Overcharging - the voltage will read 15+ volts?

Tested the alternator tested fine exchanged anyway still same thing got new battery same thing. But once in a while like today it went up to 15+ volts then dropped to 12 and would not come back up. I put truck in neutral while driving turned truck off and restarted it went to 15+ then two minutes later dropped to 12 and stayed. Lights flicker even when volts are normal now truck at idle in park the rpm is going up and down from 600 to 700.

Good advice, it helped my problem :)

Ok, so this is driving me insane.

I have a 03 1500 sierra with a 5.3, I am having the weirdest charging problem, when coming to a stop my alternator drops to around 9-10 volts and it'll usually stall if I don't throw it in neutral and bring the rpms up, as long as I brake in neutral it's fine. The battery seems to be charging well and it always restarts, i've checked all the connections and they are clean and tight. This is the clincher though, I put a new alternator on yesterday assuming that the reg was gone, and it's the same thing, it's almost like the truck is running directly off the alternator, it gets warmer than it should, but the battery seems to be charging, it starts well and will charge at 14.4 at idle, it works fine until it gets warm as well. Its strange. Ive tried looking this up on the net and nothing, i'm going to use the load tester at work to see if the battery is pooched, any advise would be appreciated.

I never thought of that, if I go out and start it right now cold, it charges like it should and running down the road its fine, I put it in gear earlier and turned the lights on and my kc driving lights which draw a lot of power and it dropped right off, i'm just assuming that the battery if it's good should power everything better than that though, i'm wondering if it's holding a false charge, i'll know better tomorrow after I load test it, or take it to the dreaded dealership and have them scan it, thanks for the reply

Also note those years had some issues with faulty battery cables. Maybe check the cables at the battery. It may look fine and clean but if you skive back the insulation four to six inches you may find a lot of corrosion as well.

Hello, I have a 2007 GMAC Sierra 1500. While driving the battery light came on and the dash displayed 'charging system failure'. The battery was tested and it tested good. Charging system tested while vehicle was running and it failed, no charge. Alternator was removed and bench tested and it passed all test. Reinstalled alternator was reinstalled, checked all terminals and wire connections to make sure they were secure. Dash continues to display 'charging system failure'. The dash volt meter shows that it is charging at 12 volts.

Check belt make sure its not slipping

I have an obd 2 scan tool that I link to my phone and it reads the same as the dash.
I have googled and cant find any way to test the alternator field generation circuit, how do you test that yourself? And where is it located? And how to fix it?