Specific 8-speed shifting question

Dunc · October 4, 2016

I'm well aware of the issues with the 8L90. My question is does anyone with the 8-speed trans (even better if you have a 6.2L) experience a ridiculously harsh downshift from 8-7 on the highway between. Harsh to the point the entire car jolts and the interior trim sounds like it wants to come apart. The jolt could be best described as: say you are in neutral in any automatic car, you rev the engine to say 2k rpm and then shift into drive before letting the RPMs drop. That jolt you'd feel is literally how harsh the 8-7 shift is and not just sometimes, but everytime. Other shifts e.g. 7-6, 6-5, 2-3, etc are butter smooth and great.

I just wanted to check before I go in to take the service manager for a ride to show him the issue on Thursday. Without going into what's already been done I will say that this will be my 4th trip for the transmission (it's been completely replaced as well.) The last time the tech told me it drove like every other customer's car and there was nothing else for him to do because it was normal. There is just no way I believe that because GM would not sell a single Sierra if this was normal. I'm ok with dealing with some minor quirks in shifting and an occasional clunk. I realize things aren't perfect and I'm not expecting the moon or exaggerating a small non-issue. This is just way over the top bad and unacceptable.

BlueCrayon · October 4, 2016

Yes mine (6.2L 8 speed) has done this several times....I feel like when you let off the gas and then give moderate throttle and at the same time might be switching back into v8 mode....on the plus side, this was mostly in the first 2000 miles, now at 4000 and I don't think it's done it more than a couple times since. The bigger problem I have is downshifting into 1st coming to a stop, it shifts hard sometimes.

Dunc · October 4, 2016

Yes mine (6.2L 8 speed) has done this several times....I feel like when you let off the gas and then give moderate throttle and at the same time might be switching back into v8 mode....on the plus side, this was mostly in the first 2000 miles, now at 4000 and I don't think it's done it more than a couple times since. The bigger problem I have is downshifting into 1st coming to a stop, it shifts hard sometimes.

Agreed with the coming to stop thing. Mine does that, but that is something I can live with as it doesn't always do it. As for the 8-7 shift, I can even preload things by accelerating in 8th and then just barely given it enough to switch it to 7th and the trans feels like I neutral dropped it. It also does it if I just get on the throttle immediately. It does it everytime no matter what I do differently trying to make it stop yet all the other gear shifts are fine which is more than enough to tell me that 8-7 isn't normal.

ltzsilverado · October 4, 2016

I have the same issue. I have the 5.3 tho. 8/7 shift is always hard like where you feel the whole rear end shake and it just can't be good for the gears. I love the 8 speed I just wish gm could learn how to program these damn things better

Z45 · October 4, 2016

Mine never has done that in 27,000 miles.

Dunc · October 5, 2016

So last night going to my soccer game the 8-7 shifts were just as smooth as the other shifts. I drove no differently and was trying to get the harsh shifting to happen but I was unsuccessful. On the way home it started doing it again like normal. I'm at a loss for now. Hopefully more is found tomorrow at the dealer.

Z45 · October 5, 2016

So there is nothing wrong, its probably just the way the computer is responding to the way its driven. Not blaming you, its just that you are driving a computer.

If a problem it would occur with regularity, not occasionally.

Dunc · October 5, 2016

So there is nothing wrong, its probably just the way the computer is responding to the way its driven. Not blaming you, its just that you are driving a computer.

If a problem it would occur with regularity, not occasionally.

It is regular. Last night was definitely a fluke. It does it on every 8-7 shift since the transmission was replaced roughly 5,000 miles ago. This issue was likely masked since the original trans wasn't in spec and therefore I wouldn't have noticed it. If it was like this from day 1 I would have taken the truck back within the 1st week for a refund.

I can also make it do it by putting the shifter into M8. Then when I press the button down once into 7th gear it is just as harsh which would eliminate my foot causing anything. Press the button again into 6th gear and it's a smooth transition.

Bayou-AllTerrain · October 6, 2016

I've only got a little over 1K on mine but haven't had a single harsh shifting problem yet. Neither when coming to a stop or the 8-7 like you are asking about. Maybe they have updated tuning on the 17s. My 2014 5.3 did the 3-2 or 2-1 thing occasionally.

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Sir Dan · October 7, 2016

2016 5.3L 8 speed here. I have the harsh 2nd-1st downshift as well. It's like it can't decide if you are going to stop or get back on the gas so it waits till the very last minute and your wheels are nearly stopped and then wham! It just seems like they cannot get the shifting profile right...ever. There is talk of a CVT drivetrain coming soon for GM light trucks and that might be the only solution to all these shifting woes which seem to be getting worse the more gears that they add to try and get better MPG.