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Old 06-22-2007, 10:05 PM   #1 (permalink)
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Jeep 4.0L electric issues

okay well here is a short story of this jeep, my gf's dad bought it brand new in 98 and he has a rubicon express long arm kit on it when they first came out, and last month she bought it from her dad since she's always liked it and he was selling it, anyway my problem is with the alternator, a couple days ago while i was driving it, i noticed the battery gauge jump all the way to 19volts, luckily i was rolling into the parking spot so I shut it off, and turned the jeep back on it was fine. on tuesday of this week, the jeep wouldnt start and she called me while i was out of town working near about in tears cause she couldnt get to work, well she just kept cranking it and stepping on the gas and it finally started, and later that day she noticed the the battery gauge jump to 19 so i told her to shut it off wait a sec then turn it back on and it was fine, it has done this several times since, so I changed out the alternator with a spare we had laying around, even though I had a thought that it was more likely the voltage regulator, I found out today that it is externally regulated by the ecm, which is bad news if it is the voltage regulator. so since i'ved replaced the alternator i havent noticed any spikes, i had the alternator tested after i took it out and it passed. so my question is what would cause this kind of spike in voltage, and why she had a random no start condition? she also has very common overheating problems that kind of concerns me, there is two electric fans on there, one is thermistacally controlled and the other one is controlled on a switch, besides header and cold air intake, there is no other engine mods.
thanks for reading and helping if you've gotten this far.
-Jack

Last edited by 4runnerboy; 06-22-2007 at 10:06 PM.
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Old 06-23-2007, 09:35 AM   #2 (permalink)
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so far your right about your alternator and pcm controlled voltage regulator. just because the parts store tested the alternator doesn't mean anything. i always have them test it at least 10x back to back if not more. it obviously is an intermittent problem.

as for your "common" overheating. if it's the stock radiator, or stock style replacement, then replace it with an all metal radiator. the stock ones are metal/plastic tanks that start leaking right where the metal and plastic join. when they start leaking like that, they will cause intermittent or specific overheating depending on the situation.

but of course make sure you also check the rest of the cooling system.
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Old 06-23-2007, 11:59 AM   #3 (permalink)
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sorry I forgot about the radiator, it is a Hi flow radiator and a super flow water pump, it's all set up to not even get close to overheating but whenever we do in town driving it still heats up and we have to turn on the secondary electric fan. I just wish I knew why it starts to warm up like it does.
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Old 06-24-2007, 01:33 PM   #4 (permalink)
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so you have 2 electric fans and no mechanical??? any chance your electric fan isn't pulling enough CFM, or maybe the motor is going and you just haven't noticed it because the fan still turns.

if you're overheating at slow speed, but not on the highway, the only "real difference" between your equipment is the airflow.

either that or i'm not thinking right now cuz i was up till 3 am fishing.
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Old 06-24-2007, 05:19 PM   #5 (permalink)
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on the voltage spike:

check the GROUND connection on the battery. make SURE it is clean and tight. my dodge truck did the same thing with spikes where the volt guage would peg-out and the lights would get really bright, dash lights get bright, radio get real loud. totally freaky man

i replaced my battery ground connection and have had no prob since. i thought it may be the computer but held-out on replacing that $300-$500 item till i tried everything else.

on the over heating:

make sure your radiator is good. they don't last forever and get clogged with travertine etc. also consider replacing with a 3-row (core). why doesn't it have a mechanical fan and does it have a fan shroud?
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