No power to fuel pump inertia switch, 91 MTX

Discussion in 'Gen 1 & 2 - Electronics & Audio' started by tompumped, Sep 15, 2020.

  1. tompumped

    tompumped SHO Member

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    I haven't had time to find wiring diagrams. I have a lot going on right now and I could really use the help or guidance.
    This is my first 91 and the wiring is different than the three 92s i've owned. Full disclosure I just got done installing a different motor with the same original complete engine harness. I did a 3g alternator upgrade using the factory alternator charging wire harness from a '95. I was able to use the yellow/white wire with 20ga fusible link from the 95 alternator charge wire harness and solder that to the original yellow/white wire over by the starter solenoid. I completely eliminated the original charge wire, and the yellow/white wire with fusible link that was attached to it. I also did a gen 2 starter upgrade. It will crank, the S terminal for the starter is part of that 95 alternator harness.

    I can not get power to the fuel pump. If I put power to the pump at the intertia switch it primes. The CCRM is coming on and sounds as it should. Turning on for a few seconds then the fuel pump relay kicks off. This 91 also has a yellow wire with 20ga fusible link that is directly attached to the positive battery terminal. The CCRM will not activate without that wire connected.

    The brown wire that you connect to the ECM test terminal to get the CEL to flash to count codes should be a ground IIRC but when I check for continuity or use my power probe it isn't.

    When I have more time I will see if I can get the fuel pump to activate from that ECM test plug, but I need to find a pinout of it. I will also double check the voltage readings at that plug, but I think I had low voltage there. The ground is connected to the back of the intake, the ground that is part of the harness for the ecm test plugs is also connected.

    I have no clue what i'm missing or what I did wrong. It's not the ecm/ccrm, I have known good spares. Thank you in advance.
     
  2. zoomlater

    zoomlater SHO Member Supporting Member

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    tompumped likes this.
  3. tompumped

    tompumped SHO Member

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    In the stickied how-to thread the person stated that you have to provide power to the old alternator charge wire. I did try putting power to it initially and I thought it would've caused the fuel pump to prime, but that wasn't the case. I falsely assumed he did this, because at the solenoid end of the charge wire there's a yellow/white wire that is spliced into it. I had the same yellow/white wire attached to the larger 1995 charge wire so I never applied +12v to the old charge wire. If i'd cycled the key while providing power to the old wire it would've worked as it should.

    The way I found out it was that was I grounded the fp terminal at the EEC test connector and then applied power to the old alternator charge wire. The strange thing was I was getting five volts at the fp pin on the eec connector. When I would check the voltage there it would cause the relay to cycle, and i've never seen anything like that happen. Same thing when I grounded the terminal.

    I started to tear into the harness to try and find the splice where the old charge wire powers up whatever wire it does and I can't find it. I think if I find the splice number in alldata I can then find it's location without stripping all the tape off the loom and searching. That's the first and last time in my life I wont try and find the wiring diagrams first. It only took a few hours to figure it out but it could've all been avoided.
     
  4. tompumped

    tompumped SHO Member

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    If anyone happens to know where the splice is on that alternator charge wire that would be great. I can't find my prints that show the splice numbers. I'd rather find the splice and just put voltage to it than leave that charge wire powered. If I can't find it I doubt i'll tear apart the whole loom to find it,but i'd rather do this the right way.
     

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