Tranny Trouble

lownoma98 · January 5, 2009

I have a 1996 GMC 2500 auto. The truck will not shift out of 1st gear. However if if unhook the battery for a couple hours then it will shift fine. But after a day or sometimes less it will start acting up again. Usually then it will shift to 2nd very hard and jerk, then not shift any more. I was wondering if I need to get a new computer or what. I dont think it is the transmission because I would not think unhooking the battery would do anything to the tranny. Thanks for any help you can provide.

grosss · January 5, 2009

I have a 1996 GMC 2500 auto. The truck will not shift out of 1st gear. However if if unhook the battery for a couple hours then it will shift fine. But after a day or sometimes less it will start acting up again. Usually then it will shift to 2nd very hard and jerk, then not shift any more. I was wondering if I need to get a new computer or what. I dont think it is the transmission because I would not think unhooking the battery would do anything to the tranny. Thanks for any help you can provide.

Sounds like your truck is in "Limp Home Mode" for the transmission. The computer detected something was wrong with your transmission and switched into the mode to prevent further damage to the tranny (is your check engine light not on?). By unplugging the battery you erase the volatile memory on the computer and it boots back up in the normal setup, until it detects the problem again, and goes back into limp mode. I'd hope you've done this already but check your transmission fluid level with the dipstick and top it up, and take the truck to a shop to have it looked at.

pennzoil29 · January 5, 2009

Mine was acting up similar to what you are describing. I took it to a tranny shop, and they scanned it for me. The "SES" light was not on, but he got a low voltage to the "TPS" (throttle postion sensor). I replaced that, and everything is back to normal. Good luck.

lownoma98 · January 6, 2009

Thanks for the help guys I will try both of these and let you know.

gnet158 · January 6, 2009

I have a 1996 GMC 2500 auto. The truck will not shift out of 1st gear. However if if unhook the battery for a couple hours then it will shift fine. But after a day or sometimes less it will start acting up again. Usually then it will shift to 2nd very hard and jerk, then not shift any more. I was wondering if I need to get a new computer or what. I dont think it is the transmission because I would not think unhooking the battery would do anything to the tranny. Thanks for any help you can provide.

I'd take it to the dealer and have your trucks PC reflashed. Or borrow a buddies handheld programmer and see if that makes any difference. But it's your trucks PC.

trocb · January 7, 2009

I have a 1996 GMC 2500 auto. The truck will not shift out of 1st gear. However if if unhook the battery for a couple hours then it will shift fine. But after a day or sometimes less it will start acting up again. Usually then it will shift to 2nd very hard and jerk, then not shift any more. I was wondering if I need to get a new computer or what. I dont think it is the transmission because I would not think unhooking the battery would do anything to the tranny. Thanks for any help you can provide.

I'd take it to the dealer and have your trucks PC reflashed. Or borrow a buddies handheld programmer and see if that makes any difference. But it's your trucks PC.

I am having a similar problem. Is it possible that the pcm/ecm could lose its flash and not show some kind of codes? thanks Cort