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1994 Jeep Grand Cherokee bad batter? Or alternator?

Tiny
BFDFF94
  • MEMBER
  • 1994 JEEP GRAND CHEROKEE
  • 4.0L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 264,000 MILES
When I am driving using the a/c the voltage hand goes down to around 12 and at night with the lights on and a/c it goes down farther than that everything works fine and it runs good but that hand just has me puzzled lights aren't the brightest in the world but everything functions properly what could be wrong? And it does throw a check engine light most of the time as well.
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Tuesday, May 12th, 2015 AT 6:53 PM

1 Reply

Tiny
CARADIODOC
  • MECHANIC
  • 33,873 POSTS
Ignoring the Check Engine light can lead to a simple and inexpensive problem becoming an expensive one. Have you had the diagnostic fault codes read and recorded?

Your description of the symptoms suggests the alternator has a bad diode. If so, it will only be able to deliver exactly one third of its rated current, and that's not enough to meet the demands of the entire electrical system under all conditions. The charging system should be tested with a professional load tester for maximum full-load current and "ripple" voltage. With one of the six diodes bad, ripple voltage will be very high.
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Tuesday, May 12th, 2015 AT 7:26 PM

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