Background; I installed a 6L90e in a 08 VMax that came with a 4L70e. After a few small issues I got it sorted out and it worked well. But one fine day the TQ converter stopped locking up. Not all the time, it seemed to have a mind of its own. It would lock up 10-15% of the time tops but usually only after a restart of the engine. Meaning I'd drive it somewhere shut it off and the TQ would engauge after starting it. That didn't mean it would stay engauged though. So after scanning it, I find the TCM isn't commanding the TQ lock up. (This is with the stock GM file) I was thinking of replacing the TCM but my friendly GM tech says the the ECM commands the lock up. But I think the ECM provides the signals for RPM, TP, and MAP. Even though EFI doesn't have a MAP reference table for shifting I think it is a variable in shifting and TQ lock up. As playing with VVT can really mess with the shifts via causing the TVS bypass to close and build boost at very light throttle.
The tech I'm working with says he has never seen a bad TCM so he doubts thats the issue but I'm not sure what else it could be. I'm using a 08 GMC Denali PU Vin and OS. I reflashed the BCM (No effect), and was thinking of reflashing the Gauge Cluster. But the fact that it worked fine for a few months has me puzzled. The GM tech line was very helpful but couldn't offer any options that worked either.
That's the end of my Rant..
Thanks