Just bought a rebuilt trans for my sons 1994 Chev P/u. Installed it uneventful. Upon test drive , good rev, good 1 st and shift to 2 nd but when it shifts into 3rd, converter unlocks . HELP!!!!!!!! Need Advice as to what to check. ? Seems electrical. Was hot dynoed at builders shop.

 

The torque converter should not be locking until you are going about 40-45mph in third gear.

If it feels like it goes to neutral when it should be shifting to third then that trans is coming back out. It can be a valve body issue, mechanical or electrical, the apply piston could have been assembled incorrectly, its sealing rings could have been damaged upon rebuilding. Too many possibilities to try to guess at.