1991 stock D250 diesel. 416000 km. Excellent truck.

It seems for some time now, I have had an irratic, occasional, refusal to upshift to OD. New TPS! Smooth shifting. Only occasionally does it refuse to upshift. On a certain hot day(+44 celcius / 110 F.) it would not shift. Other days, it is just tempermental. Today, no load, it won't shift. Occasionally, when I deliberately turn off dash OD button and then turn button back on, it will not shift back to OD. Once, I tried putting it in neutral while coasting and then back to OD and it went it !!!! Usually a day of rest brings things to normal .......... hopefully !! What is wrong??

Thanks,
Bob V.

 

I'm not an expert on trannys but here is something to check. There is temperature sending unit in the fluid line to the cooler. It may be giving the PCM a signal that the transmission is too hot. Our trucks cut the OD off when the transmission fluid reaches 280 degrees F. It may be a wireing problem , bad sending unit or the worst case, the PCM is going away. Good Luck

 

Mine will do the same thing after 1k miles or so, I just have to reset the voltage on the TPS(lower the voltage in my case). I'm not sure why the voltage creeps up on mine, cant find anything loose.