350SD - Transmission Downshifts
 

I've combed the vacumn charts threads but haven't found a description of this exact issue.

When the car is cold, all is well. Shifts are smooth and not hard or clunky at all. Once it's been on the road for an hour or so, the transmission seems to want to downshift, or stay in lower gear. I can be going down the freeway at 65-70, and suddenly she'll be down in third. She won't upshift to 4th until I get the Rpms >3500, and then she'll pop into 4th...but if I slow down at all, back to third she goes.

Where should I start looking for the problem? Feels like the vacumn modulator if I were to guess, but I don't want to guess. Thoughts?

Shift points are controlled by two things: The downshift button under the loud pedal (unplug if it appears to be stuck on/down), and the bowden cable / downshift cable which is attached to a bellcrank on the firewall and then to the throttle linkage bracket on the intake manifold, to the injection pump.

If the downshift button isn't the problem, try un-hooking the bowden cable and sliding it in/out by hand to see if it is binding. I have a used one.

Vacuum only affects the firmness of your shifts.

I have the same problem with my 91 350 diesel and I have been driving it this way for a couple months now. I have tried every position with the bowden cable, and I have plugged, unplugged and even checked the switch under the accel pedal. None of this has stopped the problem and I was wondering if anyone has any other ideas, because it seems it MUST be a vaccuum issue. thanks for any advice.

Updated
 

THe Triple B went to the shop to have this looked at. Shop went over it, said "Hmmph". Trans shifts fine and so they are not thinking trans itself, but maybe that the Transmission Control Module is getting hot and intermittently failing. Requested I attempt to verify by pulling off the side of the road, switching off and then restarting the car, to see if it would clear itself. They couldn't recreate the problem because they could not drive it long enough. It only happens after I drive it for <100 miles, which, with my job, is about every day.

Anyone know procedure to bench this module?