( a couple of amps etc.) turned way up and suddenly the volt gauge drops down and now my alternator makes a high pitched whine. When I put it in gear the gauge drops pretty low and goes up or down with the RPM's. What makes the alternator whine like that? Bad bearings? Diodes?

 

If I had to guess, I'd say the noise would be the belt slipping due to the heavy load the alternator is presenting to the engine.

The clues you gave were that it was under a good load (stereo w/amplifiers) and the voltage suddenly dropped.

If it was bad bearings, the voltage (output) would not vary with rotor speed. [EDIT: Unless the bearings were so bad that it causes the alternator to slow down - but then the noise would always be present. It's one of those thing's that you'll know (belt squeal vs. bearing noise) once you hear it.]

Reducing the load (stereo) would/should reduce the load enough that the belt would not slip. No slip => no noise.

Mine does the same - heavy load (three amplifiers) with A/C and <1100rpm. I don't know why the A/C would affect this. There's not that much added electrical demand (only the compressor clutch and blower fan) that would load down the alternator so much so as to cause it to slip.

Now that I think about it, I wonder if the added load of the A/C compressor is causing the belt to slip on the crankshaft pulley. This in turn would cause the alternator to slow down. If it slow down enough (especially at/near idle), the voltage will drop. The serpentine belt will now have two significant loads to overcome - the alternator AND the A/C compressor - both driven from the crank pulley. I think that this makes more sense as to the origin of the "mad pig" squealing.

I question whether a new serpentine belt is the answer.

[This message has been edited by Stuart Moss (edited August 05, 2001).]