6L80 Harsh 2-1 Downshift if Rapid Removal of Accelertor Demand

[HammerFingers]

I rebuilt and tuned my 6L80 (by myself). HP Tuners / Bluecat, TransGo shift kit. Everything is working great except for one minor thing. When I go hard on the gas off the line and then decide to abort (say I get to 20-30 mph and quickly let off the accelerator during WOT), it gives me a harsh kick in the pants on the 2-1 downshift. If I ease it down from WOT, it does a nice soft downshift like normal. Is there a place in my tune where I need to look to tell it to go easy on the downshift even for a quick let off of the accelerator during WOT?

If I revert to a factory 2018 Silverado tune that harsh 2-1 downshift after WOT abort goes away ... but then upshifting and downshifting in general is sloppy and loose everywhere (as you would expect in the factory tune)...

[mikez71]

I'm thinking you mean 1-2 upshift?

If so, I notice it some as well since I raised the 1-2 shift schedule..

I've been wondering if adjusting the Torque Adder tables in the negative and 0 torque low rpm cell(s) would help. (adding time to shift)
That is the only part of the map that slowed down in 2015 truck tunes vs earlier tunes, everywhere else in the map, time was the same or reduced..

Or maybe the throttle follower needs tweaking.. (1st and 2nd look more like neutral than the other forward gears)
I'm just throwing this out there so someone who knows can tell me (us!).

And.. the people here who can tell you if anything is wrong, will tell you to post your tune.

[HammerFingers]

No, I meant downshift. The move from 1 to 2 (upshift) is smooth as silk even under WOT. If I abort WOT to hit the brakes quickly (because I need to suddenly stop), that is when the truck bucks on the 2 to 1 downshift (it is preparing me for slower speeds but it is doing it too aggressively). This often happens if I am giving it full pedal and then suddenly decide to stop or slow down quickly.

The 2-1 downshift is the only harsh shift I have (and that 2-1 downshift is only harsh if I quickly reduce pedal command from a WOT or near WOT state). All other shifts (up or down) are smooth at all times and all throttle conditions. The 2-1 downshift is also smooth if I am only part throttle and pull off the gas quickly to stop. If I am WOT and feather off the gas just a bit as I prepare to stop, I can also avoid the harsh 2-1 downshift.

There is a guy in another forum saying that there is supposed to be an 8th check ball placed in the valve body to avoid this EXACT condition ... (I have never heard of this.. but I am not a valve body expert...)

[mikez71]

Hmm... I don't get it. I can't imagine being at WOT, letting off the gas, and it downshifts..
If it downshifted, whatever your RPMS are @WOT, it would suddenly jump to even higher RPMS..?

Usually hitting WOT, causes the downshift(s) (unless you are in 1st.), and lifting throttle goes to upshift. ?

I've heard of that checkball before, but was under the impression it was an update in the later years.

[TransGo Robert]

If you really want help, include a log and the tune. If the computer was commanding things like it should, it would be a physical impossibility to feel a 2-1 coast downshift since the low sprag overrides. So either something is not right in your tune causing it to apply the L/R brake on coast down or something else is happening that just feels like a harsh 2-1 by the seat of the pants. It is all crystal ball diagnostic at this point with no log to look at to see what is really happening. As for the #8 checkball it has nothing to do with that nor did it apply only to certain model of vehicle, it is an update that was done to all of them no matter what it is in.

[HammerFingers]

Update: Well, you guys are not going to believe what the issue was... I had a local tuner look at my logs, and the first thing he said was, "Why did you not send any logs of the issue when the truck was warmed up?" and I said, "Huh?" He said my truck was getting to a max temp of 166 in the logs and to replace the thermostat before we did anything else.

I thought cooler engine temps would be a good thing!? Cooler means things are better, right!? The truck was not throwing any codes. No codes mean it is all good, right? Turns out my thermostat was stuck open, and on these colder days in WI, even after running for hours, the truck just would not warm up to the operating temp of 210 (which I never knew was a "problem"). I replaced the thermostat, and now the truck quickly warms up to 210. Although I can get one or two harder shifts while the truck is warming up, once it gets to 210, I cannot duplicate the shifting issue anymore at operating temp.

And even more unbelievable is I have taken some MPG measurements on a very familiar piece of highway I travel to work, and I seem to have picked up 1.5-2 MPG ... just by replacing the thermostat.

[Tfoggy]

Good deal !! Yup operating temps will help all settings get to their "normal" mode
I was going to suggest looking closely at your 2-1 downshift mph tables as well

[mikez71]

I've wondered about this setting...
Screenshot 2024-12-10 225840.png

But, seems like if the engine hits 166degF, the trans should be able to reach the same temp.. which should be in normal range.
which does not appear to be affected by that map.

But honestly, I do not believe in a 2-1 downshift after lifting from WOT.
Makes zero sense (to me). Share datalog or it did not happen!

Are you calling it a downshift because the revs drop?