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Discussion Starter · #1 ·
I have a 1998 Jeep Grand Cherokee with the 5.2L V8 and the other day I was sitting in it (it was running) then it randomly started beeping at me, I looked at the dash and it said check gauges, then I looked at the battery gauge and it was down all the way. I turned the Jeep off and on and then it was fine. it's done this a few more times, once while driving and the rest while either idling or going slow and after a few seconds it resets and it's fine again. The alternator has been replaced earlier this year and the battery isn't old enough to be going bad. Could the belt be slipping? Thanks.
 

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Sounds like a bad voltage regulator to me. Those are the exact symptoms I had when it died in my 5.9 a few years ago. It is built into the PCM, so your best option is to get an external voltage regulator kit.

That being said, diagnose first. A year old rebuilt alternator can be bad, and a not-so-old battery can die. Also check the wiring.
 
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I learned about the above mentioned bad new battery the hard way. I looked for a voltage drain on my wifeys car, ignoring the newish battery for half a day while looking for a cause. Then finally I charged up the battery with the battery disconnected. After a full charge I could watch the volts drop on my meter. The battery was warranteed and replaced and no more drain.
 

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Discussion Starter · #4 ·
Sounds like a bad voltage regulator to me. Those are the exact symptoms I had when it died in my 5.9 a few years ago. It is built into the PCM, so your best option is to get an external voltage regulator kit.

That being said, diagnose first. A year old rebuilt alternator can be bad, and a not-so-old battery can die. Also check the wiring.
I was able to look while it was doing it and the belt isn't slipping. It also doesn't have any trouble starting or anything like that. I'll look into the regulator next
 

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When the PCM cannot control the alternator as it see's fit, for some reason it drops the voltage gauge like that.... Most likely either another bad alternator or a bad regulator in the PCM.
 

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Discussion Starter · #6 ·
When the PCM cannot control the alternator as it see's fit, for some reason it drops the voltage gauge like that.... Most likely either another bad alternator or a bad regulator in the PCM.
I'll try finding one of those voltage readers later to see if it is the regulator or not. Is there any way to fix the one in the PCM so I don't have to go through all of the wiring to get a new one installed?
 

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There is certain procedure on how the voltage regulator is measured and checked. It's more than just measuring the alternator output voltage.

As for repairing the regulator inside the PCM, you're looking at buying another PCM if you don't want a standalone regulator. W7ring a standalone is very simple.
 

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This guy will tell you how to test the regulator.

 

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Powertrain diagnostics manual has the proper proceduce on how to measure/ diagnose the voltage regulator operation. I can take screenshots & link them here when I get home.

My manual is for a TJ/XJ, but the procedure is the same in a ZJ as well.
 

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Discussion Starter · #10 ·
Powertrain diagnostics manual has the proper proceduce on how to measure/ diagnose the voltage regulator operation. I can take screenshots & link them here when I get home.

My manual is for a TJ/XJ, but the procedure is the same in a ZJ as well.
That would be great, I'm in no rush
 
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