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Thread: Alt vs pcm/ecu

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    Default Alt vs pcm/ecu

    My 97 has a charging issue. The 6 year old optima went bad (i think) so I replace it and also replaced the battery tray so I could go with a 34 group battery. Drove it around, everything seems great. Next day…..totally dead.Voltmeter says 13.3 v engine running….hmmm. So I charge the battery and drive on over to the local O'reilly where I had them test the battery and alternator. Test says…battery good/ low charge, alt good. So i figure maybe it's the battery temp sensor, so I remove the battery and tray and low and behold the BTS wire had gotten under the tray and smashed a bit. I buy a new one and install and re-install the fully charged battery,put the VM on and read 13.9v at the battery engine off, 13.9 engine running…turn on all accessories…13.2v….damn. I also checked for AC current, .02 or so. It should be putting out more than 13.2 with accessories on? This model is voltage regulated at the computer. There are no other electrical symptoms other than seemingly low alt output. Belt tight,no squeals,good connections,no corrosion,no broken wire that I can see. Anyone know how to determine if the problem lies in the computer?
    Thanks in advance.

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    Default Re: Alt vs pcm/ecu

    The alternators, (I BELIEVE) are internally regulated... i didnt think this was done by the PCM... That being said, I would like to see 14.4 coming out of the alternator, but as long as you're in the 13+ range I think you're ok.... Even with that, you shouldn't be experiencing dead battery symptoms.
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    Default Re: Alt vs pcm/ecu

    Quote Originally Posted by NW99XJ View Post
    The alternators, (I BELIEVE) are internally regulated... i didnt think this was done by the PCM...
    Everything I have read indicated the PCM handles the voltage regulation duties.

    A previous owner had put an external regulator on my XJ that went bad and upon research it seems it was done by the PCM.
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    Default Re: Alt vs pcm/ecu

    As far as I know, yes the PCM does regulate the alternator on the 96 and up rigs. That being said though, a shaky test results still point to an alternator problem in my opinion
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    Default Re: Alt vs pcm/ecu

    Update:
    Problem still not resolved. I removed the alternator and took it to a reputable alt shop, he went inside the unit and determined that the brushes were bad, repaired for 40, tested at over 100 amps. Wow, I was really pleased. I rushed back home and installed the alt and put on a new belt. Drove it around, all seems fine. Next day....dead as hell. I have already replaced the battery since the old one was 6 years and had a bubble on top. I now fear that the new battery is toast after having been totally discharged twice. Starting to suspect a parasitic drain.

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    Default Re: Alt vs pcm/ecu

    Could be you have one. You might disconnect at night and check the next day
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    Default Re: Alt vs pcm/ecu

    Something could be drawing on it and you might not be able to see it. As Steve said disconnect at night and if battery is good the next day that is a good sign.
    The next step would be to disconnect the positive battery cable and place a multimeter between the battery and the positive cable. Then start pulling fuses, when the voltage drops you have found the circuit that is drawing on the battery and can start checking the components in that circuit. When you are going this, if you have an underhood light, take out the bulb, also if you are doing this with your door open.. as you likely would be the inside lights will draw and give you a false reading. Turn them off or go ahead and pull that fuse first and keep it out.
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