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1996 Peterbilt 378 Speedometer Issues

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by stewtrk23, Mar 11, 2025.

  1. Mar 11, 2025 #1
    stewtrk23

    stewtrk23 Bobtail Member

    2
    0
    Mar 11, 2025
    0
    I have a 96 Peterbilt 378. Changed out the engine from a Cummins M11 to a Cummins ISX15. Speedometer keeps going out with a check engine light. Changed the speedometer already. Usually happens when loaded and driving a long distance at a constant speed. Sometimes will come back on when do local work with more shifting. Any advice what it can be?
     
    stewtrk23, Mar 11, 2025
    stewtrk23, Mar 11, 2025
    #1
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  3. Mar 11, 2025 #2
    bjytech

    bjytech Light Load Member

    239
    220
    Nov 9, 2013
    Ohio
    0
    Need to check the speed sensor on the back of the transmission and check to make sure the yolk coming out of the trans is tight, if it's loose that can cause loss of speed signal. after that time to start chasing the wires from the speed sensor up to the ECM. Being a 96 it could have a old 4 wire speed sensor, if so one set of 2 goes to the speedometer and the other set goes to the ECM.
     
    bjytech, Mar 11, 2025
    bjytech, Mar 11, 2025
    #2
  4. Mar 12, 2025 #3
    stewtrk23

    stewtrk23 Bobtail Member

    2
    0
    Mar 11, 2025
    0
    I checked the speed sensor and yolk looks tight. I’ll check the wires next chance I get. Thank you
     
    stewtrk23, Mar 12, 2025
    stewtrk23, Mar 12, 2025
    #3
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