Ok so i just recently replaced the trans in my 94 ecsb gmc with a 4l60e out of a 95 that i know was a good transmission. i personally drove the 95 to the shop to do the swap. Now, 1st test run, good trans installed, the truck doesnt want to come out of 2nd gear, so i slow down manually downshift to first, and it works, truck goes into 1st, but still no 3rd or OD, i get back home and the truck is idling horribly and eventaully dies. So, i load it up on a trailer and carry it to a transmission shop, try to show the guy what it was doing and the truck runs flawlessly shifting thru all gears and everything. i replaced the map sensor to take care of the idling problem, test run it, again flawless no issues. Next day i try to run it and im back at square one, truck is doing the exact same thing as it was on the initial test run, no 3rd gear or OD, and idling terribly. Im stumpped, any ideas? IAC? TPS? EGR? Would any of these affect the trans? Ive heard some of the shifting is engaged by vacuum but not sure. Any feedback is greatly appreciated.

 

What transmission codes were you getting? Those two year units can be a little tricky because the electronics have changed. PWM was introduced in 95 and in most cases the valve body and the separator plate changed as well. With the electronics the pump has changed as well. IMO I would get the right year in the vehicle.