Ok, I have had this problem for ever... amd I have posted before, but I still haven't figured out this stupid problem!

When I start the car and it is cold, the volt meter is high, 13-14 volts. After the car warms up however, and if I am at a low rpm or idle; it is negative. If I then rev it, it will go back up to positive, either for a second, or it stays there for a while.

This is the second alternator to do this, and it might be bad luck... I don't know; but I think there is another problem that I can't find.

Please help!

 

The 2nd alternator I put in was a junkyard pull with ? miles on it... but I'm still skeptical on both having the same problem. I'm affraid to buy a new one cause I might fry it!

Also, Advance Auto Parts has the Delco 105 amp for $80, is this suitable?

 

My car behaves the same way. Starting the car drops battery voltage due to the heavy load from the starter. The regulator in the alternator raises the charging voltage to charge the battery slowly tapering off as you drive. When the battery is back up the alternator output drops and system voltage drops back down a bit. At idle, the alternator is not turning fast enough to put out full voltage, so the battery supplies some of the power untill you speed up , or the regulator kicks in and raises voltage. I see this effect at night especially when lights, heater etc are on. I think the problem is primarily in the speed of the switching circuit at which the regulator in these alternators turns on. Been thru 3 alternators on these cars over the years and they all acted the same.

A stock alternator is rated at 105 amps. Make sure that is what you got from the yard. Rating is stamped somewhere on the housing.

Check the battery voltage with a separate voltmeter with the car running and compare it to your dash gauge. Maybe the dash gauge is off a bit.

If your are running a 2000 watt sound system, the stock alternator isn't gonna cut it. 2000 watts divided by 13 volts is 153.85 amps. 105A-154A = -49 amps. Doesn't leave much for the fuel pump, injectors, etc. You probably need a 200 amp high output alternator..

 

Hmm, all good advise. I'll have to look into that alternator rating, thanks!