I don't think anyone outside GM knows exactly what's going on with these A8 transmissions. I had the shudder, but not the delayed engagement or rough shifts. Mine started shuddering at 9300 miles and had the TC replaced at 13000. It was fine for about 500 miles then started shuddering again. I put a Range Technology V4 eliminator on it and it ran fine, albiet in V8 mode only. The TC did not exhibit any other symptoms.

 

Maybe it has something to do with vibration or harmonics in V4 that GM is still trying to get a handle on.

Interesting. How many miles did you drive it with the Range before selling?

 

Could be, but in the engine or transmission?

2000 or so. It was actually pleasant to drive, but you have to pay attention to how long the car sits with the Range plugged in.

 

Looking at the data, 2015 isn't looking so hot.

 

That's what I've been trying to wrap my head around. The A8 exhibits none of the issues in V8 mode. The second I moved the lever to "M", thus defeating V4 mode the symptoms instantly disappeared. Move the level back to "D" and here come the shudders. So what is it about V4 mode that causes these torque converters to fail? Programming issues would be resolved with a reflash but this is a mechanical problem. I think your theory is a good educated guess. Only GM knows for sure.

That's a question that I would love to ask Tadge.

 

I remember watching a video in which Tadge was explaining what they had to design into the car in order to mitigate the vibrations generated when the engine is running in V4 mode. Can't remember now what it involved but perhaps this is what is causing the problem in addition to issues with the TC and stator support. Who knows? Maybe someday.

 

^^^ Only that they used a transmission with a conventional torque converter. Not the design of the converter itself, (which appears may contribute to the problem).

From R&T on the Z06:
"GM wanted to maintain cylinder deactivation on the LT4, and a traditional torque converter helps dampen the vibrations that accompany the transitions between V8 and V4 mode."

 

What's there to be impressed with?

Being a very technical guy who has experience driving a variety of newer TC autos and DCT's across quite a few platforms, I'd love to hear a technical answer of what they are impressed with.

That's correct, they were more or less forced to use a TC auto because of the ability to micro slip and dampen the transition.

 

Can't speak for everyone, but I am impressed with the speed and smoothness of the shift changes and how when left in "Drive" in Sport or Track mode, the transmission has the almost uncanny ability to select the right gear for the immediate situation. Also, full throttle shifts and the Performance Algorithm Shifting (PAS) & Performance Algorithm Lift Foot (PAL) programming is pretty cool.

Maybe I'm too easily impressed.

EDIT: Not a very technical response, I know.

 

No, buts it's ok.

PAS/PAL and holding gears in a sport mode or algorithms based on throttle input is not unique to the 8l90, unfortunetly. Also has everything to do with programming and the TCM, not so much the transmission, itself.

Programming gimmicks that make the cool features. I could say the same about Lexus' Aisin in the new F's (from the LS460) and the ability to utilize G sensors to optimize downshifts. Lexus also claimed the Aisin 8 speed was the "worlds fastest" at .1 shifts.

As I mentioned in an earlier post, maximizing the upshift speed to "beat" the PDK was it's only claim to fame, and that was all programming/controller. The upshifts are fast, but only at WOT in D.

TC's auto just aren't DSG/DCT's. You can't overcome the mechanical advantages wth programming/controllers, IMHO. The shifts are close and GM, Aisin and ZF have maximized the TC auto and made it as good as can be.

But I still think the slight edge will always go to a DCT, despite the lower gears not being as "user friendly" and smooth.

 

Harmonics or vibration from the engine being transmitted to the A8 and causing the TCC lock-up mechanism to malfunction.

 

Could you please actually read the posts before commenting? I used 49% as an example of someone saying its not a problem because "most" people haven't had an issue.

You want I want??? I want GM to actually give a @%#&@#% about customers that they've lied to and caused lots of headaches for. How about they actually correct things that they already admitted is a problem? How about they give some indication that they actually want customers to ever buy another car from them again.

Go away?? I PAID THE SAME MONEY EVERY ELSE DID AND ACTUALLY EXPECT THE CAR TO DO WHAT WAS PROMISED. So really, how about you go away since you contribute absolutely nothing?

 

I don't think it is unusual to expect owners to post everything when their car performs as it was designed to perform. That would be a pretty boring forum. These forums are supposed to provide a venue to discuss and resolve issues and problems like mature adults. Let's continue to do that.

 

^^^^^^^^^^^^^^^^^^^^^^^^^

exactly !

 

So maybe the transmission is micro slipping when there is no need to do so because the engine is in V4 mode and not transitioning between the two. I wonder if that is software controlled or uses a mechanical signal to time the micro slip.

 

I don't think so, and I agree--or maybe we're both too easily impressed. I'm very pleased with the performance of the A8 for the reasons you stated and would be a happy camper if it weren't for the issues I've encountered over the past two years, which is the V4 shuddering and the delayed engagement. The latter has not been a big issue in my car and happens only about every three months and actually seems to be improving over time. If my A8 stopped the shuddering and had a delayed engagement every so often I would label it a quirk and live with it as long as it didn't get any worse.

Amen and pass the collection plate.

 

It uses programing to electronically control fluid pulses that quickly engage and disengage the clutch causing "slip" that will reduce the shudder. Its likely not physical slip, but slip caused by quickly engaging and disengaging the clutch. However, some physical slip will also occur, but they try to reduce this because of clutch wear. Shudder is occurring because of V4 mode. There are other engine applications that cause vibrations where micro-slip is used to mitigate, such as small displacement and turbo. The micro-slip is probably not needed for V8 operation, but under low load it could be used to allow locking the torque converter earlier in order to improve efficiency. BTW, this issue is all about meeting the new CAFE/GHG standards by 2025.

There are valves controlled by solenoids that open and close to make the fluid pulses. I am just starting to look into this more closely, but I am assuming the valves are in the transmission and then the fluid flows through orifices in the TC.

I am guessing one problem could be that fluid flow in the TC isn't working as intended. Either not designed for all conditions being encountered, inconsistent manufacture of the TC, or partially blocked fluid flow. The other problem could be wrong choice of clutch material or it wasn't consistent during manufacture. Something I am not sure about is if the shudder is clutch chatter or the system not mitigating the vibration caused by V4. Maybe both issues are occurring making the problem even more complicated.

This is something I just started looking at in my spare time so I could be wrong and I welcome being corrected if someone has better info.

 

Sounds like a plan. But people actually do post about all the good stuff when a car first comes out. After that, not so much.

 

^^^ Very true. Posts soon after ownership tend to be very positive until the first problem arises, then we start to see the negative posts. This is human nature and is to be expected.

 

pulse width controlling the TCC could cause that i bet?

"hmmmn....hmmmn.....hmmmn....hmmmn.. .." no noise, just an oscillating sensation that i've experienced from time to time. but mine is so new i've noticed it when it had only 60 miles on it chalked it up to breaking in, maybe trans learning (locking converter too early) etc.

only on this once stretch of parkway and cruising from 65mph up cruise on or maintaining speed and slightly accelerating.

i even felt something like that accelerating slightly onto the same parkway stretch.

it happened a couple times when i would be in say 4th or 5th gear (in auto, coming off of a stop light at entrance to parkway) then gave it some gas, not alot or i should say not enough to kick the TCC out it upshifts just a weird oscillation reverberating throughout the car felt through the seat cushion. maybe that's what it is.. the TCC pulse width?

i'll be able to test it more in few months. if they wouldn't void my warranty i would have my friend who is a tuner play with the settings would love to try that but GM will have to address it should it happen again..

Only thing for this on my Z06 is traveling outside of this area several times I did not notice any of it.. so strange. might just be driving style vs programming like someone mentioned to get mpg's out of these transmissions?

my 2016 Sierra has an A8 in it and it has a noticeable oscillation at 75mph cruise.. that is the difference i've noticed so far with the Z06.. the truck does it on every highway in PA, NY NJ.. at that speed and beyond. it's probably there at lower speed but the NVH is not making it pronounced enough to notice... change gears? same, neutral? no change (the truck not car but same thing, no change when i tested the Z06). so that eliminates the TCC not kicking out on that truck at least.. killing that theory..

if you want to talk about a huge problem for GM? go on the truck forum it has 700 pages and 1.47 MILLION views. and thread is so deep it's insane. for 14-15-16 Silverado/Sierra. mind you the A6 and A8 same problem there leading one to believe it may actually be the rear or Driveshaft to thin? all different body styles, drivetrains etc.

I recall finding a couple guys having it rectified by new rear diff install. one guy had it done TWICE before it fixed the problem. new replacement rear defective but third rear worked.