Howdy, I have a 1991 GMC sierra K2500 light duty and I wanted to swap to a 4L80E. I currently have a 700R4 but it's leaking bad and I want something that can handle a little more power. I wanted to go to a junkyard and find a K2500 with the same engine and a 4L80E and get take the wiring harness and ECM to install in my truck. would this work?
Yes, as long as it's intact. Before you pull the trans out ,drop the pan and make sure there's no kibbles and bits as well as burnt clutch material (a small amount is normal, large amounts are not).
Once you get it to your shop, remove the pan and yank out the transmission wiring harness, replacing it with the ROSTRA harness that works for those units. The factory harness is notorious for leaking through the case connector and shorting out (you'll be in limp mode and only have rev and 2nd gear, if so).
If you have to replace the electronic pressure control solenoid because the one in the replacement 4L80E is bad and the replacement 4L80E is a 1991-1993 unit, you'll need to do some adaptation to the valve body and plate to run the updated EPC made by Borg Warner (Bosch made the 91-93 EPC which had a vented design and built in cleaning cycle commanded on by the PCM at various points; 94+ eliminated this cleaning cycle).
You can verify the year on the ID tag at the passenger side-rear of the transmission if you end up not pulling it out of the vehicle yourself.
