I just bought an 89T AT. After driving 30-45 min and the car is good and warmed up it will not shift into overdrive and RPM's will increase. If I turn the OD off i will cruise at 45mph at 32-3500rpms. I was able to run up the rpm to about 4500 and release throttle and it shifted. If I drop the hammer at 50mph it will down shift ok. Any ideas of the problem?
Thanks,

try changing the fluid and filter!

I used a little Lucas oil stabilizer and changed filter. I will check it out and see if that helps. The old fluid looked pretty good though. How much fluid should the tranny take?

Change fluid and filter did not help. Any other suggestions? Can someone who knows about these trannys give me there phone #? Thnks