I have an L86 swapped into a Jeep with an 8L90 (RPM Extreme harness). Both came from a wrecked donor '18 Sierra 1500 Denali so the engine and trans have always been together.
After swap trans would not engage when shifting to RNDL(1-8). Fluid levels were good. After some checks it was determined the internal pump wasn't making pressure so the pump was changed, along with new torque converter, and internal harness. New complete seal kit was used and system was changed over to Mobil 1 LVP HD. When the transmission connector is unplugged (round plug) from the transmission, it pulls hard in manual valve body override (reverse and manual 1). When the transmission is plugged into the harness it won't engage.
I swapped in a new TCM (T87a grrr) and downloaded the GM TIS2/SPS calibrations into the new TCM. I am able to successfully go through the fast learn process in GM Tools TIS2/SPS (Not HPT). The transmission will bark the tires during the relearn with my foot on the brake so I know the transmission is strong. However, when I cycle the ignition off to allow the TCM to drop relearn commands and adopt the new fast learn calibrations the transmission still does nothing in reverse and drive. In GM GDS2 I can see the commands from the range selector commanding the voltage changes in the transmission, and there are no MILs on TCM.
My setup right now is bone ass stock E92 with VATS disabled in the ECM (ECM has all the latest software from GM as of 12/29/20). I have swapped in a new locked T87A so it is very stock with latest software and GM calibrations for VIN and trans TUN. I do still have my HPT unlocked T87A TCM.
Has anybody ever encountered a stubborn 8L90e that will not move when commanded through ranges? Any help is appreciated. I have access to TIS2Web, GDS2, SPS, and HP Tuners. Thanks!