Just wanted to circle back on my experience with the GDE trans tune and GDE after taking some time to do some additional tow tests. My memory was fuzzy about which shift/RPM drop was the problem, turns out it was the 1-2 upshift and not TCC lock as it was the first RPM drop, not the second.
So, I went to the barn today and hitched up the horse trailer to test D, L, and T/H modes. Here's what I experienced and what I've passed on to GDE.
Anyway, GDE is going to try and reproduce the harsh 1-2 upshift while towing but without T/H enabled. Even if there's no fix, I can at least work around the issue by changing trans or T/H settings to run where it and I are both generally happy. Going back to the stock trans tune is a last resort, and the main test is going to see what our off-road trailer tows like once it arrives next year. It's much lighter than the other trailers I tow/have towed and I'll do whatever needs to be done to make sure the truck is happy towing it as we're planning on putting a lot of miles behind us with it.
- D, T/H disabled, trailer hitched up, starting from a stop with like 30-50% throttle there's a thud/bump as it upshifts from 1-2. TCC lockup is fine.
- D, T/H enabled, trailer hitched up, starting from a stop with the same throttle input the thud/bump is much less pronounced and I don't see it as an issue. The difference here is that with T/H enabled the transmission isn't short-shifting for the 1-2 upshift and is holding 1st gear longer. The 1-2 upshift is firm but not harsh. The grade braking will downshift to the moon with minimal brake input even on flat ground.
- L, T/H enabled, trailer hitched up, slowing from 55 to a stop it doesn't trigger grade braking on flat ground like in D. Apparently it only goes into rage mode in L with T/H when there are actual hills and it thinks it needs to downshift because the truck isn't slowing fast enough?
It's interesting because my 1st to 2nd shifts are fine and I can feel (and see on my idash) when the TC locks and it's pretty smooth. the shifts are the same whether I am towing my trailer or not. I'm running the same TCM rev you are.