A8 woes
Safety Car
Joined: Aug 2012
Posts: 3,960
Likes: 1,106
The Z06 with its LT4 engine seldom operates in AFM mode so the problem is far less frequently seen with it although the problem will probably appear at higher mileage. My A8 equipped Z06 stays in manual mode to avoid AFM with its undesirable side effects.
Pro
Joined: Mar 2008
Posts: 549
Likes: 22
From: Castle Pines Colorado
If the torque converter strategy was used just during the transition it would be no problem but what GM is doing is continually "modulating" the mechanical torque converter clutch during V4 operation to reduce felt vibrations from a V8 engine running as a V4. The mechanical clutch (aka "lockup clutch") in the torque converter is continuously being switched between its locked and unlocked states many times per second with the strategy of reducing vibration but it appears this is likely causing very rapid wear of the clutch and we start seeing the RPM variation (and feeling the shudder) as it no longer operates as intended.
The Z06 with its LT4 engine seldom operates in AFM mode so the problem is far less frequently seen with it although the problem will probably appear at higher mileage. My A8 equipped Z06 stays in manual mode to avoid AFM with its undesirable side effects.
The Z06 with its LT4 engine seldom operates in AFM mode so the problem is far less frequently seen with it although the problem will probably appear at higher mileage. My A8 equipped Z06 stays in manual mode to avoid AFM with its undesirable side effects.
Instructor
Joined: Nov 2014
Posts: 203
Likes: 88
From: South Hadley/Boca Raton MA/FL
Has anyone looked into the active fuel management process and engineering? On my 16 Z51 I get the same fluctuation in RPM's as well. It's only when transitioning from V8 to V4 mode. I question if what people feel is truly a surge. If you're really in-tune with your car you can tell when it's changing but it is definitely nothing abrupt and most people can't feel it.
I'm thinking the AFM adjusts the torque converter because the available power is different. The reason I think this is because I have a 16 Yukon with the 5.3l and the 6 speed transmission in it and it does the exact same thing as my vet. It's only when shifting from v8-v4. No surge problems, sometimes stumbles to find the right gear when you come to a stop and don't fully stop but everything feels right about it.
What do you guys think? Could the AFM engineering be designed to release the torque converter slightly because of the transition from V8-V4?
I'm thinking the AFM adjusts the torque converter because the available power is different. The reason I think this is because I have a 16 Yukon with the 5.3l and the 6 speed transmission in it and it does the exact same thing as my vet. It's only when shifting from v8-v4. No surge problems, sometimes stumbles to find the right gear when you come to a stop and don't fully stop but everything feels right about it.
What do you guys think? Could the AFM engineering be designed to release the torque converter slightly because of the transition from V8-V4?
First attempt with the dealer to remedy it failed because my Magnaflow exhaust was defective and constantly triggered CEL's. By the time I resolved that issue I left for Florida (with the car).
Flash forward to now @ 27K miles, Magnaflow has been replaced with Billy Boat and all functions perfectly on that end, but RPM fluctuations and lugging are worse. It's at the dealer's now, and has been for 3 days. They tried to re-program the A8 first, have now moved to the triple flush procedure and I should hear something today or tomorrow.
I don't feel the triple flush will solve the issue and told them so, but I understand they must follow GM procedures to cover themselves and I expect that the TC will then have to be replaced. If they try to come back after this saying that this is normal (after 22K miles), there is going to be a nuclear explosion.
Safety Car
Joined: Aug 2012
Posts: 3,960
Likes: 1,106
"so with this in mind is it a problem with the TV or AFM? Again it's happening on non A8 transmissions."
I think the problem is the basic method of trying to reduce AFM vibration although I am not a fan of AFM in any form. A torque converter mechanical clutch can be built to withstand this kind of environment but it probably is large and heavy enough that it doesn't fit well with packaging and fuel economy goals. Your Yukon is heavy and has significant wind drag so I could see it creating rapid wear during AFM. I vaguely recall when the 8L90 (i.e. A8) was announced there was a blurb about its new "squashed" compact torque converter and this could be the reason the A6 (6L80) used in the 2014 years seems to be holding up to AFM better and doesn't have the varying RPM issues afflicting the 8L90 but under heavy use conditions (like your Yukon) the weakness of the older converter clutch design (of the 6L80/6L90) are also revealed. I don't think anyone has noticed the problem with AFM and the A8 right off the lot but it does appear like it is very likely to happen with age with variance depending upon how it is driven.
I think the problem is the basic method of trying to reduce AFM vibration although I am not a fan of AFM in any form. A torque converter mechanical clutch can be built to withstand this kind of environment but it probably is large and heavy enough that it doesn't fit well with packaging and fuel economy goals. Your Yukon is heavy and has significant wind drag so I could see it creating rapid wear during AFM. I vaguely recall when the 8L90 (i.e. A8) was announced there was a blurb about its new "squashed" compact torque converter and this could be the reason the A6 (6L80) used in the 2014 years seems to be holding up to AFM better and doesn't have the varying RPM issues afflicting the 8L90 but under heavy use conditions (like your Yukon) the weakness of the older converter clutch design (of the 6L80/6L90) are also revealed. I don't think anyone has noticed the problem with AFM and the A8 right off the lot but it does appear like it is very likely to happen with age with variance depending upon how it is driven.
Last edited by NSC5; Apr 6, 2017 at 11:26 AM.
Safety Car
Joined: Aug 2012
Posts: 3,960
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SAE link: http://articles.sae.org/12782/
Melting Slicks
Joined: May 2012
Posts: 2,904
Likes: 315
From: Cedar Falls Iowa
The TC lockup actually modulates on and off the whole time its in AFM for real?
I don't understand why they have the TC do that. The M7 obviously isn't able to do this, and the shudder is only marginally more noticeable when switching the AFM on and off. It's not a big deal. They could have made the A8 remain in lockup while switching between modes, and it would've felt just like the M7. They would have pissed off alot less people and saved millions in labor tearing stuff up to swap out all these TCs.
I don't understand why they have the TC do that. The M7 obviously isn't able to do this, and the shudder is only marginally more noticeable when switching the AFM on and off. It's not a big deal. They could have made the A8 remain in lockup while switching between modes, and it would've felt just like the M7. They would have pissed off alot less people and saved millions in labor tearing stuff up to swap out all these TCs.
Melting Slicks
Joined: Mar 2013
Posts: 3,104
Likes: 626
From: Baton Rouge LA
GM used the "triple-flush" method early on, like two years ago, and it apparently wasn't successful on most cars. Tadge admitted this in his answer to the A8 questions in his sticky thread. They went from replacing trannies to the flush, then replacing TCs with the flush, and now back to flush only. As I said in this reply in another thread (see #41), my feeling is that GM is taking a beating on all the TC replacements and are going back to the flush-only method, hoping it'll fix a few cars, at least temporarily. The calculation may be a reduction in overall costs. My 2¢ with no evidence offered to support the above.
Last edited by iclick; Apr 6, 2017 at 01:14 PM.
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Le Mans Master
Joined: May 1999
Posts: 6,517
Likes: 1,304
From: Windermere FL
I'm just curious. Has anyone with these problems tried using something like a Range AFM disabler module or tune out the AFM using a Diablo Tuner? Does it eliminate the tranny issues of shuttering and rpm fluctuations?
I had my A8 replaced with 100 miles on the clock but I have been running the Range module. Makes me wonder if I unplugged it if I would start to notice these issues.
I had my A8 replaced with 100 miles on the clock but I have been running the Range module. Makes me wonder if I unplugged it if I would start to notice these issues.
Last edited by Flame Red; Apr 6, 2017 at 01:51 PM.
Le Mans Master
Joined: Sep 2012
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From: Hagerstown MD
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Since the trannies are warranted for 5 years and either 50K or 100K miles, I expect GM will see a whole lot more as folks accumulate more miles. This issue is not going away anytime soon.
Melting Slicks
Joined: Jun 2016
Posts: 2,845
Likes: 521
From: Eastern WA
See SAE link with new 8L90 "squashed torque converter" design to fit in same space as the 6L series. I wouldn't be surprised if some of the more robust converters of the 6L series start suffering issues for owners who hold onto them long enough.
SAE link: http://articles.sae.org/12782/
SAE link: http://articles.sae.org/12782/
The TC lockup actually modulates on and off the whole time its in AFM for real?
I don't understand why they have the TC do that. The M7 obviously isn't able to do this, and the shudder is only marginally more noticeable when switching the AFM on and off. It's not a big deal. They could have made the A8 remain in lockup while switching between modes, and it would've felt just like the M7. They would have pissed off alot less people and saved millions in labor tearing stuff up to swap out all these TCs.
I don't understand why they have the TC do that. The M7 obviously isn't able to do this, and the shudder is only marginally more noticeable when switching the AFM on and off. It's not a big deal. They could have made the A8 remain in lockup while switching between modes, and it would've felt just like the M7. They would have pissed off alot less people and saved millions in labor tearing stuff up to swap out all these TCs.
I wish GM would publish some technical info about this so we don't have to guess.
Drifting
Joined: Nov 2014
Posts: 1,782
Likes: 106
From: Salado TX
I'm just curious. Has anyone with these problems tried using something like a Range AFM disabler module or tune out the AFM using a Diablo Tuner? Does it eliminate the tranny issues of shuttering and rpm fluctuations?
I had my A8 replaced with 100 miles on the clock but I have been running the Range module. Makes me wonder if I unplugged it if I would start to notice these issues.
I had my A8 replaced with 100 miles on the clock but I have been running the Range module. Makes me wonder if I unplugged it if I would start to notice these issues.
Race Director
Joined: Oct 2005
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From: Plymouth Massachusetts
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Also since the A6 will have a different TCM with different tune parameters. It may not be tuned to use the TCC the same way. I have seen posts on this forum with C7 A6's having the same problem, but there are a lot fewer C7 A6 on the road so maybe that's why you see less of it.
Melting Slicks
Joined: May 2012
Posts: 2,904
Likes: 315
From: Cedar Falls Iowa
Also since the A6 will have a different TCM with different tune parameters. It may not be tuned to use the TCC the same way. I have seen posts on this forum with C7 A6's having the same problem, but there are a lot fewer C7 A6 on the road so maybe that's why you see less of it.
Maybe its less of an issue in the M7 because its not switching in and out of V4 as much. Also the connection between engine and tranny may not transmit vibration the same way.
I wish GM would publish some technical info about this so we don't have to guess.
Maybe its less of an issue in the M7 because its not switching in and out of V4 as much. Also the connection between engine and tranny may not transmit vibration the same way.
I wish GM would publish some technical info about this so we don't have to guess.
You're probably right that there's some other factor like how the A8 transmits the vibrations different. If the TC lockup modulates, then it sounds like they're trying to cancel a harmonic...
Melting Slicks
Joined: Jun 2016
Posts: 2,845
Likes: 521
From: Eastern WA
It does seem like they are concerned about a harmonic. Does HP Tuners give you the ability to modify or to turn off the lockup modulation? Or does it just turn off lockup altogether? I would like to see it tested to see if there really is an issue at all.
Nailed It
Instructor
Joined: Jul 2011
Posts: 197
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Just picked up my wife's from the dealer. New T/C. It's better for sure, but still has problems. The car seems to "lurch" every so often. Like it slips between gears for second. We got 17,000 on the ticker. 2016 bought Nov, 2015.
Burning Brakes
Joined: Nov 2016
Posts: 774
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The key to this puzzle is for all of you tech/motor heads out there is to figure out what does the C7/A8 have different than any happy go lucky Silverado A8, Camaro A8 etc. Their numbers are way higher with no big problems.
I think it's electron more than TC exchanges. Others here have said the same thing. Problems still lerking AFTER a new TC put in. I don't believe the tranny is ONLY bad in Vettes but OK in other GM vehicles!
It's got to be a gizmo glitch.
I think it's electron more than TC exchanges. Others here have said the same thing. Problems still lerking AFTER a new TC put in. I don't believe the tranny is ONLY bad in Vettes but OK in other GM vehicles!
It's got to be a gizmo glitch.
Le Mans Master
Joined: Sep 2000
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