Quote:
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.
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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.