Hello,
  My alternator went out last week; before it did, the battery gauge went up to 18 volts in the red for about 5 minutes, then down to -9v before I got home. I replaced the alternator with an Autozone rebuilt 80 amp. Since then the battery gauge is showing 13 volts and drops to -9v at idle with the a/c on.
Question: Could I also have a bad voltage regulator and/or could I have a bought a bad alternator?
  I have a good red top Ultima battery in the vette now.
Any thoughts?
Thanks!
Phill's Vette2007-06-01 17:52:48

You may have a short somewhere, and the regulator is trying to keep the voltage up to make up for the loss. After a while, the regulator will "shut down" due to heat...
The fact that it did the same thing with the old alt. as the new, suggests the alt. is not the source of the problem.
Does it only do this when you use the A/C, or does it just do it at random?

Seems to drop more with A/C on but still drops without A/C. I drove it around tonight with the A/C and headlights and it didn't drop below 12v at idle. I hooked up a charger to to for a few hours before driving; the battaer voltage before the charge was 12.4v and 14.9 after charging. Only at idle was it dropping way low?? Driving speed I get 12 - 13.5v
For about 3 months before this, all the lights on and in the car would have a slight flicker; thats now gone with this new alternator.
Does this sound like a short and/or ground issue?
Thanks!

I think I'd check all the connections, as far as grounds from the batt. to the frame, and the frame to the engine first.
While driving, there should be more than just 12v showing, but you really can't trust the dash gauge. Put a v-meter on it, and see what is has that way. With the lights & A/C going, the alt should be putting out minimum 13v while driving. Much less than that, and it's not going to keep the batt. at full charge. At 12-12.5v driving, it's all running off of the battery.
Check those grounds, and the positive cable on the starter for good, solid connections.

I will do this tonight.