Medium and Heavy Trucks
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Hi my name is***** will ba glad to try to help. Check the VSS Vehicle speed sensor on the rear of the transmission and the wiring for corrosion or damage. Most commonly it is a faulty sensor but check the wiring first. When installing a new sensor, thread in hand tight to just when the sensor lightly bottoms out and then back it off 1/2 to 3/4 turn and lock the jam nut. Let me know if we need to dig deeper into this.
Thanks for the update. Are there any fault codes logged in the ECM? Have you had the DDEC software connected to read live data for switch status and parameters? If the ECM is getting speed signal along with the speedo head itself, there can be a problem with the gauge. But that would not affect the cruise control.