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sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 22803
Experience:  Chrysler/Dodge/Jeep Master Certified, ASE Master and Advanced Certified, Trans and Hybrid Specialist
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2002 jeep Liberty: new battery..light comes..alternator is pushing 14

Resolved Question:

I have a 2002 jeep Liberty with a battery indicator light on. I installed a new battery a week ago and the light comes on intermittently. When the light is off the alternator is pushing 14 v. When light comes on it pushes 12v. I understand that regulator is controlled by pcm and the alternator has tested ok. My mechanic installed a new (used) pcm and the car, as usual, drove for about 30 minutes and the light came on again. And what happens next is car will go so far and die. I assume not enough power to run the vehicle and everything shuts down. However, I quick charge will get it back on the road but the indicator light will come back on and the process starts all over again. Help? I am tempted to just change out the alternator evevn though it tests ok.
Submitted: 5 years ago.
Category: Jeep
Expert:  sprinkles08 replied 5 years ago.

Hello and welcome to JustAnswer!


I'm not sure how the alternator was tested, but the only time it can be reliably tested is in the vehicle with the engine running and the battery light is on. If it was tested off the vehicle that's never a usable test for an intermittent problem, and if it was tested when the battery light was off then obviously it would have been charging fine.


The problem is either the alternator or it's wiring, it's quite easy to test the system at the time it's acting up. Nearly all charging problems such as yours are simply caused by a failed alternator. If you have a test light there is a terribly simple test you can do at the time it's not charging. Unplug the two wire field connector on the back of the alternator and insert a paperclip into one of the terminals. Attach the alligator clip to the paperclip, and then touch the second terminal in the connector with the probe end of the test light. If the test light lights up but the alternator isn't charging then it's bad and needs to be replaced.

Customer: replied 5 years ago.
The alternator has been tested correctly by myself and mechanic in default mode and when indicator is off. I personally drove this car for a couple of hours last weeken and i couldnt get the warning light to come on. I has been very inconsistent hence the installation of pcm.
Expert:  sprinkles08 replied 5 years ago.
If it was tested when the battery light was off then it won't tell you anything. The only time you can test it is when it's not charging. If the light is off then it's charging fine and there's nothing to test. Test it while it's not charging and I'm quite sure you'll find that the inputs are good and the alternator is the problem.
Customer: replied 5 years ago.
Im sure the mechanic tested alternator correctly but would a faulty alternator act so inconsistently?
Expert:  sprinkles08 replied 5 years ago.

Absolutely, they normally charge intermittently before failing altogether, just as yours is doing. 99% of the time your symptoms are caused by a failing alternator.

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