Battery light on

2002 GMC ENVOY
178,000 MILES • 4.2L • 6 CYL • 2WD • AUTOMATIC
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JOSH USHER
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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.
Mar 20, 2019 at 6:09 PM
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ASEMASTER6371
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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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Mar 20, 2019 at 6:44 PM
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JOSH USHER
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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.
Mar 20, 2019 at 7:49 PM
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ASEMASTER6371
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Do you have battery voltage at the wire from the battery?

Roy
Mar 20, 2019 at 7:54 PM
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JOSH USHER
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The fusible link? Showing 12.3. Battery is partially drained at 12.3 right now.
Mar 20, 2019 at 8:00 PM
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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
Mar 20, 2019 at 8:08 PM
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JOSH USHER
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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.
Mar 20, 2019 at 8:11 PM
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ASEMASTER6371
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You are welcome.

Always glad to help.

Roy
Mar 20, 2019 at 8:14 PM
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JOSH USHER
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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.
Mar 21, 2019 at 8:18 AM
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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
Mar 21, 2019 at 9:38 AM
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JOSH USHER
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No, I grounded the grey field wire.
Mar 21, 2019 at 9:56 AM
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JOSH USHER
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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.
Mar 21, 2019 at 10:18 AM
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ASEMASTER6371
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Sounds good.

Roy
Mar 21, 2019 at 10:58 AM
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JOSH USHER
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Swapped out the re-manufactured for a new one and all works well.
Mar 21, 2019 at 12:22 PM
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ASEMASTER6371
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Good deal.

Roy
Mar 21, 2019 at 1:27 PM