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Thread: Spastic tach

  1. #1
    Senior Member mriulvr's Avatar
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    Spastic tach

    My tach works fine until I have been riding for a while and then starts spastically bouncing around above 3k rpm. I have checked and cleaned all grounds, did a quick check for worn wires, checked the pink wire from the ECM to the gauge cluster, done the wiggle test, etc. What is odd is that I am not getting any codes, the RPMs from logged rides are normal, no spastic bouncing in the log, and runs fine. Probably not a real issue but the tach needle is driving me crazy!

    Narrowing it down to a worn wire or bad connection somewhere, which will take some time to check. Then today I got to wondering what input the ECM uses for the output to the tach. Maybe the issue isn't the output but the input (Garbage in garbage out). Does anyone know what input is used by the ECM for the tach output? I tried figuring it out from the wiring schematic but it is beyond limited electrician interpretation skills.

  2. #2
    Senior Member Cooter's Avatar
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    From what you're saying, Bill () is that the ECM is seeing the proper signal, so the signal is ok. You also said the pink wire and grounds are OK. Whats left is the gauge cluster itself.

    The needles aren't cable driven like the old days where a bouncing speedo or tach is just a mechanical fix. The needles are attached to stepper motors and they will do nothing if they lose power, not drop to 0. They would stay in place. Which leads me to believe (using your info) that you should remove the gauge cluster, carefully check that connector at the pink wire pin, and trace that on the SMC board to the stepper. Look for bad solder joints because it sounds like a temp driven loose connection.

  3. #3
    Senior Member mriulvr's Avatar
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    Thanks Cooter. I feel like Bill after chasing this issue. If your suggestion fixes it then I will stop complaining about you crawling up my skirt, or just complain a little less.

  4. #4
    Senior Member 34nineteen's Avatar
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    There is a dyno loop in the harness you could get an inductive signal from. Hook up an external tach to it and monitor the signals. If both are bouncing, then you likely have a bad signal. If one stays steady and the other bounces, then you know you have a problem with the cluster. You could probably use ECMDroid to check this but that interface is a bit slow and may not be quick/precise enough to register the "bounce".

  5. #5
    Senior Member Cooter's Avatar
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    Quote Originally Posted by mriulvr View Post
    the RPMs from logged rides are normal, no spastic bouncing in the log,
    Remember when Spastic Logs broke up in the 80's? after the lead singer went straight.

  6. #6
    Senior Member Barrett's Avatar
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    You've been chasing this problem in excess of 2 years.
    http://www.badweatherbikers.com/buel...tml?1562020439

    Simply put, it's either a failing dash cluster...cluster feed connector....failing ECM....interrupted ECM harness connections.

  7. #7
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by mriulvr View Post
    spastic bouncing in the log
    Back in the day I met this dancer named Cinnamon who was all about spastic bouncing on the log (if thats what you wait to call it).

    Oh wait, you said spastic bouncing IN the log. My bad. Carry on.

  8. #8
    Senior Member Cooter's Avatar
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    Isn't it weird how parent just KNOW their little baby is going to be a 'dancer' and name them accordingly? Hmmm, mysteries of the universe.

  9. #9
    Senior Member mriulvr's Avatar
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    Quote Originally Posted by Barrett View Post
    You've been chasing this problem in excess of 2 years.
    http://www.badweatherbikers.com/buel...tml?1562020439

    Simply put, it's either a failing dash cluster...cluster feed connector....failing ECM....interrupted ECM harness connections.
    I know ^^^^ and have been trying to narrow down instead of randomly throwing parts and $$$ at it. Started two years ago but haven't had much time over the last 18 months while stuck working longer hours from home in the midst of the pandemic. I've just recently had some time to retackle the tach. Just drawing on the wisdom and insight of others to help identify the root cause by eliminating potential root causes.

  10. #10
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by 34nineteen View Post
    There is a dyno loop in the harness you could get an inductive signal from. Hook up an external tach to it and monitor the signals. If both are bouncing, then you likely have a bad signal. If one stays steady and the other bounces, then you know you have a problem with the cluster. You could probably use ECMDroid to check this but that interface is a bit slow and may not be quick/precise enough to register the "bounce".

    Hey look ! Diagnosis procedure^^^^^



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