[EMTNLSS]

I've only had this bump happen once or twice when I had under a few thousand miles on it. It happened when starting out slow and going from 1-2. I now have about 23k on it and haven't had it happen since those first couple thousand miles.. 1-2 shifts can be a little weird sometimes when it's cold but once it's warmed up I haven't really had any issues.

[Kise]

I have this issue occasionally and let me tell you, its disgusting. used to happen to my 13 regal GS so bad I thought the transmission would go out. it only happens when things like going really slow up my driveway and it hardly shifts out of first, once I move around on the street a bit a downshift will come in hard. im glad im not the only one with this issue though lol.

[SixgenSix]

Mine JUST starting doing the BUMP when coming to a stop and right before I come to a complete stop there is a clunk from possibly the driveline/rearend? I imagine that bump is when the transmission downshifts into 2nd. My car has 10,600 miles on it.

Back around the 250 mile mark, the dealer did the update to the programing of the transmission.

I'll be making a appointment with my dealership.

[Risky Justice]

I have experienced this in my car as well, but I know it isn’t the transmission downshifting...because I have a manual. My thought was one wheel coming off the ground briefly, but it always felt more like a little tire slip to me. It could be described as a bump feeling too though.

[LtColumbo]

The other day, going into the same driveway that usually sets up the BUMP, but I had to stop just before entering the driveway because someone was taking up all the space as they were coming out, there was no bump. Then at some point when releasing the brakes and pushing on the gas...BUMP! Yep, after coming to a complete stop! This has nothing to do with lifting a wheel on my car.

[LtColumbo]

I turned off T/C and stability control. Going into this same driveway, I only felt a very slight bump.

Then the next time I went in, with those turned back on, I looked at the dashboard as I entered the driveway. As I drove in, got the bump, the stability control light was flashing! I can't see any reason for that to intervene.

Something else for you guys to try.

[Drsagacity]

It would be interesting if you manually shift where the “bump” normally occurs. Manually shift down to first earlier and see if the problem still happens.

[kkriskal]

I have a 2018 2SS Automatic and mine does the exact same thing. I did a lot of mountain driving last week in Colorado (Ouray, Silverton, Breckenridge, Aspen etc.) and while going up the mountain, everytime there is a sharp curve and you slow down and just when you gently, very gently put your foot on the gas, it does that as if it is not sure which gear it needs to be and just downshifts in a big mad rush to give you that boost. So looks like how the software works.

[njweatherman]

2018 2ss, automatic. I feel the rear-end bump (like getting rear-ended) every now and then while stopped at a red light and then taking off.

[JaxChris]

Edited for clarity:

If you're talking about "almost" coming to a complete stop as you turn into your driveway, it is a bug in the wheel sensor reporting causing a premature downshift on slow right hand turns. It's not a mechanical issue and affects all automatic Camaros, including the ZL1. Instead of downshifting 3-1 at 0mph+1 second, it is downshifting 3-1 at 2.5mph when making tight slow turns. GM made a lot of performance related tweaks to certain models, in the name of lap times, but at the expense of driving pleasure.

The wheel sensors are not being averaged in 'normal' operation. Instead gear selection is made using the slowest drive wheel sensor only. During tight turns you have a problem. You just started moving forward from a stop, the car tries to do a 1-3 upshift quickly when in ice/touring/sport to minimize unnecessary torque, then you make a tight turn (driveway or parking lot) and now the rear inner tire is rounding off to 0mph while the front outer wheel is moving at 4+ mph to turn you. You just completed the 1-3 upshift and now that one sensor is causing the TCM to perform a 3-1 downshift, as if the vehicle is deadstop. And the vehicle starts to lung, the downshift causes torque to increase and the vehicle either jerks or chips the rear inner tire in response. If you're in a parking lot you plan to move more than the 35-40 ft of your driveway, so your foot stays on the gas and you upshift again. If you're in your driveway, you've conditioned yourself to just left off the gas and hover the brake as the car pulls itself up the driveway effortlessly in 1st gear.

You can try a couple items to test it.

1. Come in wide and a little faster to keep the rear inner wheel moving and then tap brake once you're straight again to avoid the 3-1 downshift
2. Place the vehicle in manual mode and place yourself in 2nd gear prior to making the entrance (the 2-1 downshift doesn't seem as premature as the 3-1 downshift -- likely because the car is able to idle in 2nd but not 3rd)

In an effort to shave small amounts of time off those lap times, GM appears to have chosen slowest wheel as the value for triggering downshifts. This has a benefit in 'Performance Mode' of deciding to downshift right as you cross the apex of a turn. Traction control and stabilitrak read each wheel sensor in order to function, but the transmission acts based on a single value provided by the BCM. It doesn't change which value is read regardless of which mode you are in.

Anything with the T87 or T87A TCM have this low-speed turn-in jerking. It would've been nice if with the T87A that GM would've added a different measurement for Performance Mode which would've used slowest wheel speed and then all other modes relied on average wheel speed (like the speedometer does). Sadly, I don't think the memory space was provisioned for dual mode source values. None of the settings I've seen HPtuners unlock for the Camaro show the transmission parameter source values. Could be that requested values are hard-coded and there is no way for us to tell the TCM to ask for something different.

[cmitchell17]

Originally Posted by JaxChris

Edited for clarity:

If you're talking about "almost" coming to a complete stop as you turn into your driveway, it is a bug in the wheel sensor reporting causing a premature downshift on slow right hand turns. It's not a mechanical issue and affects all automatic Camaros, including the ZL1. Instead of downshifting 3-1 at 0mph+1 second, it is downshifting 3-1 at 2.5mph when making tight slow turns. GM made a lot of performance related tweaks to certain models, in the name of lap times, but at the expense of driving pleasure.

The wheel sensors are not being averaged in 'normal' operation. Instead gear selection is made using the slowest drive wheel sensor only. During tight turns you have a problem. You just started moving forward from a stop, the car tries to do a 1-3 upshift quickly when in ice/touring/sport to minimize unnecessary torque, then you make a tight turn (driveway or parking lot) and now the rear inner tire is rounding off to 0mph while the front outer wheel is moving at 4+ mph to turn you. You just completed the 1-3 upshift and now that one sensor is causing the TCM to perform a 3-1 downshift, as if the vehicle is deadstop. And the vehicle starts to lung, the downshift causes torque to increase and the vehicle either jerks or chips the rear inner tire in response. If you're in a parking lot you plan to move more than the 35-40 ft of your driveway, so your foot stays on the gas and you upshift again. If you're in your driveway, you've conditioned yourself to just left off the gas and hover the brake as the car pulls itself up the driveway effortlessly in 1st gear.

You can try a couple items to test it.

1. Come in wide and a little faster to keep the rear inner wheel moving and then tap brake once you're straight again to avoid the 3-1 downshift
2. Place the vehicle in manual mode and place yourself in 2nd gear prior to making the entrance (the 2-1 downshift doesn't seem as premature as the 3-1 downshift -- likely because the car is able to idle in 2nd but not 3rd)

In an effort to shave small amounts of time off those lap times, GM appears to have chosen slowest wheel as the value for triggering downshifts. This has a benefit in 'Performance Mode' of deciding to downshift right as you cross the apex of a turn. Traction control and stabilitrak read each wheel sensor in order to function, but the transmission acts based on a single value provided by the BCM. It doesn't change which value is read regardless of which mode you are in.

Anything with the T87 or T87A TCM have this low-speed turn-in jerking. It would've been nice if with the T87A that GM would've added a different measurement for Performance Mode which would've used slowest wheel speed and then all other modes relied on average wheel speed (like the speedometer does). Sadly, I don't think the memory space was provisioned for dual mode source values. None of the settings I've seen HPtuners unlock for the Camaro show the transmission parameter source values. Could be that requested values are hard-coded and there is no way for us to tell the TCM to ask for something different.

Thanks for the explanation! Its so nice to actually get real answers. What do you think GM's driveability benchmark was? It seems like they only care about wind noise and other NVH stuff but driveability is the last priority. I may be biased since I haven't daily driven enough other cars like Toyota's, which are supposedly perfect in every way, but if I wasn't used to it and had a passenger it would be embarrassing driving in stop and go traffic at 5mph when letting off the gas and all the backlash in the driveshaft and rearend is taken up and all the negative torque in first gears throws you forward then slams you back. And then you have the really poor and very inconsistent shift quality of the 8L80 that decides it wants to shift way too hard at unnecessary times. The rolling downshift 4-3 clunk I think can be solved by not allowing the TCC to be locked during braking downshifts. Unfortunately I can't adjust the backlash in the differential.

Another NVH issue that they seemed to completely ignore is the drive to neutral to reverse clunk/rattle that feels and sounds like something is broken. I assume this is the rubber isolated carrier bearing allowing the driveshaft to clunk up and down. I drove a new 2017 ATS and it appeared to have the same issue but a lot less noise and clunk. I can't believe they overlook these issues, in this case it appears they may have been trying to isolate the driveshaft from a highspeed vibration.

I didn't mean to go on this long of a rant but I wish I would have noticed these things before I bought the car. I don't think my car is an isolated issue I think they all do it you just have to daily drive one enough to pick it up.

Anyway thanks for the explanation again, very interesting to know why this kind of stuff happeneds and what kind of trade off it is.

[KingLT1]

I believe some of the issue is the converter unlocking. When I tuned my trans to not lock the converter until 8th gear part throttle the issue went away. This is how they should of came from the factory being it's easier on the converter and much smoother driving. The downside is it lowers fuel mileage...and is the only reason why GM programs the converter to lock up in the lower gears. Sometimes the converter will hang on too long while locked and jerk when you slow down or round a corner. I also believe some of it is related to what Jax Chris mentioned, but I do know changing how the converter locks seems to have eliminated the issue for me.

[arpad_m]

Originally Posted by JaxChris

Anything with the T87 or T87A TCM have this low-speed turn-in jerking. It would've been nice if with the T87A that GM would've added a different measurement for Performance Mode which would've used slowest wheel speed and then all other modes relied on average wheel speed (like the speedometer does). Sadly, I don't think the memory space was provisioned for dual mode source values. None of the settings I've seen HPtuners unlock for the Camaro show the transmission parameter source values. Could be that requested values are hard-coded and there is no way for us to tell the TCM to ask for something different.

JaxChris, thank you for the detailed explanation, it's absolutely enlightening and makes total sense. On the same note, are you sure this hasn't changed or been adjusted during MY 2018?

The reason I'm asking is that I observed this exact behavior to the dot with an 8L45 every single time when making a sharp right or left turn into my inclined driveway, but my current 8L90 has been somewhat surprisingly "bump-free" from day one, on the same roads and the same driveway doing the same maneuvers. I don't have to give it any special treatment, it just shifts properly and the car never jerks or bumps, regardless of the speed of approach,deceleration rate or turn angle/radius (I did perform a few experiments to make sure).

[cmitchell17]

Originally Posted by KingLT1

I believe some of the issue is the converter unlocking. When I tuned my trans to not lock the converter until 8th gear part throttle the issue went away. This is how they should of came from the factory being it's easier on the converter and much smoother driving. The downside is it lowers fuel mileage...and is the only reason why GM programs the converter to lock up in the lower gears. Sometimes the converter will hang on too long while locked and jerk when you slow down or round a corner. I also believe some of it is related to what Jax Chris mentioned, but I do know changing how the converter locks seems to have eliminated the issue for me.

Yes not allowing the TC to lock when braking fixes the issue. I guess allowing the TC to be unlocked lets any bind up to just spin the TC backwards without reversing the whole driveline and have it bump and clank. I guess this shouldn't be much of a problem if the 8L80 shifted better but I think on especially the 4-3 it is engaging the oncoming clutch to fast and causing it to bind up and reverse for a split second. Its funny that if you brake a little harder it dosent do it and same goes for if you just let it coast to a stop without the brake. Hopefully if I can get a unlocked TCM hp tuners has the ability to force the TC to unlock during downshifts since I like the TC being locked while crusing. I also wonder if the TC is locked during part throttle upshifts as well and if we can force it to unlock during upshifts to get better shift quality.