2002 GMC Sierra Alternator Voltage Regulator Problems

Kirkathon25 · November 7, 2012

I have a 2002 GMC Sierra 6.0L 2500HD that i am trying to sell. The alternator went on it last week so i replaced it with a reman that turned out to be bad, it had low voltage coming out of it when i tested it (3.5-4V). They replaced that one with the same thing and tested it first to make sure it was good before handing it over. I installed that one and am having the same problem. The truck runs until the battery dies with no codes with the alternator averaging 3.5V of output according to my voltmeter. I am getting relatively pissed about this since i have replaced many an alternator and have never had so much trouble. Any help would be appreciated, cant find any threads that are specific to this problem.

scooby5121 · November 7, 2012

Has to have approx. 14.5Volts average output.

Scooby

Kirkathon25 · November 7, 2012

Yeah i know, i dont know what could possibly be the problem, all wires are properly hooked up and not corroded.

Kirkathon25 · November 8, 2012

Does anyone know how to test the voltage on the plug for the alternator excitor? i think that the problem might be with the wiring for the regulator since i just put an alternator out of another truck that i absolutely know for sure works flawlessly and got the same problem. The plug itself has two wires running into it a brown and white.

GMmytruck · November 8, 2012

Had this problem in an old ford. :throwup:Took it in 3 times with the same problem. 3 alternators. I told them it can't be the alternator. They said their big 'magic' tester didn't lie. I finally ended up taking it to a small repair shop and the mechanic had a small hand held tester. He checked it for a minute or two and ended up pulling hard on one of the wires in the wiring harness and lo and behold it was broken. He fixed the wire and no problems after. Hope this helps a little.

Kirkathon25 · November 8, 2012

The wire seems tight in the plug however i just tested the voltage coming out of the two wires on the plug and there is voltage coming out of the brown wire but not the white, what i am unsure of is if this plug is ac or dc power? if the plug regulates voltage then i believe it is ac.

Kirkathon25 · November 19, 2012

Ok, its been weeks but i have found something out that might help, but i have no idea what it means. If i connect the alternator directly to the positive terminal it works great. The contacts inside that red box are all clean and the original jump wire from the alternator has continuity so i have no idea why it wont work hooked up that way. The only way the alternator charges properly is when my wire connects the positive terminal of the battery to the alternator directly.

citydriver · November 21, 2012

Check for voltage drops across the cable connections, connect the gound probe of a multimeter on the negative battery terminal, then check the voltage of the battery, then work your way down the system to the alternator. The voltage should be the same across the system. You can do this with the ignition ON and the engine OFF( not running ) Check all cable connections and fuses.

Jason Hall · February 8, 2016

Ok, its been weeks but i have found something out that might help, but i have no idea what it means. If i connect the alternator directly to the positive terminal it works great. The contacts inside that red box are all clean and the original jump wire from the alternator has continuity so i have no idea why it wont work hooked up that way. The only way the alternator charges properly is when my wire connects the positive terminal of the battery to the alternator directly.

Did you ever get a solution to the problem?

I've got very similar issue. I've replaced battery and alternator and my voltage gauge reads 14-15 volts and then randomly drops to around 12 which will not charge the battery and run the vehicle. The truck has always started seemingly with the same amount of cranks but it cranks slow for the first 1-2 revolutions. Now it has not cranked at all for me once. I don't want to be left stranded in the future!!!

davester · February 8, 2016

Sounds like the actually cabling is bad, either the wire within the cable or between the cable ends and the connectors. I'd suggest replacing the wiring from the alternator to the battery and then try again.

There's a reasonable chance the battery is still fine, just not fully charged because of the iffy connection.