Why is my GMC Sierra 1500 not shifting?
Why is my GMC Sierra 1500 not shifting? If in fact the tranny you put in is good, then the possibilities are that you have a wiring problem from your TCM to the tranny or your TCM is messed up. A good tranny shop can run stand alone testing to find out what you actually have going on. 41 people found this helpful. How to check your GMC Sierra 1500 transmission? An the TCC solenoid the signals from the power control model tells it when to shift check the Orange an yellow wire on the harness that connects to your...