I recently swapped a 4l80e in my 2004 RCSB silverado. Rebuilt the trans myself and it was my first one. Sonnax boost valve, reemed and replaced AFL valves, new clutches and steels, dual fed the direct, billet input shaft, billet forward hub, rollerized output. Triple disc converter. Followed the Nick's Transmission videos on youtube very closely during the build. Segment swapped the p59 ECM. It has reverse and at least has first in D3, D2, and D1. I do not yet have a gauge to check line pressures, should be delivered today. When the shifter's in Drive, HPT shows D4 and 1st gear commanded but the truck won't move. I've only driven it out of the garage and a few feet back and forth in the driveway. Dip stick has been calibrated 1/8" above the pan rail and trans has about 13.5 quarts of fluid. Re adjusted the shifter cable with no change. All I can think is a valve body issue at this point. Any insight?

I'll report pressure values when the gauge arrives.

 

Sounds like the forward clutch isn't clutching, for whyever. Could be something about the valve body or could be the clutch itself. Could also be, the low roller assembly, which is a sprag, is put in backwards. But I'm no expert, just my best guess.