My 91 D250 automatic transmission intermittantly shifts in and out of overdrive by itself at highway speeds, even when coasting down hills with no foot on accelerator. Other than that, the transmission shifts solid, with no problems. What will cause this? And what needs to be done to fix it? Any advice would be welcomed.
Thank you.

 

the tps is either bad or in need of adjustment. check out the sticky there is directions on how to adjust it. also if it is bad you can go with a potentiometer fix, it's alot cheaper than a new tps. there is also info on that in the sticky.

 

Another possibilty, assuming that you have the electronic Vehicle Speed Sensor instead of a speedo cable, an intermittent sensor can cause the OD to kick out. The computer needs to see that you're going over 32 MPH before the OD engages. If the VSS is intermittent, so will be the OD.

 

mine does the same thing,

there is a fairly large wiring harness that junctions very close to the battery terminal.

for some reason, this harness seems to migrate toward the fender... when it does my o/d starts giving me grief.

all I do is pull over to the side of the road and shift the harness back away from the fender.

I know it sounds hokey but it works every time, and depending on how rough roads I'm driving on, lasts for quite a while.

the thought of tieing into that maize of wires seems like opening up a can of worms.

I dont mind shifting a wiring harness slightly every month or two.

I can usually get it done before anybody sees me pulled over with the hood up.

rjm

 

to add to that, when my VSS sensor went out i would not have OD and my speedometer in the dash would not work.

 

How about adjustment of the bands? A tranny mechanic told me it sounds like the bands may need to be adjusted. Does this sound a possible cause? or is it most likely the TPS? Also please briefly describe what the TPS does.
Mahalo.

 

Bands only affect the 1-2 shift and reverse engagement.

TPS is the most likely culprit. Adjusting and cleaning it may cure it, or you may need to get a new one. The TPS tells the PCM how much you are pushing the throttle. The PCM uses this info along with road speed from the VSS to determine when to shift in and out of OD.