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?
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.
