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Old 03-18-2010, 08:17 PM
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Voltage Spike

I have a 99 TJ 4Cyl. Today while out and about the voltage spiked to 19 for a few moments then back to 14. Then did it again. Third time I shut it off, started it and it went back to normal. Hasn't done it again.....any ideas?

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Old 03-18-2010, 08:21 PM   #2
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Test the charging system. You might need to replace the alternator (voltage regulator).

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Old 03-18-2010, 08:23 PM   #3
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Did you notice any driveability issues? Do you have any aftermarket stereo equipment?
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Old 03-18-2010, 08:38 PM
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Drives fine. Put in a new stereo about a month ago. Nothing big just a Sony HU and new speakers.
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Old 03-18-2010, 08:41 PM   #5
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How about night driving? Any issues with the lights? It's possible your regulator is having some issues. It's integrated with the alternator.

I'd keep a close eye on it and try to replicate the issue.
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Old 03-18-2010, 08:51 PM
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Ill try it tonight. Everything else is just fine, runs good stereo works fine no whinning in the speakers no crackling. Nothing but the guage going to 19 then back, and only for no more then 10 seconds.
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Old 03-18-2010, 08:57 PM   #7
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I would load it up (stereo, light, motor, fan running, etc) and re-evaluate. If you're alternator is on the way out, I would tend to believe it would either fail under load or just stop charging the battery which you will notice when you try to start it.
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Old 03-18-2010, 09:17 PM   #8
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I would put a volt meter on the battery + to - and start the engine and see if the voltage is comparable with what the gauge in the vehicle is reading. Switch on and off various loads and change rpm's and see if the voltage spike will duplicate. When charging systems get voltage spikes like that, it's usually the voltage regultor or the gauge itself is going away. My guess is the regulator which is part of the alternator.
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Old 03-29-2010, 02:27 PM
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Well I went ahead and changed the Alternator and also installed a new battery. The gauge still spiked. I'm assuming at this point the gauge is going. Never changed one has anybody every changed one?
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Old 03-29-2010, 06:48 PM
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Update: Went out and did the Guage test, ya know hold the trip button turn on key. Guages and dash lights went through a test phase and so far all seems well. Took it out drove it 50 miles.....no volt spike so far.
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Old 03-29-2010, 07:35 PM   #11
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Look for a loose connection in the relay box. A momentary disconnect to the regulator sense line (which I'm pretty sure is part of the ECM, not in the alternator) makes it think the output is too low, so it tells the alternator to go full blast.

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