I feel the need to post here and share that this info seems to have fixed (at least temporarily) my wife's 2017 Chevrolet Suburban with the 6L80.
We first had issues with this vehicle at 120k miles. On the interstate on a 5-6 upshift the transmission gained another neutral and the CEL illuminated. Pulled off to the side of the road, turned it off and back on, and everything was fine. Arrived home to find that we had a P0796 code for Pressure Solenoid C Stuck Off. Started doing some research on this and found mostly "it's toast replace transmission posts" as one often does on automotive forums anytime an automatic has an issue.
I checked the fluid level and condition and it was black and smelt terrible. Parallel to this while researching, I found this post on justanswer.com with GM's guided diagnostics:
https://www.justanswer.com/chevy/ij1...olenoid-c.html.
So at this point I order 4 gallons of Valvoline Maxlife Multi-Vehicle Dexron VI and 2 transmission filters and go about doing a flush in my driveway per the GM guided diagnostics. P0796 code is gone, clean fluid, no material in the pan. So then we drive it and start to very occasionally get a rumble strip sensation when lugging the engine at ~1000RPM in 5th-6th. So, back to the internet. I eventually find a thread on bobistheoilguy.com detailing the rumble strip sensation and how GM's fix is to flush 3 times with Mobil Synthetic ATF LV that is specific to Dexron, and one guy added a bottle of Lucas to the fluid and it went away. In an act of desperation I added a bottle of Lucas and while it improved it, it did not fix it. Further research led me to one guy on a forum somewhere say "just turn off lockup in 1st-4th and disable sliplock in 5th and 6th." This was a bit of a eureka moment for me so I download a 6L80 tune from the repository and start investigating how to figure this out.
After loads of research and some help from the above videos I diagnosed our issue as TCC slip, and want to better detail what is going on in this post as the above videos don't do a great job of it. The long and short of it is that GM has calibrated the TCM to pretty much always partially lock the TCC's wet clutch, allowing anywhere between 20-40 RPM of slip when cruising down the highway. Eventually this is causing early TCC's to overheat and warp, thus causing too much slip. GM's factory settings for the PID controller on the modulated line pressure are both too low and too slow to overcome a worn clutch after ~100k miles, and that is why many owners are now having the issue.
So, the tune I have attached to this post disables torque converter lockup altogether in 1st through 4th in an effort to preserve the little bit of life left in this TCC clutch, as well as demanding zero slip while in 5th and 6th lockup. In addition, I have increased the apply ramp rates so that as more slip occurs, pressure is added to the clutch much quicker than factory settings and increased the minimum pressure demand when in lockup to 417-470kPa. There is a noticeable improvement to drivability with this calibration and also very little slip reported by the TCM.
I'm unsure what the root cause of my P0796 code was but at this point it appears to be fixed. My assumption is that the deterioration of the TCC clutch caused the dirty fluid and contaminated the pressure solenoid line, allowing me to find this failure before it reached the point of no return. I don't want to advertise this as a fix once the rumble strip effect is there; I plan to install a new torque converter ASAP. I do feel this will mitigate the TCC clutch issue for the time being and will be utilizing this calibration with our new torque converter.
I also disabled AFM at the same time I changed the TCM settings, mostly just to prevent lifter failure.
I hope this helps someone else who is in the same situation I was 2 weeks ago.
Here is a log file at cruise with factory settings:
Cruise_Factory.png
Here is a log file at cruise with these settings:
Cruise_SlipFix.png