I have a '97 F350 diesel that won't shift on its own from 1st to 2nd gear once its warmed up. I have to do it manually. Transmission rebuilt 50,000 miles ago.

I've taken it 3 times to the transmission shop. Everything looks clean and good in the pan - spotless really. Computer gives no codes. Today they changed the accumulator, but it still doesn't shift.

Any suggestions?

Do you have OD and is it flashing? I had a similar problem with my 97 and it turned out to be a short in the over head light that was sending a strange code to the computer. you might try to remove the light and see if fixes the problem.