Transmission in limp mode.

revdevil · 04-05-2008, 10:49 AM

02 GMC 2500HD W/Allison Trans Problem

Help please.

I have a 02 GMC 2500HD SLE 4x4 with the 8.1 and Allison trans and it is locked in first gear.

Here are the symptoms:
Service Engine Soon light is on.
4 low will not engage (push button selector)
4 high engages but stays in 1st gear
Loud clunk when I go from Neutral to Drive.
Also my D (drive) indictor is not lit up but all other postions will.
And when I put it in park the power locks stay lock when they are supposed to unlock.

I don't have OBD reader so I know I will have to take it to a dealer and get raped on the service costs but any help or suggestions would be appreciated.

Thanks
Revdevil

**deeretracks** · 04-05-2008, 10:54 AM

Could be the NSBU. switch.

**BigRedChevy** · 04-05-2008, 11:49 PM

I agree probabaly the NSBU switch.

01lb72500 · 04-07-2008, 04:42 PM

x3 on nsbu

revdevil · 04-08-2008, 11:48 AM

So what exactly is the NBSU switch? Same as PRND?

**Bryan@ATS Diesel** · 04-08-2008, 02:59 PM

Yep.....its the white electronic board on the drivers side of the tranny that has the shift cable feeding into it with two electrical plugs.