I have a Lebaron 1991 V6 3,0L with a 4-speed automatic transaxle.
It shifts nicely until 45 minutes of driving has passed. Then the transaxle is at full operating temperature I believe.
After 45 minutes of driving the gearbox is reluctant to shift into 3rd and 4th gear. The car will also gave a more significant engine-braking feeling. I have to push the car over 49mph in order to have it shift. The phenomenom comes and goes, depending on load I think.
I have done the following actions:
- Full renovation of the gear-box (it was close to total break-down)
- Exchange of coolant temperature sensor and oxygen sensor
- Exchange of the transaxle computer to a new (reman) one.
- Exchange of the transaxle compuer relays to new ones
- Cleaning all connectors related to the engine and the engine computor (external inputs to the gear-box computor, such as soolant temperature, throttle-position and engine rev)
I still have the problem.
Any ideas?

 

If you had full renovation of the gear box I take that as a rebuilt transmission, new fluid etc. You may want to try putting a transmission cooler in front by radiator. Like the kind used for towing just to make sure the tranny doesn't over heat. You can never seem to keep them cool enough.