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Discussion Starter · #1 ·
I have an '02 GMC Sierra 2500HD with the 6.0L

I was on a long road trip. Out in the middle of no where my battery light started intermittently going on and off. I immediately checked the volt meter. It was normal at 14v. I keep driving. They light eventually stopped coming on. About 150 miles later the light started doing it again, this time the volt meter had dropped about 2-3 volts. I was still about 120 miles from home so I turned off my stereo, my AC, and didnt even use cruise control. I was determined to make it home. I made it home but before I stopped at Schucks and had them run there charging system test. All they could tell me was that I had 11v at my battery and it needed to be charged. I made it home. I pulled out the alternator and had it bench checked. They guy at Napa said that it was bad so I replaced it.

After I replaced it I thought that would fix the problem but it didnt. I just got the new aternator installed and went for a short drive. The battery light is still intermittent. The volt meter in the truck reads 14. When I hook up the multimeter it reads 13.8 with no accessories running. When I turn on everything, AC on max, Stereo and Lights on bright, it drops down to 13.1 but never drops lower. So the alternator is working but what could cause the battery light to be intermittent. When I say intermettent I mean that it comes on for 3 seconds, goes out for 5, comes on for 2 seconds, going our for 3. Its always coming on but never in a constant pattern.

The battery was diconnected from my truck form about 3 days so I would assume that would of cleared any codes that was in the system.


Any suggestions? Should I not drive it? I'm pretty stumped
 

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Hmmm have your battery load tested if you dont have a load tester. There is a chance that it has a bad cell.... Otherwise you really have me.. I know that i had a pin push out on my 06 Silverado that caused the same thing. So I would start there if your battery is good to go.
 

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Discussion Starter · #4 ·
Hmmm have your battery load tested if you dont have a load tester. There is a chance that it has a bad cell.... Otherwise you really have me.. I know that i had a pin push out on my 06 Silverado that caused the same thing. So I would start there if your battery is good to go.
What do you mean by had a pin push out?

I havent had my battery tested yet. I'll have to do that tomorrow. I am home with the kids now. Thanks for the suggestions. I'll update when I get the battery tested.
 

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This was a dealer repair at 400 miles.. anyhow supposedly the wiring harness that pluggs into the Alternator had a pin that had pulled/pushed out so there was no contact on the plug.
 

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Discussion Starter · #7 ·
Brought it in today to have the charging system checked again. They say that the Battery and Alternator are both good. They say the only thing that was off was when I went to start my truck my battery was at 10v. It didnt seem hard to start or anything. Drove it most of the day. Light is still intermittently on and off. Mosty on. Volt meter never dropped at all. Didnt get to have the codes pulled if there are any, I guess someone borrowed Schucks reader and neglected to return it. Got a friend that is going to check to see if any are stored tomorrow.
 

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Discussion Starter · #8 ·
Is everyone pretty much as stumped as me on this one? I have driven it since I replaced my alternator. I havent noticed anything abnormal except the battery light keeps coming on.
 

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YEAH i sure am.. Sorry man. Did you check the wiring harness etc? As i mentioned, that really is the only place i can think unless you have a short in the cluster.. Which is possible. Otherwise everything works like it should sooooo...
 

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Discussion Starter · #10 ·
Buddy just pulled some codes, I had 3.

P1638
C0222
C0281
 

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Go through and check your wires.

I had a wierd Alternator/Battery issue a few weeks ago.....
Replaced the Alternator, Battery was fine and strong, but still the problem persisted.

I played with the wires (seperating the Alternator/Charging system wires from the rest of the wires) and then pulled them away from the engine block/TBI.....and that corrected my problem.
So I am guessing I still have either a weak or broken wire in the system somewhere, but for now that aspect of the truck is operating as it should be.
 

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I did a bit of looking into this, and according to GM, the P1638 sets if the PCM is not getting the correct signal for the alternator load, and they suggest checking the connections at the alternator and at the PCM. The conditions for this code to set are that the truck is off with key in 'on' position, and the PCM is getting a signal of 5% load or greater, or that the truck is running and the PCM is getting a signal of less than 5% alternator load (either one for a time of 6 seconds or more).

The P1638 is the code setting off your charge light, and for your sake I really do hope it is a bad connection somewhere, as this is GM's diagnostic procedure:
1) check alternator (new alternator should rule this out)
2) check for bad connections (let's hope)
3) replace PCM (could get pretty costly on an 02 out of warranty)


I would guess the C0281 is probably unrelated, as it is for a short in your brake light switch on your brake pedal. Again, check the connections, make sure your brake lights work, clear the code. It shouldn't come back unless your ABS kicks in while there is a short in the switch.

C0222 also goes to your ABS system - the right front wheel speed sensor is not sending a signal. These sensors fail fairly commonly on these trucks, and isnt too difficult of a fix.

I hope this helps, and I wish you the best of luck in getting this issue sorted out.
 

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Discussion Starter · #14 · (Edited)
I have checked the connection at the alternator. Where exactly is the connection to the PCM?

It would be just my luck to have to replace the PCM. Is that something I can get at a salvage yard?
 

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I know that this is a long shot that anyone is still on this forum but did you ever get the issue figured out, was it the pcm? I am having the same issue with my 2002 tahoe. Any help would be greatly appreciated.
 

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I wasn't around in 2010; if I had been, I would have suggested changing the negative battery cable.
Like your phone, the truck is using a smart charging system, it should fast charge to 80 percent and then slow charge to 100 percent.
There is a load monitor built into the neg cable, I suspect the monitor may be failing.
 
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