Alternator Stops Charging At High Rpm

sicsiksix · August 5, 2009

Well I've been having some charging issues lately on my 2002 gmc 2500hd 6.0L Bit of history, about 15,000km ago I replaced my old alternator with a 130a model, changed the tensioner and belt (both). It had been working fine, although I was noticing my headlamps dimming when I go WOT and the RPMs hit 4000 or above. Now a couple days ago I was on a roadtrip and I pinned it up a hill and my volt gauge dropped and the battery light came on. So I stopped and managed to replace the alternator under warranty with another 130amp Delco rebuild (New tensioner and belt again). Once the new one was in, it seemed fine until I got above about 3000rpm and then it stops charging. Most of the time when I let off the gauge comes back up and everything is fine. So far I've load tested the battery twice, checked all grounds and even added another one directly from Alt. to Neg. battery. Checked to see if the Alt was leaking A/C voltage, tested the alternator (works fine at idle/low rpm, then just drops to zero above 3000rpm), made sure the belt is not slipping. I'm starting to lean towards my PCM maybe...I backprobed the brown wire going into the alternator and it drops to under 1volt when I go high rpm. Not sure what to try next, any ideas?

jf781 · August 7, 2009

Not sure if this would be it but is the regulator built into the alternator or is there another one located somewhere in the system?

2000Sierra6.0L · July 21, 2010

Just wondering if you found the cause of your voltage dropping? And if you or anyone else knows of a fix.

I'm having the same issue i think w/ my 2000 Sierra 6.0. When i get on it hard getting on the highway the battery

light will come on and on occasion the voltage will drop also. The alternator was replaced about 4 years ago.

[email protected] · July 21, 2010

Just wondering if you found the cause of your voltage dropping? And if you or anyone else knows of a fix.
I'm having the same issue i think w/ my 2000 Sierra 6.0. When i get on it hard getting on the highway the battery

light will come on and on occasion the voltage will drop also. The alternator was replaced about 4 years ago.

Since you're bringing back this thread, It's a sign one of the diodes is starting to go. At high RPMs, the regulator can't handle the high voltage so it shuts down until the voltage is manageable. This Ac current that's allow in your system can also cause ECM problems. But, I'd still like to know the OP's fix for his problem.