GM 8-Speed Truck Transmission Tune (Low speed clunk)

[dantsr24]

Anybody have any tunes that they have completed for the GM 8-speed transmission they would like to share. I have a 2017 Sierra with a 6.2 that I am trying to tune. I purchased the Tuning School Transmission book and made adjustments, but still having a hard time with the low speed downshift from 2-->1. Anyway that can be smoothed out? Any help would be appreciated.

[fifty]

This I would love to know. My 15 yukon 8 speed does this

[staytuned]

I think GM would like to know how to fix that as well

[cmitchell17]

Haha it's seems like everyone else, zf and Asin, doesn't have this problem, they must be stupid apparently and just sabtoge the TCM to make random clunks and uncessary hard shifts.

To the op, I haven't looked at a truck 8 speed TCM tune much but I know the Camaro has at 0% throttle to apply the TCC in gears 2-8. I would change gears 1-6 all to 318mph, apply and release, this will let the TCC be unlocked on coasting/braking downshifts and absorb the sorry TCM programming that GM programs that makes the torque reverse from the mistimed clutch applications. I know you were talking about the 2-1 and I think the stock tune probably has the TCC already commanded off, but if you didn't know the 8l90 goes from 3rd to 1st on coasting downshifts.

[dantsr24]

So you saying only change the values to 318 at 0% throttle for those gears? Did this work for you?

[chameleon]

I actually just introduced this problem messing with the bluecat shift timing tables as opposed to what i had before, trying to figure out what exactly contributed to that.

[cmitchell17a]

This worked for me mainly on the 4-3 clunk, technically the clunk (torque reversal) is still there but since the TCC is unlocked it gets absorbed through it.Capture.PNG

[chameleon]

Setting the shift timing aka what speed the truck downshifts.. i had 2-1 and 3-2 all set at 4mph under like 12% throttle.. raising that up a tad pretty much eliminated it, so if it downshifts while your still rolling a bit you dont feel it but if it delays a tad and does it while your standing still then your going to feel that clunk and it was annoying as all hell lol

[dantsr24]

Awesome! What do you have your other tables set to? Pattern A & B?

[chameleon]

Awesome! What do you have your other tables set to? Pattern A & B?

their actually filled with some shift pattern i was using before which is a bit higher than what i have now, I still havent figured out what triggers the pattern A or B, so their both set the same but different than "normal" and so far i only notice the truck use the normal tables.