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Thread: Bad PCM??

  1. #1
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    Default Bad PCM??

    Hello, new to the forum -- Need some help with a troubled XJ

    2000 Cherokee 4.0 -- 225,000 miles

    Jeep starts but misses terribly -- Barely runs at idle -- runs when on throttle but very rough, misfires badly

    When I bought it it needed the dual cats at the manifold -- cali emissions

    bought the walker setup, re-welded all the joints, and installed all new O2 sensors because I didn't have 2 of them

    So now that it's quiet The jeep is misfiring badly -- Changed plugs to champions, tested Ignition coil pack -- All Coils OHM out equally -- then tested spark with a steel brake line connected to ground -- All coils firing strong

    After plugs I started getting codes -- One was injectors -- open circuit cylinders 2 and 4 -- I tested the wiring with no real results -- could never get a reading on my Volt meter -- I changed injectors -- upgraded to the ford mustang ones

    Now with new injectors the injector codes are gone -- New code -- P0122 -- Throttle Position Sensor Low voltage

    In an attempt to diagnose the TPS -- The 2 outside terminals on the plug 5 volts in and ground -- the center terminal on the plug is supposed to be TPS signal out -- I get 4.32 volts there with the TPS unplugged from the wiring harness

    4.32 volts -- Only hooked up the the PCM -- Now I search wiring diagrams and find which pin is connected to TPS input on the PCM -- Pin #23 -- With the PCM wiring harness unplugged I get 6 volts on PIN # 23 -- I also get 7 volts on the pin which is supposed to be 5 volts to power the TPS -- I forget the pin number

    So now what? -- Is my PCM fried? I don't even know where to look now...


    Thanks for your thoughts -- hope you can understand this

    Jer

  2. #2
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    Default Re: Bad PCM??

    I would check a couple of other sensors for input. It may be that you have fried it if it is putting out more than 5V
    230 hp 4.3L Chevy
    Built 4L60E with manual lockout
    Atlas 4 speed
    Griffin radiator
    On board air
    Warn 9.5ti on custom bumper
    7.5" RE front and bastard pack rear on custom long arms
    30 gal gas tank
    lockers
    And a bunch of other stuff

  3. #3
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    Default Re: Bad PCM??

    It's funny -- With the plug disconnected at the PCM I get 7 volts at the pin that is supposed to supply the TPS with 5 volts -- When I plug the harness in I get 5 volts at the TPS as I should

    The big problem is I'm getting voltage from the PCM at the terminal thats meant for TPS input

    I would think that, with the TPS unplugged, I should get 0 volts at the terminal thats supposed to read the voltage from the TPS -- It's unplugged -- why is there voltage there?

  4. #4
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    Default Re: Bad PCM??

    I don't know. Sounds like you are getting voltage from something that shouldn't be giving it
    230 hp 4.3L Chevy
    Built 4L60E with manual lockout
    Atlas 4 speed
    Griffin radiator
    On board air
    Warn 9.5ti on custom bumper
    7.5" RE front and bastard pack rear on custom long arms
    30 gal gas tank
    lockers
    And a bunch of other stuff

  5. #5
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    Default Re: Bad PCM??

    correct -- and it sure seems like it's coming directly from the #23 pin on the PCM

    I guess it's as simple as that -- PCM fried

    I certainly hold out for some thoughts from others for a few days -- The XJ is a play toy -- in no rush

  6. #6
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    Default Re: Bad PCM??

    It happens, they can go south on you
    230 hp 4.3L Chevy
    Built 4L60E with manual lockout
    Atlas 4 speed
    Griffin radiator
    On board air
    Warn 9.5ti on custom bumper
    7.5" RE front and bastard pack rear on custom long arms
    30 gal gas tank
    lockers
    And a bunch of other stuff

  7. #7
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    Default Re: Bad PCM??

    SO I pulled my PCM -- took it apart -- No visible damage or burnt parts -- I'll do some testing tomorrow


    Can someone test their TPS connector for me on a running XJ?

    Pull the connector from your TPS -- with a voltmeter test the center terminal on the connector with the key on -- Positive probe on connector -- negative probe to a good ground somewhere ---Tell me if you have voltage there with the wires disconnected from the TPS



    Thanks, Jer

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