I've got a 2007 GMC Sierra Classic Crew Cab 5.3 Z71. It's the 4l60e/4l70e. My transmission went out on me while driving the other night. I drove home in second.

When put in Drive or 3rd from a dead stop, the truck does not move. 1 & 2 both work from a dead stop.

When I accelerate through 1 & 2, I can manually shift to 3rd, and manually shift to D and get into overdrive. Upon deceleration below the speed range of 3 or 4, it once again acts like it is in neutral.
None of the gears are slipping. The shifts are like they were before the incident.

Any idea what's going on? What system is responsible for engaging 1st gear when the car is stopped and put in drive or 3rd?

 

if your transmission was having mechanical problems i doubt you would be able to manage the way you say you have been. try replacing fuses and resetting your ecu by disconnecting your battery for a while.

 

This transmission is in what is called "limp mode". Limp Mode can be caused by several things.....mechanical failure, electrical failure, and the biggest one..........HEAT........I would have the computer scanned and make sure the tranny is not getting too hot