I recently purchased my first diesel (1991 ¾ ton, automatic). The unit has been great but now I seem to be having tranny trouble. Yesterday, when I was driving at about 100 kph the truck abruptly lost power and behaved like the tranny was slipping so I pulled over, however when I got back on the road everything seemed normal except the tranny won’t shift in overdrive.

I have done a bit of research on this site and it seems that there could be problems with the governor pressure solenoid and or transducer or the throttle position sensor. I’m not sure if I’m barking up the right tree so I would like to hear what you guys think.

Any help is appreciated!

 

Could be you hit the "magic" 270*F and the PCM disabled the OD...

 

The TPS is most likely. Take a look in the sticky thread at the top of this forum, there's info there on adjusting the TPS.

The OD can be hotwired to test for mechanical/hydraulic trouble. Ground the wire that's NOT blue down at the OD solenoid connector on the trans and that should give you OD. If that works, it's just a control issue.