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Chris (aka- Moose)
Chris (aka- Moose), Technician
Category: Transmissions
Satisfied Customers: 1594
Experience:  16 yrs. experience with auto, manual, & CVT drivelines
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I have a 2004 GMC duramax with allison transmission. allison

Customer Question

I have a 2004 GMC duramax with allison transmission. allison 1000 It will get stuck in fourth gear and not shift up or down. After clearing the codes it will shift fine one time but as soon as you let off the throttle it will not downshift and stay stuck in fourth gear. It will also shift fine several times when is it warming up, but when it is at operation temp it gets stuck every time.
Submitted: 8 months ago.
Category: Transmissions
Expert:  Edward replied 8 months ago.

Hello, Eddie here and I will do my best to help. Do you know what code or codes it is setting by chance? Apprixmate mileage on it, and has the tranny ever been overhauled yet? or out? Thanks.

Customer: replied 8 months ago.
I dont know the codes, mileage is approx 150K, it was doing ok and leaking trans fluid out of the rear housing thru a small crack and we replace that and now the shift problem. I am sure they are related but not sure how.
Customer: replied 8 months ago.
all of the filters were changed when the trans was out of the truck to replace the rear housing
Expert:  Edward replied 8 months ago.

Alright, It's interesting that it shifts fine cold, but warm it locks into 4th and won't downshift. If you can check the codes, it'll be like a code that starts with a 7 like PO735 or PO745 something like that. Any generic OBD II tester can get the codes out, that would tell me alot, if there is shift solenoid codes, or communication errors. I believe it to be in the valve body/ECM control. since it does have all of it's gears cold. Is there any possible way you can get me the codes?