Allison Tranny Issue/question On 01 Hd 2500

MTIrish98 · June 19, 2009

Gentlemen,

I recently purchased a used 2001 HD 2500 Sierra with about 59K miles on it. I really like the truck, but I may be experiencing one of the problems I read about before I bought it.

I have the 8.1L gas engine w/Allison Tranny. I had read of a problem specific to '01s where the tranny isn't always in the gear the automatic gear selector says it is in. Once I was in 'D' but going down the highway at 3500 rpms. The second time I couldn't find reverse at all. In both cases shutting the truck completely off and then restarting made the problem go away.

Is this a programming issue? I appreciate any tips or ideas! It doesn't seem like a big deal now, but I can imagine some bad places not be able to find reverse during hunting season.

ss502gmc · June 19, 2009

Welcome!! Yup it sounds like youll need to join the club of having to get a new NSBU switch installed. It should be ok for a little while but once it starts acting up on a regular you eventually wont be able to drive it anymore. If it hasnt done it already you will experience the tranny shifting itself into 3rd gear on its own "fail safe gear" and a screwed up prn321 display. Its a pretty simple fix, i installed one on my truck in about 30 min. The part was a genuine Allison part and cost about 200$. Goodluck!!!

MTIrish98 · June 21, 2009

Welcome!! Yup it sounds like youll need to join the club of having to get a new NSBU switch installed. It should be ok for a little while but once it starts acting up on a regular you eventually wont be able to drive it anymore. If it hasnt done it already you will experience the tranny shifting itself into 3rd gear on its own "fail safe gear" and a screwed up prn321 display. Its a pretty simple fix, i installed one on my truck in about 30 min. The part was a genuine Allison part and cost about 200$. Goodluck!!!

By simple do you mean simple for a GMC Enthusiast, or simple for a guy who can work his way around a lawn mower engine?!?

I do get the screwed up PRN321 display occasionally now.

ss502gmc · June 22, 2009

It really is pretty simple. The switch is mounted on the drivers side of the tranny and its where ur shift cable mounts to so theres a couple mounting screws that hold it to the tranny and a nut that holds the shift cable to the switch but the most important thing is to make sure when you install the new one you must make sure the shift cable is in the same position as the old switch but if you buy the allison genuine part it comes with a line up clip so you dont screw it up but also before mounting the new one u may have to sand/file the burr on the shaft so that it doest scrape the inside of the new switch. the hardest part i had was getting the new one on without forcing it because of that burr but my first time was only 30-40 min. Goodluck!!

spackelman · June 27, 2009

It really is pretty simple. The switch is mounted on the drivers side of the tranny and its where ur shift cable mounts to so theres a couple mounting screws that hold it to the tranny and a nut that holds the shift cable to the switch but the most important thing is to make sure when you install the new one you must make sure the shift cable is in the same position as the old switch but if you buy the allison genuine part it comes with a line up clip so you dont screw it up but also before mounting the new one u may have to sand/file the burr on the shaft so that it doest scrape the inside of the new switch. the hardest part i had was getting the new one on without forcing it because of that burr but my first time was only 30-40 min. Goodluck!!

The hardest part for me was getting the wire harnesses off the old switch. Overall though, took about an hour.