2012 Silverado Transmission Issue

[AlljackedupZ71]

Hey guys, just wanted to run this by anyone reading today, see if anyone has seen this before.

 

Friend had his '12 6 speed rebuilt after it lost all gears. Shop rebuilt it, and got a new converter. Transmission when put into drive will free wheel, then after a moment, will slam into 1st.. and then works just fine until you come back to stop.. same deal. Also, does not do this until the truck warms up a bit. The guy then "supposedly" replaced the valve body, same issue. New TCM, same issue... tried another GM torque converter and same thing. Has given up on the trans, and refunded him the cash. 

 

Any ideas?

 

I really feel like it's a valve body or pump issue, but have not seen this exact issue. Anyone had this issue? Does NOT slip in any gear, the delay is the only issue. 

 

Thanks for any input.