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Unread 02-10-2013, 09:48 PM   #1
MTBlue
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Bad Alternator? HELP PLEASE!!!

Hey guys,

So I left my lights on all day. Oops! Obviously, jeep ain't starting of its own free will. SO, use my gf's 98 TJ to jump it. Use my brand new jumper cables, and try to start 'er up. No dice. It cranks and gives me all the dash lights like it does when you first turn the key to start it, and eventually it gives me the mileage display along with airbag and seatbelt lights (the rest die out) but it still doesn't turn over and fire. So we leave it for another 10 minutes. Come back ~20 minutes from when we first put the jumper cables on, and it does the same thing. Now, I try giving it some gas when I'm cranking it, and it turns over and starts. When I let off the gas, though, the RPMs fall and it dies. I am able to drive it (drove it to school where I will formally diagnose and repair it), but it dies every time I let the RPMs fall. Scary trip, I will say that! I think it's a bad alternator, but I really don't know. Just an educated guess, really.

Sorry for the book! Any and all help is appreciated!
-Evan

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Unread 02-10-2013, 09:52 PM   #2
ZeeJay1997
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What is the battery gauge doing during all of this?
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Unread 02-10-2013, 09:57 PM   #3
MTBlue
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Looks to be going where it should, eg. up to about 14, but I never really stopped to look at it until it was running, unfortunately
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Unread 02-10-2013, 10:46 PM   #4
MTBlue
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Hmm.. Looks like others have had this problem too. Might not be the alternator after all. Could be a dead battery. So I'll probably just test both the battery and alternator tomorrow and see what happens. Hope its the battery and not the alternator. The battery that's in it right now is Walmart-status, so I'll replace that regardless, just a matter of sooner rather than later.
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Unread 02-10-2013, 10:57 PM   #5
Pat97tj
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Make sure it's not just a bad conection or an old cracked wire. I had a similar problem, replaced everything and all it was was a bad wire.
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Unread 02-10-2013, 11:37 PM   #6
MTBlue
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Make sure it's not just a bad conection or an old cracked wire. I had a similar problem, replaced everything and all it was was a bad wire.
Copy that! I'll run through it all when I get a look at 'er tomorrow at school. Thanks for the advice!
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Unread 02-11-2013, 08:43 AM   #7
MTBlue
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This is getting weird. I got to school this morning, put the key in the ignition, and just for ghits and shiggles, tried to start it. Lo and behold, she fired right up. So, since it was close to zero here last night and she had been sitting outside, I turned the defrost on to clear the frost off my windows and took it for a spin around the parking lot. It drove fine for about a minute, then the RPMs dropped and I had to floor it to keep it from dying again. After that, though, it seemed to do fairly well. I pulled it in by our shops and am going to run some electrical tests after lunch. I'll keep this thread updated with what I find I guess...

-Evan
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Unread 02-11-2013, 10:05 AM   #8
Technohead
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This is normal behavior. The computer was just relearning. Starting it up and shutting it off a few times seems to speed the process up.
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Unread 02-11-2013, 10:16 AM   #9
McCloudsZJ
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Yup. If you're battery is really really dead, the Jeep will have trouble staying alive. Get the battery good and charged (or replaced) and after a couple start/stop cycles of the computer relearning, you should be set. A load test will tell you if the alternator really is suspect. I was experiencing similar problems when my alternator couldn't keep the (brand new) battery charged. After testing it, I replaced the alternator, and everything is good.

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Unread 02-11-2013, 10:52 AM   #10
MTBlue
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This is normal behavior. The computer was just relearning. Starting it up and shutting it off a few times seems to speed the process up.
But I haven't done anything to it yet... it just sat overnight. So what changed? I can see that happening if I replaced the battery, but I didn't do anything to it between last night and now. Could the battery have fully charged between now and then? By just running it maybe? Or magic!
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Unread 02-11-2013, 01:22 PM   #11
Technohead
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But I haven't done anything to it yet... it just sat overnight. So what changed? I can see that happening if I replaced the battery, but I didn't do anything to it between last night and now. Could the battery have fully charged between now and then? By just running it maybe? Or magic!
Yes you did do something. You went through enough start-stop cycles for the computer to relearn. You should be fine now.
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Unread 02-11-2013, 01:38 PM   #12
MTBlue
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Originally Posted by Technohead View Post
Yes you did do something. You went through enough start-stop cycles for the computer to relearn. You should be fine now.
ooooooohhh... That makes sense! I'll give it a try.
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Unread 02-11-2013, 03:08 PM   #13
MTBlue
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Sure 'nuff! Both the battery and the alternator passed the tests, so I guess it was the computer just "relearning". Interestingly, though, my alternator was charging a little low. I think these are supposed to be at around 14.6 and mine is at 13.8. So ill probably end up replacing that sooner rather than later.

Thanks for the advice guys! You nailed it Technohead!
-Evan
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