My allison did some strange things a couple of times today. After about a 90 mile trip for business the truck sat for about 2 hours when I pulled out to leave I noticed when shifting I did not have the PRD321 being lite up as normal so when I pulled into traffic I know I was I Drive it would not shift out of third gear I stopped and went through all the gears and reverse would lite up but not drive and sometimes 3rd would lite up and when I cycled the key on I noticed that 1st would luminate and flash Pulled out again and this time it shifted fine all the way into overdrive and when I slowed and dropped it into 3rd and back into drive it wouldn't shift again. I pulled off and checked the connections on the nsbu switch and it has not done it since. Could it be the nsbu switch.By the way it is a 01 with 82,000 miles on it

That would be the first thing I replaced, good or bad. They are know for acting up a lot.
mike

One thing to note is that these are on Nat'l back order from GM and Allison. Guys, I have seen a rash of these lately and it wont be long and there will be no parts avail...at least for a time being..