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Ford F150 PCM Power Diode

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  • Ford F150 PCM Power Diode

    Any Ford Techs on here that can tell me the the purpose of the PCM power Diode in the Fuse Box under the Hood. Drove the truck in the shop put a set a brake pads on and when ready to test drive it would not start no fire no fuel pump on injectors pulse. After 3 hours of hunting and head scratching found the power diode burnt. put a jumper wire on it and cranked right up. Why is this Diode There "before" power goes to fuses and relays? Any Ideas what caused it to burn out. I've Never seen this before.
    I've learned that we are responsible for what we do, unless we are celebrities.
    Bobby


  • #2
    Re: Ford F150 PCM Power Diode

    morning Bobby, just trying to remember what a diode does and I am thinking it only lets power go in one direction.
    If they take a power "bump" in the opposite direction of the flow of voltage they will fail
    (I just googled this, hope it helps)

    PM me, hows things

    Rob
    home is where we drop anchor

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    • #3
      Re: Ford F150 PCM Power Diode

      Depending on the year, it was a chronic issue for Ford.
      Depending on the year, they had different types.

      All did the same job, protect the system from reverse voltage.
      (Some blew for no reason)
      It can go anytime there is a reverse voltage, bad ground in the shop, triggered through the lift, spark from an air chisel on the frame, welding, jumper cables hooked up wrong, high powered battery charger hooked up when it was on high...numerous issues to 'speculate'.
      YOU NEED IT, get another one.

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      • #4
        Re: Ford F150 PCM Power Diode

        1FB I orderded one from dealer. Just trying to understand where the reverse ,high voltage, enough to blow it came from. Its a 1997 4.6 early 6/24/96 build date. Every thing I can see is stock no power amps for radio or anything out of the ordinary. If it was that high a voltage would it not have blown a fuse first. Just looking for cause and effect. I will put it back in and see what happens.Thanks again
        Rob we got sunshine today first time in a while dont want to stay in the house will PM Later. Thanks we still be kicking!!!
        I've learned that we are responsible for what we do, unless we are celebrities.
        Bobby

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        • #5
          Re: Ford F150 PCM Power Diode

          Auto it could of been a voltage/amperage spike. Check around the left fender for wires that are chafed as just about every wire in that truck goes there. Its a well known problem spot on fords.

          High voltage will not pop a fuse but high amperage will. We run the same fuses in our 24/48 volt systems as we do in our pick ups.
          Canada more square miles of awesomeness per person than any other country on earth.

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          • #6
            Re: Ford F150 PCM Power Diode

            Thanks PJ Looked at those wires didnt see anything out of the ordinary. Put in a new diode and he left. Showed him where it was and he left to go get a spare just in case.Did notice he had a brand new muffler and tail pipe. Might have been from the welder as 1fb said.
            I've learned that we are responsible for what we do, unless we are celebrities.
            Bobby

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            • #7
              Re: Ford F150 PCM Power Diode

              I doubt that it was reverse voltage that killed it. More likely it was overtemperature that did it. That could have been due to excessive FORWARD current for too long a time. Power diodes are pretty hardy beasts, but sometimes they are specified at too close to the operating current - that is, not enough room for error. Replace it and you should be fine.

              You might also spot check during normal use to see if either the diode itself is getting hot, or if - for some reason - the area in which it is located is getting heated up. A permanent fix would be to get something really hefty - say a power diode such as used in lift trucks - that has greater capacity than the stock one.
              Coolbreeze -- '86 Four winns/170

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