Speedometer jumping all over the place

Discussion in 'Kenworth Forum' started by Mach, Jan 10, 2022.

  1. Mach

    Mach Light Load Member

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    Nov 19, 2017
    Knoxville Tennessee
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    09 T370 with the cummins ISC (pacer px8) and an Eaton Fuller 10 speed. Been having this issue for a while now but it is getting worse. Speedometer will jump to 80 and stay there or just go all over the place. Happens when driving and parked. When driving the cruise control will continue to work even if the speedo suddenly goes whack. The truck will still accel or decel when needed and the odometer will still turn at the correct speed. Also the speedo will never read less than the speed of the truck.

    When parked, it will go everywhere and on occasions it will trip the ELD into driving even when not moving. I can't tell what cause it to trip to drive as the speedo can bounce for 0 - 80 - 40 - 30 - 0 - 80 - 10 and so on and it won't to rip it. Hell it can sit at 80 for a solid minute and not trip but then randomly it will and stay driving for several minutes before it thinks the truck stopped. I usually have to turn off the eld if I know I'm going to be stopped with the truck running. Id turn it back on and there be 4 - 6 in identified driving events that I'd have to reject to keep the logs clean. Also note that with the key on, the issue stops. So the engine has to be running for it to start

    I've tighten the yoke nut and bought a new speed sensor but it was the wrong one. I haven't gotten the right one yet but I sit here thinking that it can't be that because cruise control and the odometer will always work correctly, which tells me the ECM is seeing the right speed. But then the speedo and the eld is controlled by the ECM. So when the speedo acts up, why does it occasionally trip the drive time?

    I honestly have no idea. I'm still going to replace the speed sensor but I just don't think that will fix it

    Video in action

    20220110_163352.mp4
     
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  3. Big Road Skateboard

    Big Road Skateboard Road Train Member

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    How many speed sensors? Since the cruise control continues, apparently the ECM is seeing the speed constant, ruling out loose tailshaft or bearing issue.

    Either 2 different sensors, and likely so. Or, 1 sensor feeding ECM, and ECM supplies info to truck. If so, there's an issue with interference between truck and ECM.

    If 2 sensors, bad sensor,or wiring to truck.

    I'm leaning towards ECM send info to truck.
     
    Last edited: Jan 10, 2022
  4. Mach

    Mach Light Load Member

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    Nov 19, 2017
    Knoxville Tennessee
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    As far as I know, there is only one seasor at the output shaft for the trans. It is a 4 pin plug for that sensor ( I got a 2 pin buy accident).

    If it's ECM, is that even fixable without replacing the entire computer? And of it's interference, Lord knows how to find what is doing it lol
     
  5. Big Road Skateboard

    Big Road Skateboard Road Train Member

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    I don't think the ECM would be the issue. Possible the 4 wire sensor has 2 wires for ECM, and 2 for truck. My assumption would be the wiring to truck has an issue, either from the ECM or the sensor, not an actual ECM issue.

    Simple diagnosis with a wiring diagram and laptop to see what the ECM, ABS, and truck see.
     
  6. Mach

    Mach Light Load Member

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    Nov 19, 2017
    Knoxville Tennessee
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    Good points. I'll try to see what I can find for a diagram and also when I see my mechanic next, I'll have him hook the laptop up and see what it sees. Not looking forward to having to replace what ever wireing harness that maybe if that's the issue. But at 800k miles I wouldn't be surprised if something is worn down to the wires.
     
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