Allison tranny slippage

[skyking600]

I have a 2002 GMC 2500 Duramax with Alison. It just turned 50,000 and just had my first problem this last weekend. After driving 100 miles at 65 mph I pulled over to take a leak. I let the engine idle and when I got back in after a minute I shifted into drive and the truck moved about 10 feet before slipping out of gear and not moving any more. At the same time it slipped out of gear, the battery indicator light came on and all the engine gages went to zero for a half a second. I shifted back to park and then into drive again only for it to do the same thing. The engine would rev up but the truck wasnt going anywhere and the bat light and gages did the same thing. I called Onstar and no error messages were being read by them. There I was stranded in the middle of the desert ready to call a tow truck when I thought about turning the engine off and restarting. I did, and that worked for the rest of the trip and since then. I called the dealer and they said they had never had heard of that problem before. Has anybody else had this problem and know what is causing it? . Thanks

[Joe Duffiney]

Just taking a wild guess but could it be NSBU switch?

[DmaxMaverick]

Welcome to the Forums, Skyking600!

While the NSBU is the usual suspect, sounds more like a voltage problem. If the batteries are original, they may be on the way out. Other options are grounds, poor connections (batt. terminals), poor grounds, and/or alternator (less likely). When the NSBU fails, there is usually a glitch in the gear selection display.

[skyking600]

Thanks guys for the input but what does NSBU stand for and what does it do? I am a aircraft mechanic but I am not familiar with the automotive acronyms. As for the batteries I replaced them 2 weeks ago. Maybe I got a bad battery that internally shorted but the problem has not reocured since.

[DmaxMaverick]

NSBU = Neutral Start Back-Up. It's the device the gear selector cable connects to, on the side of the tranny. The older model (pre-2003) would pack up with snow/ice, and get stuck. I don't think this is your problem. In most cases of problems with them, the PNRD321 display would either disappear, flash, or not be correct in some way.

New batteries? Disconnect both of them, wait about 30 minutes, and check voltages. They should be the same, or within .2 V. These rolling supercomputers are very sensitive to voltage issues. What does your volt meter indicate on a cold start? You should see 3 stages...9-10V (GP/Htr), 11-12V (Htr), and it should move up to 14V+ after several seconds. If it's taking a long time to creep up to 14V+, you probably do have a bad battery, or poor connection. There was a software update to address the voltage regulator, which is controlled by the PCM. What computer updates have been done since new? If none, it may be in your best interest to have them done. Even if you have to pay for labor time, it'll be worth it. There were some other very effective updates that addressed the TCM (shifting behavior), and many engine parameters.

I moved this thread to the 2500HD/3500 HD Trucks & Drivetrain (http://www.thedieselpageforums.com/tdpforum/forumdisplay.php?f=13) Forum.

[Mark Rinker]

Sure sounds like a bad battery on its way out to me. The scenario was perfect for it to happen; extended cruise (underhood heat), desert (+heat), stopped and left to idle (+heat soak).