My 92 cummins with an automatic 518 has been shifting it self in and out of overdrive at random times and speeds. It does it whether in on the peddle as well as off the pedal. Could that be the throttle positioning sensor, or linkage, or is there a celonoid to control that? Any ideas? (tranny has been rebuilt with heavier aftermarket parts)

 

Sounds like the tps sensor to me.

 

x2 on the TPS. Put a POT on it and be done with it.