Well, I started with no reverse, with the "range shift inhibited" on the DIC. Then it didn't want to go in drive. Took it to Brownsville, TX to the GMC dealer, and had the NSBU switch replaced. No good. Once I get it in gear, it does fine until I switch the engine off. Then; no reverse, hard to get into drive. BTW, don't take your truck to the dealer in Brownsville, as mine is the first Allison they had ever worked on. Will take to Kingsville tomorrow, as they say they have two Allison certified technicians. Any ideas?

Sounds like low fluid or prime/loss of prime. See if you can get some codes.

Diagnosed yesterday as bad ignition switch. Apparently there is a separate switch that provides voltage to tranny. Voltage was low. New switch today, tranny works fine. Neessen Chevy GMC Kingsville, TX Seem to know what they're doing.

I have a 02 and that sounds like a problem I had the dealer had to do some research and found that the wiring to the modgel or computer on the trans was faulty because the wires were not heavy enough after replacing them I have not had anymore problems