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Old January 28th, 2013, 12:09 PM   #1
Flexin' XJ
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03 Sierra 5.3 Not Charging

My headlights have been dimming for a while when at a low RPM. Figured the alternator was on its way out but was always charging. I replaced the battery in September of 2011.

Sunday on my way into work my "Battery Not Charging" message was being displayed on the DIC. Figured the alternator finally crapped out. When I left work, it seemed fine. No message. I left for work this morning and as soon as I got on the road the headlights went way dim and the battery light came on and the message appeared again. The gauge showed between 9 and 14volts. I had to keep putting the truck in neutral and rev it to get the battery voltage back up for a short while.

Got to work and ordered an alternator. Put it in and same problem. Checked battery connections again and found the negative terminal was stripped and kinda loose. Got the parts store to warranty it and I have a new battery. Still not charging.

If I test the alternator with a DVOM, it has about 11.9v output at the battery. The same if I touch the output terminal at the alternator and the alternator case.

Seems like the computer isn't telling the alternator to charge. Any ideas? Bad PCM? Anybody have a wiring schematic for the charging system? I can't view photobucket at work.

Thanks.

Notes: Truck not charging. New alternator and battery today. Fusible link good. 11.9v at alternator. PCM not telling it to charge? Have battery power at DLC.

Last edited by Flexin' XJ; January 28th, 2013 at 01:03 PM.
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Old January 29th, 2013, 09:17 AM   #2
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Repalced alternator connector thinking I have a broken wire at the connector. Worked for a minute then the voltage dropped in small incriments until it was about 11.5v.

Tested brown wire at the connector
Disconnected - KOEO - 9.88v
Backprobed - KOEO - 1.3
Backprobed - KOER - 1.3 (no switching)

I have read somewhere GM uses a 5v signal to turn on the alternator. Why am I reading 9.8?

I am really stuck. I don't have access to GM diagnostic info anymore.

My other option is maybe the new alternator is defective. But without proper diagnostic processes I don't want to keep throwing stuff at it.
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Old January 29th, 2013, 09:37 AM   #3
87mopar
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Pcm Issue? It's rare but I have seen it. IIRC the voltage is controlled by the pcm
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Old January 29th, 2013, 09:41 AM   #4
hawkscj5
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with those values i would venture to say the alt. is defective.
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Old January 29th, 2013, 10:08 AM   #5
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PLIS wiring plug on the alt? jumper 12v to the S terminal and see if your lights perk up. If not, I'd suggest checking grounds and having your battery/alt tested.
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Old January 29th, 2013, 10:58 AM   #6
Flexin' XJ
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Quote:
Originally Posted by 87mopar View Post
Pcm Issue? It's rare but I have seen it. IIRC the voltage is controlled by the pcm
That is the worst case for me since I have already spent the money on an alternator. PCMs are rare so it wasn't my first thought thta it would be bad.

Quote:
Originally Posted by hawkscj5 View Post
with those values i would venture to say the alt. is defective.
I just got another alternator from the parts store and am about to install it so we'll see.

Quote:
Originally Posted by deerebowtie View Post
PLIS wiring plug on the alt? jumper 12v to the S terminal and see if your lights perk up. If not, I'd suggest checking grounds and having your battery/alt tested.
Already checked the grounds, cables and wiring. Battery is brand new yesterday as is the alternator.
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Old January 29th, 2013, 11:49 AM   #7
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Well I installed the replacement alternator and it seems to be partially working now. If I am driving it is up around 14v on the gauge. But if I am stopped it drops down, but not as far as it has been. I am chalking it up to the truck basically running off the battery since yesterday and it was low. Hopefully this fixed it.
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Old January 29th, 2013, 11:52 AM   #8
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It may be a long shot, but my 99 tahoe was giving me some charging issues a year ago. I replaced the battery and alternator with no success. I checked the grounds and cleaned the conections, but still a no go. I actually decided to run new ground cables just because. When I took the main ground off, I cut back the protective covering on the wire and the thing was corroded beyond anything I have ever seen! It has not giving me problems since. Might be worth a shot?
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