Well you nailed it. PCM was hanging the solenoid release. I actually changed the trans again and had the exact same symptom before I found it. The logic was commanding the shift, but the quad driver wasn't shutting off the solenoid cleanly.!!!!!
I did check the line pressure and it was in spec. Yes it's a stock ls1. The pcm would command the shift but it took 3 seconds after the command at light throttle to actually shift. I just finished rebuilding a good core to put in the car so I will find out if the pcm drivers are the issue. If...
I recently did a stock rebuild on a 4l60e that came in with a failed band and 3-4 clutches. I tested all the usual suspects, and replaced the 3rd accumulator check ball. With everything installed to spec I installed the unit to find no 2-3 shift at wot, and delayed 2-3 shift at part throttle...
Particularly at wot. Pulled it back out, clearances are still in spec. Before I pulled it I put another 3rd accumulator capsule in it and swapped the valve body with no difference.