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Battery light on

Tiny
JOSH USHER
  • MEMBER
  • 2002 GMC ENVOY
  • 4.2L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 178,000 MILES
Battery is not being charged. Changed the alternator, connector plug that goes to the alternator. Checked continuity between PCM connector and the alternator plug, both tested good. Checked continuity between the bolt on the back of the alternator and the battery post, tested good. Not sure what to do here. Any help would be greatly appreciated.
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Wednesday, March 20th, 2019 AT 6:09 PM

14 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good evening,

I attached a diagram for you to view.

Did you verify battery voltage to the red lead at the alternator?

Beyond that, you will need a scan tool to see if there is a duty cycle being sent to the alternator from the PCM. The PCM controls the voltage regulator turning it on and off according to load.

Roy
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Wednesday, March 20th, 2019 AT 6:44 PM
Tiny
JOSH USHER
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  • 8 POSTS
With the key in the on position I am reading 5 volts going to the alternator at the red wire. I have a scan tool but it’s not sophisticated enough to get that far into it I don’t believe. The last thing on my list is the PCM but I don’t want to just order one, still hoping It can be resolved without having to buy one.
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Wednesday, March 20th, 2019 AT 7:49 PM
Tiny
ASEMASTER6371
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Do you have battery voltage at the wire from the battery?

Roy
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Wednesday, March 20th, 2019 AT 7:54 PM
Tiny
JOSH USHER
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  • 8 POSTS
The fusible link? Showing 12.3. Battery is partially drained at 12.3 right now.
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Wednesday, March 20th, 2019 AT 8:00 PM
Tiny
ASEMASTER6371
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Okay, at this point you need a scan tool to see if the PCM is turning on the voltage regulator or not. It looks like the PCM but needs to be confirmed.

Roy
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Wednesday, March 20th, 2019 AT 8:08 PM
Tiny
JOSH USHER
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  • 8 POSTS
That’s what I was thinking too. Wasn’t sure if anyone knew a trick I didn’t know, haha. Thanks for the help Roy.
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Wednesday, March 20th, 2019 AT 8:11 PM
Tiny
ASEMASTER6371
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You are welcome.

Always glad to help.

Roy
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Wednesday, March 20th, 2019 AT 8:14 PM
Tiny
JOSH USHER
  • MEMBER
  • 8 POSTS
Is 5 volts sufficient to power on the alternator? I connected a temporary ground to see if it would come on but still wouldn’t.
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Thursday, March 21st, 2019 AT 8:18 AM
Tiny
ASEMASTER6371
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You need battery voltage to the heavy red wire.

The voltages from the PCM are pulse width modulated signals. That's why you need a scan tool to see those signals.

Did you ground the 5 volt wire? If you did, good chance you damaged the PCM.

Roy
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Thursday, March 21st, 2019 AT 9:38 AM
Tiny
JOSH USHER
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No, I grounded the grey field wire.
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Thursday, March 21st, 2019 AT 9:56 AM
Tiny
JOSH USHER
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  • 8 POSTS
I am taking the re-manufactured alternator back to the auto parts store. Made them test it three times before I left but going to test it again. Will post the results.
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Thursday, March 21st, 2019 AT 10:18 AM
Tiny
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Sounds good.

Roy
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Thursday, March 21st, 2019 AT 10:58 AM
Tiny
JOSH USHER
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  • 8 POSTS
Swapped out the re-manufactured for a new one and all works well.
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Thursday, March 21st, 2019 AT 12:22 PM
Tiny
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Good deal.

Roy
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Thursday, March 21st, 2019 AT 1:27 PM

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