My 91 w/automatic can't seem to decide if it wants to be in overdrive or not! On a slight grade at 55 to 65 it will constantly shift in and out of overdrive. If I let of the gas or give it more, it will stop for a while. I have a '90 parts car with a broke trans that I could use to swap out parts with. I just dont know what to swap! Thanks for your help!

 

There is a button on the shifter....make sure it is off and the light is on when you dont wanna feel the shift shocks of the tranny going from OD to a gear.....if it s off..the light is on...and it still does it......then maybe there are some worn bands in the tranny

 

when was the last time fluid was changed?
check to see if the fluid is low.

your TPS is ok right?

 

Fluid was changed a few thousand miles ago. Not sure what the tps is? Throttle position switch? What effect does this have and how do you adjust it?
Thanks

 

ok then check fluid level...make sure it's at the right level.

throttle position sensor...it doesn't go bad much. but how much gas u give will determine if the car should downshift or not.

 

I wonder if the fluid level maybe overfilled? I had a similar problem on my 88 ranger. The fluid was overfilled and it caused it to have the same problem....but eventuually the overdrive stop working. I would check it ASAP if I were you.

S