2016 Yukon 8l90 3 to 1 harsh downshift/jerk forward

[2NY]

I have tried everything I could between google and here to get this 3 to 1 downshift to quit lurching/jerking forward when coming to a stop. It's clear in the logs it happens at the moment it shifts down to gear 1 from gear 3. The only thing that has gotten it slightly better is making it shift from gear 3 to 2 then to 1.

I've used most of the advice here to disable the TCC unlock and locking through gears 1 through 6. I've tried both upping and lowering the shift pressures, desired shift times 3-1, and disabling the CT downshift. I'm very new to this all but I will say everyone's advice has gotten me a butter smooth trans from what it was. Even the shop I had the trans rebuilt at and a new TC installed couldn't get it right.

Thanks in advance!

[Alvin]

The timing of the downshifts are crucial.. Have you put them back to stock to see if it helps. Post a tune file

[2NY]

The timing of the downshifts are crucial.. Have you put them back to stock to see if it helps. Post a tune file

Yeah. Stock was the reason I started messing with it initially. The only thing that has gotten anywhere near a better feel is going 3 > 2 > 1 rather than 3 > 1.

Tune file attached

yukon_transtccunlock.hpt

[Estep40511]

There is an old update from GM for this in the software side. The first gen 8L90's had this issue. Believe the programming changes some pressures and shift timing. Dealt with this here at work for years when they first came out.

[2NY]

I'll have to check that out. Do you know where I can find that software?

[2NY]

Finally getting back to this. I've tried putting the tune back to stock with no success in this being fixed. Here is what I currently have setup in the tune.

[TransGo Robert]

Coming in late on this one, have you ever posted a log of all the transmission parameters capturing the problem when you come to a stop? I would like to look at it just in case this is not a command issue.

[2NY]

I'll get one in the next couple of days

[2NY]

Coming in late on this one, have you ever posted a log of all the transmission parameters capturing the problem when you come to a stop? I would like to look at it just in case this is not a command issue.

I did more research and found that many recommend (or it's at least necessary) to do an adapt reset / adapt preset after pressure changes. I adjusted some pressures on the 3 to 1 downshift, ran the adapt reset/preset, and then did a C1 learn. I did that a few days ago, and so far, it seems to be working much better. The first 15-20 downshifts of the day were always the worst, but it has gotten much better. There's still a subtle bump on the first couple of shifts, but I could deal with that.

Update tune and log file attached

[TransGo Robert]

I did more research and found that many recommend (or it's at least necessary) to do an adapt reset / adapt preset after pressure changes. I adjusted some pressures on the 3 to 1 downshift, ran the adapt reset/preset, and then did a C1 learn. I did that a few days ago, and so far, it seems to be working much better. The first 15-20 downshifts of the day were always the worst, but it has gotten much better. There's still a subtle bump on the first couple of shifts, but I could deal with that.

Update tune and log file attached

FYI I looked up 9 logs of 8L90s that I have and all of them command it to go 3-2-1 on coastdown, never 3-1 like yours. No idea why it is doing that but wanted to let you know that all the logs I have from stock vehicles this does not happen.

[2NY]

Just remembered to check back here. That's so weird! I've never seen it not that way. Do you have a stock file that you could send me? I wonder what else is different and how the MPH winds down for the 3, 2, 1.

[TransGo Robert]

Just remembered to check back here. That's so weird! I've never seen it not that way. Do you have a stock file that you could send me? I wonder what else is different and how the MPH winds down for the 3, 2, 1.

No I do not, I pretty much only look at logs that others take. I don't take or save tunes or stock files. I would look in the repository to see if you can find one.

[GHuggins]

To the OP, did you do all of the original tuning on this? You have some changes on the engine side that could be making this problem worse. You can use this one for comparison even though it's an escalade, but your rate limits and DD tables in particular can be making your problem worse. You also have some odd changes going on in the transmission calibration. Almost looks like personalization of numbers that some tuners will do to try and brand something.

[2NY]

Greg,

Thanks! I'll take a peek at this. The engine side stuff came from a tuner, and I haven't really messed with it. I just pulled his engine tune into HPT, and mated it up with my stock trans tune (or what I thought was stock) and started making adjustments to the trans side based on what I read here. It's definitely personalization on my end to serve what I wanted it to do.

A small piece being that I get a lot of chatter when the TC is unlocking and locking upon accel. Even I the highway when decel, once I accelerate again, it would lock up from barely touching the pedal and you could hear an audible each time and feel it.

[GHuggins]

Chatter? Vibration? You can raise the slip - not 0 it out like people normally say - to help alleviate that. The converter will have a shorter life, but sounds like it's already having problems anyway.

For customers having this issue I find putting the DOD tables into the regular tables will help a lot.

[2NY]

I actually had a new converter put in. It was a HD according to the trans shop that did it. I was hoping to get ahead of any issues and let them do the tear down to replace anything that was necessary.