Frodo

I've been trying to figure out the pathology of this "disease". I have an A8 and so far so good. (Frodo knocks on wood) As a lifelong diagnostician in the business machine repair world I can't help but try to figure things out.

This symptom some people are having seems to only occur in drive mode and when the engine is in V4 mode and using cruise control. Correct? Switch to manual mode, and V8 mode of course, and it seems to alleviate the condition.

Torque converters have used a lockup feature for many years. Is this lockup feature failing in V4 mode? Does engine management software do something to the transmission software to change the characteristics of the TC? Does lockup "unlock" for V4 mode? If TC lockup is supposed to happen in all modes why does it fail only in V4 mode? I'd expect the TC to operate the same in all modes.

Or could it be something else? I remember an old lawnmower that couldn't hold a steady rpm. It wandered up and down in rpm, always trying to find and hold the speed constant. Sounds like the problem we see in many Vettes with A8, to me. Like it doesn't know its going fast enough until its going too fast, then it slows, and repeats the cycle. Perhaps its a problem with V4 management.

I'm aware fluid changes have fixed some, TC replacement has fixed some, relflash the software have fixed some, and some have had a complete transmission replaced, so it does indicate the GM technicians have found something there but somehow it still doesn't all add up to me. I'd expect the TC to operate independently, not tied to V4 mode or V8 mode. Alas, I don't have training on these systems so I'm diagnosing in the dark.

Any knowledgeable people have ideas? Inquiring minds want to know.

TIA

davanz

I ditto your question Frodo. My 2016 with just 2500 miles does the rpm fluctuation in A4 steady cruise mode. I actually do not feel or hear anything happening, that is I do not hear any changes in engine sound or any subtle acceleration or decelerations. If I were not watching the tack I would not know it was occurring. It seems to me that the only thing which would be causing this is the t/c lockup clutch slipping randomly. And the one thing I keep in mind is that if there is a constant slipping of the t/c clutch, then there is some wear with miniscule particle production which over time is getting to the rest of the transmission. Is there an intended reduction of clutch control pressure in A4 mode for some reason, letting it slip some in A4? And if so, why? And, if it were a designed in feature for any reason, wouldn't GM have let us know this, instead of sending out a stream of TSB's? Like you, I wish someone who actually KNOWS the real cause and effect of this behavior would let us know. All we have seen so far is guessing and speculation, ( of which I have been a part also).

2016c7fl

sorry to say but that is exactly how my TC issue started in my 2016 Vert, second trip to dealer and they replaced TC. fluids, and who knows what else, but has been perfect since..mine started at 9000 miles. I would not accept any repair other than a total TC replacement

Good Luck

CP

I have exactly the same experience. I am waiting for the "other stuff" to show up before taking it in. At least I have to 100,000 miles (2015).

NSC5

Frodo,

I expect the root problem is the way GM modulates the mechanical torque converter clutch to reduce the transmission of increased V4 mode vibration to the rest of the powertrain and vehicle. They started using this pulse width modulation which rapidly applies and releases the mechanical clutch over 20 years ago to cushion certain shift events but the activity level hit a extreme high when this on/off modulation occurs continuously while running in V4 mode to absorb vibration of a V8 running on 4 cylinders. I expect even the upgraded clutch material isn't up to this application.

The problem rarely shows up in the Z06, probably because they rarely go into V4 mode so odds are problems with them would show up much later if at all. My Z06 is never allowed to go into V4 mode.

The most solid feeling transmission I have in any of my GM vehicles is the Allison 1000 auto in my GMC diesel pickup. When set to tow/haul mode the converter clock locks up with the shift into second and stays locked even under full throttle acceleration until vehicle speed drops far enough to force a shift back into first gear. Its unmodulated converter clutch gives it a very manual like feel in terms of power transmission. Of course given its heavy duty nature it cannot shift nearly as fast as the 8L90 (aka Corvette A8) even though it uses similar clutch to clutch architecture.

KemahGT

I am experiencing mild RPM surge in A only at this time. I only noticed the problem in the last 2 thousand miles or so. I've been in M mode ever since and everything is fine. I've been following this problem here and have Google searches where I have found even more info on Camaro and GM truck forums.

I seem to have found the most detailed info on the truck forums (sorry, I don't remember which). Those guys like to do a lot of tuning. Maybe it's just a numbers thing. My guess is 10 times more A8s in the trucks than Vettes? Maybe more?

Anyhow, there seems to be some slippage (maybe delayed engagement) programmed into the clutch when going into V4 mode. It appears this was done to help make that transition from V8 to V4 more transparent for obvious reasons. Some have said that by engaging the clutch sooner it seems to help this surging phenomenon.

Of course all of this is internet chatter. All should be taken with a grain of salt. My reason for not taking it to the dealer is simple. I want it fixed but I am still not convinced that the problem has been eliminated. Based on forum posts on this forum as well as Camaro and truck forums this is what I think we do know (again, this is the internet so it could all be fake news):

1. There is a problem with 8L90 A8 "transmissions". Make and model not important. Note transmissions is in quotes. It looks more like a TC clutch thing.

2. Apparently, a lot of torque converters have been replaced to try and fix this. Only GM knows the exact number. This number will probably never be known outside of GM except through discovery.

3. There are 3, maybe 4 different part numbers for the C7 A8 TC. This leads one to believe that design or manufacturing specs have been changed over time.

4. It seems that there are a number of cases where TCs did not fix the problem. See #3 for possible explanations.

5. Early on I recall there were some that received re-flashes to the controllers. Not sure if this is even happening anymore. Please correct me if you've experienced otherwise.

6. An even smaller number have had total transmission replacements. Those also seemed early on and may have been to fix the customer rather than the car (I'm not counting any transmission failures that were for reasons other than surge or shudder).

7. The latest TSB from GM calls for a 3 flush fluid replacement as a fix. It may still be too early to tell if this helps. I know that in at least one case it did not but that may be because the clutch in the TC was too far gone at that point.

My conclusion, and it is purely my opinion based on internet chatter, is that there is indeed a problem with the 8L90 A8 as setup in many GM vehicles. When the trans is performing as advertised it is a fabulous piece of technology. It's smooth when you want a transmission to be and bangs gears with the best dual clutch trannys when pushed. But, there is something going on that GM NEEDS to put some priority on. I know this problem can be fixed. It can't be that hard. Jeez, we put men on the moon back in 1969. We HAVE the technology!

I'm reading more and more about folks trading their vehicles in for sticks. Some are even abandoning the C7 altogether. I love my C7 but my patience will run out as the warranty runs out...If it's not fixed by then. If you think that any of this is BS, please call me out. If you have anything that you want to add, I welcome that as well.

Happy Motoring!

djnice

What NSC5 said, and I don't know why GM doesn't explain this themselves. All this BS for such a minuscule fuel savings with V4. Note that the M7 doesn't need PWM. I would like to see how much an issue the V4 vibration is if we just turn off PWM during V4.

I have the same experience with the Allison.

KemahGT

Yes, we can blame the Feds, CARB and Al Gore for all of this. I bought a high performance sports car. If I wanted fuel economy I would buy a Prius.

Make America Great Again, kill CAFE requirements!

dmvette

This is exactly how my car performed and it went in today for the flush and whatever else they do. If it still has a problem, they will contact GM for their blessing to replace the converter. We'll see how it goes.

ddelallata

Please keep us updated

YO-EL

You guys need to read the Challenger forums.,..
They have had the same issues with AFM in their 5.7L V8's for quite some time now..
But NOT their 6/4L V8's....
Most have the same symptoms leading up to failing timing chains, due to AFM.
Until some sort of design change.

Something is up with the design of this trans unit and/or TC..
Might I suggest for now, try to stay in V8 as much as possible to alleviate AFM...!

Frodo

Thanks for the information NSC5 and KemahGT. I wasn't aware of the designed "slip" in the TC for V4. I guess it makes sense if the V4 mode is transmitting vibrations into the drive train. I don't like the idea of it, though. I think I'll look seriously at the Ranger AFM eliminator.

KemahGT

No need for the Range. Just use M. It's kinda fun. No down shifts needed. The tranny takes care of that. pretty mindless . Take off and you start hearing the revs. Shift. Piece of cake.

After this is all figured out I aim to pull my engine and do a AFM delete. New cam and all. You do know that the stock cam has different lift on the AFM shutdown cylinders? WTF? This whole AFM has me so pissed off I could scream. It sounds great in theory but AFM is still stuck in the '80s as far as I'm concerned. Remember the Caddy V8-6-4 BS?

Frodo

I do use manual mode sometimes but I'm lazy. I bought the A8 so I don't have to do anything. I got over banging the gears long ago. I guess I'm not your average Corvette driver.

Pelee4me

There's a lot of threads open about this issue. Check this one

https://www.corvetteforum.com/forums...r-failure.html

MoPixBiz

Exactly, The RPM flux is almost a sure sign your T/C is shot.

dmvette

Got the car back Thur. after having the triple flush completed. I drove it home (about 20 miles) on both highway and city streets without any shuddering, . almost like when I first drove it off the dealers lot.

I am suppose to drive it for 200 miles and check to see if shudder comes back or not. I will put it to the test Friday and see if it stays shudder free.

Even if it starts shuddering right away, I was instructed by the dealer to drive it the 200 miles before bringing it back in per GM.

I am hoping that this was a simple fix but doubtful that it will stay shudder free. It is nice to be able to drive again without that annoying shudder so fingers crossed.

KemahGT

Please keep us informed. Thanks!

1SG_Ret

Funny you should mention this. Never been to their forums, but a good friend has a Challenger. I was telling him about the Corvette A8 issues and he said, "Yeah, happens w/ mine all the time. Just the way it is." I honestly couldn't believe he's acceptance of the surging and lurching w/ such nonchalance.

1SG_Ret

Rather takes the joy of ownership out of the equation when every time you go for a drive, the first thing you think about when pushing the "start" button is "I hope the car operates properly".

I know it drives me crazy even though my issues haven't been as bad as some. But my driving habits have now been adjusted to preclude this from happening (or at least mitigate it). Just blvd cruising w/ no real interstate driving w/ cruise control.