Question about 1991 Jeep Wrangler

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Alternator will not work properly

I have replaced it three times .but it will not put out the right voltage why?

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  • brklynboy187 Jan 22, 2009

    its reading 11.8 volts. and yes the battery is new the alternator is new what next?

  • brklynboy187 Jan 22, 2009

    the battery is fully charged and the reading from the alternator is the same as the battery about 11.8

  • brklynboy187 Jan 23, 2009

    hey i appreciate the response my dad is a mechanic..lol however he thinks it could be a computer what do you think? il also check the others.

  • brklynboy187 Jan 23, 2009

    what about the ecm ?could that be it?

  • Mark Egan May 11, 2010

    11.8 volts at idle, or 2000 RPM?

    If the battery is fully charged, the alternator will have less output, than if undercharged, where is could be as high as 14.8 volts (I believe the low is 12.6 volts, but don't remember for certain.).

    I also agree to check the belt, although this may be a serpentine belt.

    Are the alternators remanufactured, or are they junk yard rebuilds? And I agree with knowing where the voltage regulator is, and I think it's in the alternator too, but you need to check.


  • Mark Egan May 11, 2010

    What voltage is it putting out? Is your battery fully charged or partially charged?

    How are you measuring the voltage output of the alternator?


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I'm just thinking that if your dad is a mechanic, then have him disassemble the alternator and see if he can see anything wrong. Assuming you've wired up the alternator correctly, I am just suspect that there are no brushes in the alternator, or the regulator is bad.

Are you measuring alternator output at the alternator, or at the battery? If at the battery, and you don't have it wired correctly, then you will not see anything but battery voltage. If you check voltage at the alternator output, and it's higher than battery voltage, then there is something wrong in the wiring, either incorrectly wired, or a bad connection somewhere, as the voltage goes to the power distribution block.
Check that there is not a fuse for the alternator in the fuse panel. It may be the original alternator somehow caused a fuse to fail. It could be that there is no field power from the battery feeding the alternator through that circuit.

Posted on Jan 23, 2009

  • Mark Egan Jan 23, 2009

    ECM is unlikely. One way to tell for sure, go to a repository library and see if they have the factory service manuals (not Haines or Motors or Chiltons) for your specific vehicle. They usually have complete troubleshooting charts, wiring diagrams, everything you need that the dealers use.

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You might have them check and se that you are getting the right alternator.it could also be because of the battery not pushing enough volts back to the alternator that is keeping the reading off. have the battery tested and see if you have a dead cell or low reading on it

Posted on Jan 22, 2009

  • Ronnie Houston Jan 22, 2009

    is the voltage regulator inside the alternator on that vehicle? i cant remember right off if it is or not. does the belt have enough tension on it?

  • Ronnie Houston Jan 22, 2009

    you may need a bigger battery too i didnt think about that..cold cranking amps should be around 750 or so..

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