Drivetrain "slack" feel during transients - actually driver demand issues?

I have been dealing with the annoying things GM has done one by one. One of the last ones that is bugging me is the feel of drivetrain slop when you go from zero throttle to any throttle. Those transients lead to a jolt.

After doing a little research and playing around, I narrowed it down to the driver demand table as a possibility to help. When you go 0% throttle it commands a negative torque. So on and off the throttle you basically command negative and positive torque - engine slowing you down or speeding you up.

Now, keep in mind the negative numbers will help avoid the "run-a-way" feel and in cases where it is too positive, it may actually accelerate you if you are are 0% pedal position.

I looked at earlier truck calibrations and the 0% pedal torque was about 2x what mine is (2018). I ended up leaving the top end of the driver demand table (around 80 MPH) nearly the same. Interstate driving felt fine. The 40-60 MPH range was rough.

I increased the value to around -400 in other areas and did some smoothing. So far it seem to have made a big improvement. I am playing with the balance of no jolting while still maintaining decent speed control. The 50 MPH range is tough if it is in 8th gear. +/- 5 MPH outside of that it is fine. I have also noted improved fuel economy from better coasting, quite frankly, actually being able to coast a little before essentially engine braking. I used to be able to get off the interstate at 70 MPH and coast to a stop by the end of the off-ramp - sometimes even having to get back on the throttle. These were flat 0.25-0.50 mile off-ramps.

Anyway, long story short, I was curious if others have tried the same. If not, I am offering this up as a potential solution to see if it would help others. Here is what my current driver demand is (attached).

I should also note I left tow mode along in the 0% and 2% rows. I wanted to maintain engine braking while towing and also give me the "emergency exit" if I had issues during my tweaks. I also put Amsoil in the rear. I heard that helping others with the slop, if that was the issue, and figured why not give it a shot. It didn't really help once the fluid was up to temperature.

EDIT - for some reason I can't get a file to attach...

Not to discourage you, but no one else seems to be bothered by the slop in the drivetrain including GM, because I think they could have programmed it completely out.

To me though it is way way worse a problem in first gear at really low speeds. If the engine goes to negative torque and loads in the opposite direction, then when you go to try to apply the gas it takes a second or so if very very lightly pressing it as slow as possible to lessen being jolted forward by the extreme sensitivity of increasing torque followed by a clank noise from the driveline. It just baffles me that GM allows this to happen and didn't put in some kind of torque dampening that say only applies in first gear.

That being said the fact that the newer models are in wheel torque that should effectively add the dampening and only apply it in first and maybe second. So the torque request rate limit should take care of it but I've noticed no difference messing with it, I've also tried Cadillac values hoping it would turn my drive ability experience more luxurious haha.

The worst part about it is that sometimes I believe the torque rate request and the properly tuned driver demand tables actually do work and prevent clunks and violent jerks but it's always so inconsistent when it works or not along with the random inconsistency of the TCM and transmission clunks add to it.

I don’t have and issues outside of light throttle. I was having issues in 1st and 2nd, I was able to eliminate all that with shift points and inertia, on top of driver demand.

It seems to me they used modeling for most of these tables and just called it done. Not sure how much correlation they have done in the real world. I love how this thing drives and shifts now. Tweaking the TCC lock and slip was also another area that really helped the “driving experience”.

My truck since tuned has had a lunge forward sometimes or in reverse from a dead stop usually after a restart. Been chasing it for afew yrs now. 2015 A8 6.2 btw. Also if im understanding you correctly the tcc lockout made a big diff? For me zeroing out the slip tables and taking tcc lockout off for 1-7 gears made the truck a world of difference.

I left mine locked. I tweaked the MPH and TCC apply. The factory table and feel was horrible.

Got a tune you care to share so i can compare? Cant remember if i left my 1-6 locked or unlocked? Would it make a big difference either way? I zeroed out tcc slip why would you want to leave that on anyway?Thanks

Sure, I am having issues on here attaching files, but feel free to PM me your email.

I set my TCC apply/release to align with the part throttle downshift table, so I set it to where when coasting down (when TPS is 0%) and the transmission is downshifting, it will leave the torque converter locked but unlocked 1 mph higher than the downshift speed. So basically I made the TCC release speeds 1 mph higher than the corresponding downshift mph for that gear.

Before I had the apply and release for the 0% TPS column for 3-8th gear all at 314 mph for both apply and release, this was to ensure the TCC would be unlocked when coasting down and downshifting and would allow the unlocked torque converter to "absorb" all the sloppy shifting and tie-ups/binding (especially the 4-3 downshift). So when the TC is unlocked you can't feel the bumps and jolts and clunks and clangs from it. Only problem with this method is that if you were coasting and then you suddenly got back on the gas the TCC would be unlocked for that instantaneous moment and would allow the RPMs to jump up and then the torque converter would instantly relock, so it was kind of annoying and felt like something was wrong or broke.

The new way I did it to my surprise seems like it works, so you get the benefit of having the TC basically unlock only right when the transmission downshifts, then it relocks. This should allow DFCO to remain on as well.

So is that the tcc apply release you used? I cant tell which setting that is? I played around with the torque converter -apply/release -speed normal section and set mine run unlocked till 8th gear unless im looking at something wrong?Thanks

Not tuning related,...but I've noticed a big improvement after installing a set of Rough Country traction bars. My truck isn't lifted nor do I intend to lift it. They are a bit obtuse looking, but nothing too offensive.

By keeping the axle in a fixed position and essentially limiting the rotation upward and downward, it has made my drivetrain feel so much more solid. Almost feel like a Ram with a 5 link rear end - 'almost' lol. My trans tune is bone stock but it honestly made my 8L90 feel SOO much better. I think a lot of the complaints about the trans would be nonexistent if our trucks didn't have leaf sprung rear ends. Everyone praises and praises the Ram's ZF trans, and while it might be awesome and be tuned really well, I think a lot of it is because of the 5 link.

That soggy feeling is actually the product of a combination of algorithms that prevents transmissions from eating themselves alive in short order. I don't like it either, but it's a necessity when modern transmissions have 8 and 10 gears. To produce that many ratios, you have to increase the number of gearsets in the transmission which increases the lash in the transmission. 'Crossing lash' is a huge challenge in terms of driver demand torque shaping and managing torque during shifts. I dealt with this problem a bunch on the 9HP48/948TE when I worked at Chrysler. I remember having to plan ahead coming down the Eisenhower grade in Colorado because that transmission has a dog clutch between 4th and 5th gear that requires essentially 0Nm of torque to make the shift. Pretty sketchy when you want to downshift to 4th to slow down and your vehicle speeds up by 10mph down a steep grade lol. But that's what's required to make the transmission live.

Engines these days are pretty complicated, but transmissions are straight nasty by comparison. If you want to get rid of the dead pedal, you should be looking at fewer gears :)

I don't have any dead pedal feel or soggy feeling, not sure if you are referring to the original topic or another.

This is about transients from negative to positive torque. After cleaning up the driver demand and also a step change in the TCC slip table, I have been able to eliminate nearly all of it. The only questionable area was 0% TPS in 8th gear right before the 8-7 downshift. I can still feel a little there, but I would prefer to leave the negative torque I have for good speed control without having to shift from 8-7 sooner. My shift points for 7-8 and 8-7 are based on my most common cruising speeds and has helped fuel economy.

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Originally Posted by 15ltz6.2

Got a tune you care to share so i can compare? Cant remember if i left my 1-6 locked or unlocked? Would it make a big difference either way? I zeroed out tcc slip why would you want to leave that on anyway?Thanks

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Try these TCC tables.

The backlash I am referring to always seems to me at least is coming from the ring and pinion gear since you can either hear it or (with trucks) put it in 4WD and it goes away. I had a duramax/allison with the 11.5 14 bolt and it had so much slop it felts like literally seconds between negative torque and going to positive torque and on top of that you had the dmax/allison combo that already produced tons of negative torque it combined for a driveability nightmare. Of course this is a heavy duty truck and I am sure they build in tons of slack in the 14 bolt to account for towing heat.

It does it worst in the alpha Camaro especially when getting back on the gas from a turn you will hear the clunk/clang. However in the Camaro it is harder to reproduce and replicate and very intermittent, so maybe that's the ECM/TCM trying to do its job. So it does seem like its a extremely difficult thing for the controller to do. It makes me wonder if the "throttle progression" table has something to do with this, although I wouldn't think we would need to modify it from stock unless we had completely different drivelines.

I've tried enabling the "torque shaping" parameter in the TCM file but I can't see where it does anything, and interestingly enough, it is disabled in every GM calibration I have found, including Cadillac's where you think they would enable it.

Also to the OP, I would trying turning off DFCO to see what it does, then you could maybe raise the RPM threshold, in theory this would stop a large negative torque then instantly have to go to positive when stepping on the gas and causing a clunk/clang.

I have disabled DFCO without much change. Oddly enough, deep decel was never the issue. It was the light on/off throttle stuff. I think a lot also had to do with how fast it ramped up from 0% to 8%.

The ultra high fast ramp up rate of torque at low crawling speeds, low rpm, and low throttle is way too much from what it seems like to me. This seems like it makes the problem 80 times worse, especially when you let off and have a high negative torque. You end up getting thrown back in the seat, then your head gets thrown forward from the negative engine breaking. However, even though that high negative torque value is in the DD tables when you let off, I don't see how the engine can meet it, I mean once the engine revs down to near idle it can't cut the pulsewidth any more or it will run lean, how else could it decrease torque? Yes it can go into DFCO but then you get this discontinuous jump in torque which you can feel and it probably an over request in negative torque then, so the engine ends up cycling in and out of DFCO and I can feel every single cycle in and out.

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Originally Posted by 8850

Try these TCC tables.

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For some reason I can't open this file. Maybe it's because I'm a newb and doing something wrong.

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Originally Posted by lucas287

For some reason I can't open this file. Maybe it's because I'm a newb and doing something wrong.

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You have to download the beta version.

TCC apply/release made a huge difference in the older 4speed transmission. Haven't played with it since then, but in the 4speeds the tcc /apply/release was actually a type of overdrive you could actuate in 2nd 3rd and 4th gears. Like mentioned above, have to correlate those tcc tables with the part throttle shift speeds at different throttle positions.