Over charging alternator when RPMs are around 3,000 RPMs?

1997 JEEP CHEROKEE
400,000 MILES • 6 CYL • 4WD • AUTOMATIC
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SCXJ
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  • 9 POSTS
The charge voltage increases to around 18vdc when the RPMs 3,000. It does not stay at 18v for long, usually drops back to around 14 when RPM is less than 3,000. I verified this 18vdc at the battery terminals. The voltage gauge also shows this high voltage condition.

One more thing is after this over voltage occurs, the transmission won't shift automatically until I turn it off and on again.

Thank you
Apr 30, 2023 at 4:41 AM
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BRENDON S
  • ✓ASE Certified Mechanic
  • 653 POSTS
  • ASE Certified Mechanic
Good morning, SCXJ,

It is possible the voltage regulator built into the alternator is bad. This part isn't really replaceable so the whole alternator will need to be replaced.

Here is an article with some other tests and a lot of useful information:
https://www.2carpros.com/articles/how-to-check-a-car-alternator

Thank you,
Brendon
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Apr 30, 2023 at 5:34 AM
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SCXJ
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Brendon,

Thanks for the links and sharing your opinion on the issue. I'll replace the alternator and post the results.
Apr 30, 2023 at 9:41 AM
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BRENDON S
  • ✓ASE Certified Mechanic
  • 653 POSTS
  • ASE Certified Mechanic
Hello SCXJ,

You're very welcome. Yes, please let me know how it goes. I will be here.

Thank you,
Brendon
Apr 30, 2023 at 10:16 AM
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SCXJ
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Brendon, I just got around to replacing my alternator and you were right, all is well, it's charging my battery at 14v.. Thanks for the advice. This is a great service!
SCXJ
Jun 4, 2023 at 8:24 PM
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BRENDON S
  • ✓ASE Certified Mechanic
  • 653 POSTS
  • ASE Certified Mechanic
Good morning, SCXJ,

Awesome! I am glad you got it all fixed up and you are welcome, I'm happy to help.

Have a great week,
Brendon
Jun 5, 2023 at 12:46 AM
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