Formula52

Hi all,

Wondering if anyone has posted a Shuddering A8 on a C7. I could not find one on YouTube.

I have a 2015 A8 Coupe that actually runs perfectly with over 12,000 miles on it. Keep hoping my luck holds out but wondering what it is like. I know people have said it is like going over those reflectors in the road when driving on a smooth road.

Thanks for any video's of this.

Steve

iclick

I haven't seen a video showing shudder and if it were available it might not reflect the sensation you feel when it goes into its shuddering pattern, but there are videos on the oscillating tach syndrome. In my case both usually occurred together but not always. If either happens it should be when in Auto mode and on the highway under light acceleration and in V4 mode. In my car it was quite obvious but not consistent for the first 18 months and never when near a dealer. After that it became worse and would shudder at any time the above conditions occurred. I had the TC/fluid fix done and it worked fine for the two weeks I owned the car after that.

felkebg

Yes, I have seen one. It is in one of 492 threads revealed by the advanced search tool on this forum. Good hunting!

NSC5

Steve,
The best early indicant of problems is in V4 mode at steady cruising speed when the tachometer indicates the RPM is starting to vary markedly when it should be nearly stationary. If you have PDR record part of a highway run in V4 mode with the recording screen set for sport overlay so the tach indication is overlaid on the video and this will make it easy to see excessive RPM variation.

Make a short run with the gear selection set to manual and cruise control on driving on a flat section of interstate. The tach will be nearly dead steady. Now go back to automatic and set the drive mode to eco to force V4 as much as possible and make a run with the car operating in V4 mode. You should see a tach reading almost as steady as you saw in V8 mode because the "micro-slips" programmed into the converter clutch control for V4 mode are so frequent and brief that the normal smoothing algorithm of the tach display removes the indicated variation. But if the clutch wear is becoming bad enough then you see the hunting/erratic behavior where the clutch isn't engaging surely and at the proper time so the tach responds with a much wider variation because the converter clutch is now unlocked for a long enough period of time that converter slippage approaches that of the converter running in fully unlocked mode. As it gets worse the RPM variation and irregular clutch behavior become felt as ever worsening shudder.

It is sort of funny that part of the "logic and controls magic" that makes clutch to clutch architecture transmissions possible is the ability to instantly measure and respond to shaft speed variation from what is expected. Part of the learning nature of the 8L90 and other C2C architecture transmissions is it "knows" how a shift is supposed to feel and when actual shift feel (measured primarily by the instantaneous speeds of the input, intermediate and output shafts) differ from desired it corrects through the timing and pressure of clutch pack application and release. But the V4 mode anti-vibration strategy means there will be a near random variation between engine RPM and torque converter output RPM that normally shouldn't exist and in "normal land" this variation would set a DTC and turn on the CEL indicating unexpected converter slippage when locked but in this case the random slippage is expected and normal up to a certain point but since the converters with truly bad variation and shudder aren't setting a DTC then it appears this potential diagnostic data point isn't used for the 8L90.

subfloor@centurytrans

Here you go, videos are in post #20:

https://www.corvetteforum.com/forums...le-strips.html

KemahGT

I couldn't feel or hear anything that would show on a video. My tach did a very rhythmic dance of about 100RPM when in V4 mode indicated on the mileage screen. Pull the shifter to M and it was rock solid.

If you can see it on the tach the technician will see it on his computer when he takes it for a test drive. I wanted a triple flush but the dealer changed the TC. Loaner was provided. Took about 3 days from dropping it off to pickup.

Without a substantial redesign of the TC clutch or software change I'm sure I will see the problem again. I blame CAFE and the GM engineers that designed this whole AFM/DOD crap show.

Walkedu

Its like that **** where a man just sits and watches his wife with another man.

Why would you torture yourself just to "see". LOL

lakemg

Here's a video of mine that shows slight RPM fluctuation, but I absolutely do not feel a shudder or hear any odd noises. If I didn't witness the fluctuations on the tach, I would not know that it was fluctuating...

Gearhead Jim

Our 2017 Z51 with about a thousand miles shows a variation almost identical to yours, nothing unusual felt or heard in the car.

Does GM have any specs for how much tach variation in steady cruise is acceptable?
The only bulletin I've found seems to use some kind of vibration monitor instead of the tach.

lakemg

^^^There were some earlier TSBs that led me to believe that some fluctuation is acceptable, but I can't recall what the value was. If I recall correctly, it was tied more to torque converter rpm slippage that a GM tech would have to measure rather than tachometer variation.

NSC5

Note that most (all?) commenters in that thread are referring to a brief rattle/flutter that only occurs with LT4 engines entering/exiting V4 mode and this is different than the transmission shudder that occurs with the LT1 and LT4 and occurs during the entire time the engine is in V4 mode.

The problem cured by the supercharger belt change is more heard than felt and doesn't create the constant RPM fluctuation like that caused by the degraded torque converter clutch in V4 mode.

The V4 related RPM fluctuation is fairly minor at first but will continue to grow greater as more miles and wear occur. I suspect that it begins to wear at a much faster rate as the degree or fluctuation increases in V4 mode because now you have the situation where the lock up clutch is now constantly attempting to engage/disengage with a much greater than desired RPM difference or "slip" between the torque converter input and output shafts.

Information in some of the GM TSBs indicates the converter clutch will remain locked in V4 mode above 1600 RPM but I couldn't find definitive information about this in Corvette applications. In the Z06 this occurs at around 80 MPH, it would be at a lower speed for a Z51 or GS with the 2.73 axle ratio. An interesting test would be for someone with excessive fluctuation to drive fast enough in 7'th or 8'th gear to exceed 1600 RPM and see if the fluctuation disappears. 7th might be the best choice if you have the 2.41 ratio (Z06 or base Stingray) so that you don't have to try to convince the state trooper that you can't drive any slower or your car shudders

KemahGT

I didn't feel a thing either. I think there are many more out there with the problem. They just don't know it...yet. Many will probably discover the problem way too late. Then they'll be hit with a $2500 repair bill and maybe be pissed that it was a known issue dating back to the first 8L90 installations. Corvettes, Silverados, anything with this tranny and the brilliant AFM/DOD crap show. Yes, they get great fuel mileage, until they come apart.

Anybody ever hear of one driven until destruction?

KemahGT

I believe less than 10 RPMs is considered within limits. Need GM PC connected to see that.

davanz

Yes, according to GM bulletins anything over 10 rpm fluction is abnormal.y

Gearhead Jim

Can you give us a link to the bulletin, or post a copy?

None of the A8 people I know report less than 10RPM variation, it's typically 50-100 when in V4 mode. Not constant, it may go for several seconds with no variation and then change to 50+ variation for several seconds. None of these cars are reporting any of the other symptoms like vibration, shudder, or abnormal shifting.

GaryPitts

It'd be difficult to video the actual shudder. Trust me though, you'll know it when it happens

Gearhead Jim

True, but right now we're talking about RPM fluctuations in V4 mode with no other symptoms. If GM has a current spec about max permissible rpm fluctuations in V4 mode, it would be nice to stay ahead of the problem.

NSC5

Jim,

Here is a link to PIP5337E which provides the 10 RPM reference. Note that the resolution and smoothing of the LCD digital tach (and the similar display in the HUD) is not sufficient to show a 10 RPM variation and the RPM variation is quite pronounced once it becomes readily visible via the in car displays. Bulletin: http://www.corvetteactioncenter.com/...-mph-1300.html

On edit, I don't have shudder and I am not letting V4 mode engage in order to develop shudder but several of the bulletins note slip drops to 0 above 1,600 RPM so someone who does have pronounced shudder try cruising on the highway at a steady speed which causes V4 operation but adjust road speed or use a lower gear to see if it does disappear above 1,600 RPM. It sounds like GM may disable the clutch modulation once the engine RPM is sufficient in V4 mode for the worst of the vibration to disappear. If you place the driver mode in the eco setting it will allow V4 operation even in manual shift mode so if you want to try the 1,600 RPM test in 7th gear then be sure to set mode to eco.

Gearhead Jim

Thanks for the link.
Does anyone know if this is the latest bulletin on A8 issues?

I'm still not sure that TCC slip speed is the same as rpm variation, either way I'll check mine above 1600 rpm to see if the variation disappears.

lakemg

I'm with you...I was under the impression that TCC slip speed is not the same as rpm variation.