Bad alternator or battery? - JeepForum.com

 
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post #1 of 6 Old 07-04-2010, 08:54 PM Thread Starter
XJ96PA2
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Bad alternator or battery?

Drove my Jeep to and from work on Friday, drove fine. I went out Saturday morning and my battery was dead. Jumped it, and it started right up. Let it run for a little and then started giving it little revs to get the alt to charge the battery faster. I then had to stay on the gas, because I could tell if I let off it would shut off. When it did shut off, it wouldn't start again because the battery was dead. I figured the alternator was bad and it was only running off what little charge the battery had. I went out tonight and was going to take the alternator off a while but I figured I better check to make sure that's what it was before buying one. I jumped it but it didn't want to stay running at first, even while the jumper cables were still hooked up. I let it charge a little and then it started. I turned all all the lights on, wipers, etc. The battery gauge was right on 14. It ran for a little and then when I gave it some gas it shut off again, and the battery was dead. Now I'm not sure if the alternator isn't charging the battery or if the battery is just refusing to take much of a charge. I've had alternators go bad before, while driving, and know the symptoms, and my Jeep had none. Just a dead batt. one morning. What does it seem like it is? What is a sure fire way to check? Thanks.


2001 Grand Cherokee Limited. 4.7 V8, Quadradrive, Tow Package
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post #2 of 6 Old 07-04-2010, 09:04 PM
03GCherokee
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Sure fire way is to test the battery.
It's possible that the battery has a bad cell sucking the juice from the alternator while running.
Remember alternators charge A/C current. Batteries supply DC voltage. Running a car with a bad battery can fry a module.
Pull out the battery and have a parts store charge and test it.
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post #3 of 6 Old 07-04-2010, 09:15 PM
moonshinefuel
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A couple things come to mind for me. Is the regulator part of the alternator, or located other than in the ecu, or part of the ecu. Testing might be difficult unless you can test it under a load. If the alternator is an older one, certainly it is not doing it any favors running an alternator on a dead battery, that heats up and stresses alternators pretty quickly, they are naot made to charge batterys, just maintain voltage, which they do very well of course when the system is working correctly.
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post #4 of 6 Old 07-04-2010, 09:44 PM
nitecop
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You really need a volt meter to see whats going on.
You want 12 volts or more in battery.(not running) and when it is cranking it shouldn't drop under 9 volts at all.
but I have to ask does your vehicle have its factory battery?? if so... then I think you have your answer.

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post #5 of 6 Old 07-04-2010, 09:58 PM Thread Starter
XJ96PA2
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Quote:
Originally Posted by nitecop View Post
You really need a volt meter to see whats going on.
You want 12 volts or more in battery.(not running) and when it is cranking it shouldn't drop under 9 volts at all.
but I have to ask does your vehicle have its factory battery?? if so... then I think you have your answer.
Not sure if the battery is original, it says "Federated Auto Parts" Can't see the markings for the date or part no. or anything so regardless I'm sure it's pretty old. I just bought the Jeep a month or two ago.

I'm betting on a bad battery since the gage reads 14. It had no symtoms of a bad alternator while driving. In my last Jeep, and my car, when the alternator went bad the lights slowly got dimmer and the turn signals and windshield wipers were slow. Like I said it drove fine on Friday but wouldn't start on Saturday. Sometime this week I'll pull the battery and take it to autozone to have them charge it and test it, to be 100% sure. Not my DD so it's not an emergency. Thanks for the responses. I'll post my findings for future reference in case someone has the same questions.

2001 Grand Cherokee Limited. 4.7 V8, Quadradrive, Tow Package
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post #6 of 6 Old 07-04-2010, 10:05 PM
moonshinefuel
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Most parts stores will do that, load test the battery, not just check voltage.
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