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Ask Gm Tech (Cam) Your Own Question
Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 29457
Experience:  GM Grand Master Technician 2007. 14 year experience.
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I have a 2001 GMC 2500HD 8.1 with an Allison transmission.

Customer Question

I have a 2001 GMC 2500HD 8.1 with an Allison transmission. It has an intermittent problem during upshifting (hard). I have replaced the NSBU and the ignition switch. Fluid level is OK. I have changed the filters and there is no evidence of excessive metal in the pan. Trouble code is P0700 (general transmission)
Submitted: 1 year ago.
Category: GMC
Expert:  Gm Tech (Cam) replied 1 year ago.

Hi there, I am Cam. Can you tell me what code is set in the transmission module itself? P0700 will be reported in the engine on behalf of the transmission module .

Customer: replied 1 year ago.
I cannot read the transmission module code
Expert:  Gm Tech (Cam) replied 1 year ago.

OK, we will need to. However in the absence of the code, we can work on why its shifting hard a bit. Hard shifts are the transmissions response to a slipping condition. It increases pressures to compensate. Anything could be slipping, the Torque Converter Clutch (TCC) , any clutch and it may only be that one lip seal, one sticking valve or something like that is happening and causing it. You replaced the filter, was the magnet still on the external spin on filter?

Customer: replied 1 year ago.
Expert:  Gm Tech (Cam) replied 1 year ago.

Ok, do you still have the internal filter? Did you open it up and look for signs or a rubber lip seal, anything more than a small particle?

Customer: replied 1 year ago.
Replaced it. Nothing unusual
Expert:  Gm Tech (Cam) replied 1 year ago.

The signs can be subtle. But if nothing then it's something in the valve body, a binding valve then. Take off the valve body and clean it out. Make sure every one moves with no binding. If any do, you may need to replace the valve body.

Customer: replied 1 year ago.
Rechecked codes. I have an p1875 4wd low switch
Expert:  Gm Tech (Cam) replied 1 year ago.

that should not cause a hard shifting. That is just for the 4wd system.