Maybe someone can help me. I have a 98 C5 the voltmeter under load is staying at 12.5 with the fans on. I live in Georgia I think the summer heat is killing the car. I have drain and flushed the coolant it stays around 200 while driving and idling so no issue there. The problem is even at idle the car is showing 12.5 on the volt meter. I have a brand new red top battery and I have replaced the alternator with the OEM replacement and belt and belt tensioner. What could be causing this?

 

Test the output directly at the alternator stud. If it reads the same, it sounds like the alternator is toast. And I wont touch replacement of the original alternator. Lot's of opinions about that on here.

 

AGREE! Rebuild the OEM one that you have.. BUT,, Here is where I would start.

Engine OFF, Test the BATT Terminal on the back of the Alternator to chassis ground, Should be at FULL BATTERY TERNINAL VOLTAGE.

Test with engine running! The BATT Terminal on the back of the Alternator to chassis ground should be at full charging voltage. 13.5 - 14.7 VDC

The battery cable from the battery POSITIVE terminal goes down to the starter solenoid and back up to that Alternator BATT terminal on the back of the alternator.
The starter solenoid connection can be corroded/burnt/loose and cause this issue.

If you use A DC multimeter and put the positive lead on the alternator BATT Terminal and the Negative lead on the BATTERY POSITIVE terminal, theoretically, you should have ZERO VOLTS DC.

If you see voltage, the circuit between the battery and alternator is compromised. The more voltage you see the worse it is.

There is another wire on the solenoid main terminal that also goes back up to the alternator regulator.
Its a small red wire and it is in that small plug on the side of the alt case. That is the voltage feed back wire for the regulator. Its on PIN "D" of that plug.

 

As Rob said check that alternator output...if voltage output good at the alternator it’s time to do a voltage drop check on the positive and negative side of the charging circuit !!

 

It seems like it is getting heat soaked. I checked it from the alternator it was between 12.3 and 13.3 without a load. The battery is good. I was thinking of going to higher amperage like a 140 amp.

 

The only reason I say this is while I am driving and the air is flowing through and around the compartment the voltage goes up to 13 and higher. Its when. The car is sitting and the heat is building up.

 

So this being said without changing all the wiring what would be the highest amp alternator I could go with, and not have any issues?

 

Don't blindly replace your alternator: there are plenty of horror stories!... In any case, DON'T turn in your old alternator as core.

Take a look at this post for an in-depth analysis of the C5 alternator.

What makes this alternator so peculiar is that it is excited when its regulator receives a turn on signal of 10v at limited current provided by the PCM through the L wire. Not all regulators accept the turn on signal provided by the C5 PCM.

Another distinctive attribute described by Lionelhutz in this post:

For a potential rebuilt route, take a look at this post. It has info on the right regulators for both, 97-01 and 02-04. The price is around $30.

This other post has detailed info on the rebuild procedure and links to get the other parts.

 

As already mentioned above by Robrote, Bill Curlee and C5 Diag, do some troubleshooting first. Definitely, a voltage drop check on both, positive and negative paths, of the charging circuit.

If problems are found, check, clean with a wire brush and tight connections at the starter solenoid and battery ground point and retest.

 

You could replace it with a 300 AMP alternator. If the charging circuit is compromised, it will do the same damn thing. LOL!
TROUBLESHOOT and fix what's wrong.

 

I checked all the grounds and the starter solenoid. Everything was tight and good to go. The problem was the reman alternator. Thanks for all the tech tips I will keep you posted as to what I decide. I might do the 140 amp upgrade. It's going to cost about the same as the AC Delco alternator. I think the cheapest i found one is around 350 before tax and everything else.