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Problem with SM4 (trigger wiring)

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  • Problem with SM4 (trigger wiring)

    Hi guys,

    I have some sort of problem with my SM4 controlled car, and while im laid up with a broken neck, 3 fractures in the back and a fractured sternum, I have a lot of time to try and sort out a few issues before firing the new engine. This problem may be hard to follow, but ill explain as best I can and hopefully you guys can point me in the direction of what to check, and make some sense of it.

    When I first wired my SM4, the first fire up was perfect all worked well. The problem started when a wire was crushed that was part of the cam sync. As we were rushing to make a pass, other guys "fixed" the wiring while I got suited up. Following that, the car would not start. After checking myself, they had connected the wrong wire, yet a dispute started as everyone disagreed and thought my wiring was wrong and yet the car would only run if it was the way I believed it should be.

    That was forgotten about until switching to another ECU (another SM4) when chasing other problems and the car would not start. I reverted the cam sync wiring to how everyone else believed it should be and it fired straight up. Back to back testing, going back to my ecu, it would not run, then I revert to my way and off it goes.

    The cam sync wiring as what works with my box, is the cam sync sensor (GT101) earth connects to BOTH the black wire inside the shielded cable and also to the shield wire. If it is not like that, the car simply will not run, will crank all day.

    Now, if I put any other SM4 box into my car, from its running state, it will not go. Then if I go and remove (now I cant quite remember sorry) I have to disconnect either the shielding wire or the black wire from inside of the shielded cable and then the engine will fire right up.

    Also, when running on other peoples boxes and with the different wiring setup, the data log traces are much cleaner and straight, on my box the logs are noticeably messier.

    If it would help for diagnosing, I can confirm which of the wires is the one I disconnect to make the car run on other SM4 boxes

    I know this Is probably a bit confusing but it would be great to get to the bottom of this!

    Thanks very much for taking the time to read

  • #2
    For a GT101 sensor the black wire inside the shield should not normally be connected.

    Can you unplug the Sync sensor and measure (in turn) the voltages between the sensor ground (the shield wire) and black wire (Sync -ve) and then the sensor ground and white wire (Sync +ve)?


    • #3
      Yes that sounds correct. Any other SM4 box and it requires the black to be disconnected and it runs perfectly with a much cleaner trace on the data logger also.

      Then em for my box, black as well as the shield need to be connected for it to run. Absolutely no other changes, same loom sensor everything. Literally swap boxes one for one and have to alter that wire.

      I will certainly take those measurements as soon as I can. Might be a drawn out process with my injuries but I really appreciate the help to get this sorted!



      • #4
        Im still waiting on my new multimeter and being pain free enough to make some checks, but I have an interesting shot here.

        This is two data logs, the top one being the borrowed SM4, and having the black wire inside of the cam sync trigger wiring NOT connected.

        And the bottom pic being with my SM4 and as explained earlier the car will not run unless the black wire IS connected to the GT101. Absolutely no other changes made to car or wiring.

        The TPS in particular shows how upset the sensor circuit is


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