not shifty

I just purchased a 2001 GMC Sierra with the allison transmission. It has 50k on the engine/tranny. Today when I started it to drive home, the gear indicator lights wouldn't work, the "service engine soon" light came on, and it wouldn't shift out of first gear. Putting it in "tow" mode made no difference, nor did starting the gear shift at first gear and then moving up to drive. The fluid level was fine. Any ideas?


Senior Member
not shifty

Sounds like a computer problem. The first thing to do is to put a 'reader' on the service plug to see what the computer thinks is wrong (the cause of the 'service engine soon' light). I don't know what they go for these days, but in my 89 chevy truck, I carry one which cost about $30. Invaluable to ensure that what the repair place is quoting you is in the right ballpark... If you have OnStar, I think they can do this diagnostic operation over the phone.
not shifty

It sounds like your NSBU switch is failing. It is located on the left side of the transmission. It slides over the selector shaft and bolts on the transmission. The NSBU switch is responsible for lighting up your gear indicator and telling the computer what gear has been selected. I would first find out what the code is, like hertig stated.