Alternator not charging battery

1994 JEEP GRAND CHEROKEE
200,000 MILES • 5.2L • V8 • 4WD • AUTOMATIC
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JAKE KENDRICK
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So i have the dreaded Jeep electric issue, half the time my battery stays charged but my battery gauge goes up and down from about 7 all the way back up max charge and my lights get really bright, radio shuts off until it goes back down but here lately, mainly in the mornings but not always my alternator just stops charging battery slowly drains (my gauges quit right before) then it wont crank up. i have put a new battery starter and alternator on it and i really don't want to 300 for the computer if i don't have to any ideas? PS: shop mechanic says my alternator is charging fine but it has only been tested while hooked up to the car.
Sep 22, 2020 at 2:32 PM
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KASEKENNY
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More then likely this is the voltage regulator which is contained in the PCM.

The way to confirm this is to measure voltage on the dark green wire from the PCM. I suspect the regulator is not operating properly which is why your voltage is erratic.

Unfortunately if it were me, I would go with the PCM at this point.

Take a look at this info and we can go from there. Thanks
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Sep 22, 2020 at 5:49 PM
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JACOBANDNICKOLAS
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Hi,

As I believe you are aware, the alternator voltage is regulated by the powertrain control module (PCM). If you would have told me there were times that voltage dropped and then would return to normal, I would have first suspected a bad alternator or a connection issue. However, the spike in voltage (which can be dangerous) doesn't indicate an alt or connection issue. Instead, it indicates the PCM is not controlling the voltage from the alt at all times.

I hate to say it, but I feel the PCM needs replaced. Spikes in voltage as you mentioned can cause other issues including the potential for something to overheat and catch fire.

If you look at the attached pic, it is the entire charging system wiring schematic. Note where the voltage regulator is located.

Let me know if this helps or if you have other questions.

Take care,
Joe
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Sep 22, 2020 at 5:52 PM
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JAKE KENDRICK
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Oh, that's what i was thinking. i just hoped i was wrong (it's $300.00) do you think wiring an external voltage regulator to the battery would work? That would be a lot cheaper, but i am not sure if that would fix it since the stock voltage regulator is in the PCM.
Sep 22, 2020 at 6:53 PM
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JACOBANDNICKOLAS
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Hi,

Honestly, I wouldn't recommend it on this vehicle. The idea of installing an external regulator is more for much older vehicles.

Have you checked online for better prices? The $300.00 sounds high.

Let me know.
Joe
Sep 22, 2020 at 7:03 PM
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JAKE KENDRICK
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Found one at rockauto.com for $168.00 before the core charge. should be here Wednesday will update after i replace it.
Sep 23, 2020 at 9:53 AM
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KASEKENNY
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Good call with not going with the external regulator. I have seen a number of people try this and it doesn't work out too well.

Thanks for the update. We will wait to hear back.
Sep 23, 2020 at 6:26 PM