So I got my 1993 C3500 Dually with the 6.5l turbo with the 4L80E. My problem has been on going for some time. Sometimes it drives fine for a while and then when i stop to take off it starts to shift hard into second gear and will not go into overdrive? If i stop and reset the computer it drives fine again for a while. The last tranny i had in it did the same thing even after it was rebuilt. Computer was bad back in 2007 and replaced it. Dont know if it has went out again and causing problems or what any ideas?????

 

Since it only has a trans computer the speed sensor and TPS control most functions-I would start there and check for trouble codes.
But I am quite weak when it comes to electronic auto trans diagnosis.