8 speed auto trans issue
Thread Starter
8th Gear
Joined: Feb 2015
Posts: 8
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8 speed auto trans issue
I love my 2015 C7, but despite all the hype about this new 8 speed tranny, I am convinced that there is an issue that needs to be addressed. I hear a fluttering sound that sounds like it is coming from out of a hollow tube just below and behind the driver. It seems only to happen at around 1500 rpm. I can get rid of it by either accelerating thru it or downshifting. When cruising on the tollway at 75 mph I'm at 1500 rpm in 8th gear, and the noise is very annoying. Now if I drop it to 7th the noise disappears! This is total BS! I drove a few other new vettes on the lot w/ the GM mechanic, and they all had a similar but not as noticeable sound. He had no answer. My theory is that GM engineers programed the shift points at too low an rpm in an effort to increase mpg. I cannot believe that I am the only one w/ this issue, and I am determined to get to the bottom of this. This should not be happening. I never heard anything like this in my 08 C6. Somebody suggested that it could be torque flutter. I never heard of that. Does anyone have an idea?
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 328
From: CA.
St. Jude Donor '13-'14-'15
I also suspect the AFM is causing the "fluttering sound" Test as suggested would be to drive it in MANUAL mode and see if the same sound appears---As in manual mode the AFM is disabled---
YES however these cars are very high geared and shift extremely early--causing the engine to lug badly---But realize with VVT the engine can handle the lugging without any detonation---As a previous muscle car owner it's tough to drive around town with the engine running at 1200 RPM's --I couldn't stand it---A full tuner with EFILIVE or HP can adjust and raise all your shift points --More importantly GM LOCKS the converter from 4th gear on up which makes it lug even worse---On a A6 I always delete the lockup in all the gears but 5th and 6th---And on an A8 delete the lockup in 6th 7th and 8th--This gives you the TQ multiplication of the converter thru all the lower gears and eliminates the lugging---Plus makes the car so much more responsive when driving at P/T and flooring it
YES however these cars are very high geared and shift extremely early--causing the engine to lug badly---But realize with VVT the engine can handle the lugging without any detonation---As a previous muscle car owner it's tough to drive around town with the engine running at 1200 RPM's --I couldn't stand it---A full tuner with EFILIVE or HP can adjust and raise all your shift points --More importantly GM LOCKS the converter from 4th gear on up which makes it lug even worse---On a A6 I always delete the lockup in all the gears but 5th and 6th---And on an A8 delete the lockup in 6th 7th and 8th--This gives you the TQ multiplication of the converter thru all the lower gears and eliminates the lugging---Plus makes the car so much more responsive when driving at P/T and flooring it
Race Director
Joined: Jan 2014
Posts: 10,709
Likes: 7,101
From: Fulshear, TX
Agreed with AFM being the cause. The noise you hear is likely caused by the engine dropping to and running in V4 mode. If it bothers you that much, I'd suggest you either put the car in manual paddle mode or disable AFM. I love how they do AFM on the M7 (only in Eco mode), but disagree with it being active in all modes on the A8 cars (seems odd to me, they should at least have it disabled in Sport and Track modes).
Drifting
Joined: Jun 2007
Posts: 1,425
Likes: 238
From: Macomb MI
Add me to the suspected AFM mode. If you really want to know how awful it can sound, pull the AFM valve fuse. Welcome to tractorville.
All the above recommendations are good ways to diagnose. Also you can use the center DIC display to see when it's in V8/V4 mode. One of the Performance screen pages with the instant econ reading will display engine mode: V8 (orange), V4 (green), and V0 (white, engine off...LOL).
As others may have mentioned, you can disable AFM through a tune, but that brings other potential consequences. The most transparent and reversible is the DiabloSport InTune/Trinity. You can adjust many parameters in including AFM whether or not you install the tune. Also the recent posts by Trifecta with their tune look interesting as they can control the A8 now. The DS couldn't control the A8 initially and not sure where that stands.
All the above recommendations are good ways to diagnose. Also you can use the center DIC display to see when it's in V8/V4 mode. One of the Performance screen pages with the instant econ reading will display engine mode: V8 (orange), V4 (green), and V0 (white, engine off...LOL).
As others may have mentioned, you can disable AFM through a tune, but that brings other potential consequences. The most transparent and reversible is the DiabloSport InTune/Trinity. You can adjust many parameters in including AFM whether or not you install the tune. Also the recent posts by Trifecta with their tune look interesting as they can control the A8 now. The DS couldn't control the A8 initially and not sure where that stands.
Intermediate
Joined: Feb 2015
Posts: 30
Likes: 0
I love my 2015 C7, but despite all the hype about this new 8 speed tranny, I am convinced that there is an issue that needs to be addressed. I hear a fluttering sound that sounds like it is coming from out of a hollow tube just below and behind the driver. It seems only to happen at around 1500 rpm. I can get rid of it by either accelerating thru it or downshifting. When cruising on the tollway at 75 mph I'm at 1500 rpm in 8th gear, and the noise is very annoying. Now if I drop it to 7th the noise disappears! This is total BS! I drove a few other new vettes on the lot w/ the GM mechanic, and they all had a similar but not as noticeable sound. He had no answer. My theory is that GM engineers programed the shift points at too low an rpm in an effort to increase mpg. I cannot believe that I am the only one w/ this issue, and I am determined to get to the bottom of this. This should not be happening. I never heard anything like this in my 08 C6. Somebody suggested that it could be torque flutter. I never heard of that. Does anyone have an idea?
Now I am thinking is the tranny.
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Thread Starter
8th Gear
Joined: Feb 2015
Posts: 8
Likes: 0
It doesn't matter what mode I'm in or if I'm in V4 or V8. The sound is there around 1500 RPM. If I paddle shift down to increase RPM the sound disappears.
Thread Starter
8th Gear
Joined: Feb 2015
Posts: 8
Likes: 0
Agreed with AFM being the cause. The noise you hear is likely caused by the engine dropping to and running in V4 mode. If it bothers you that much, I'd suggest you either put the car in manual paddle mode or disable AFM. I love how they do AFM on the M7 (only in Eco mode), but disagree with it being active in all modes on the A8 cars (seems odd to me, they should at least have it disabled in Sport and Track modes).
Pro
Joined: Jan 2014
Posts: 585
Likes: 65
From: Sierra Nevada Foothills CA
From the RPM descriptions it could also be a very slight drone from the exhaust at those RPMs. When you change RPMs by shifting the noise goes away. Quite common in V8 engines with aftermarket exhausts but, for a finely tuned set of ears, may even be audible with stock exhausts.
Le Mans Master
Joined: Aug 2004
Posts: 5,259
Likes: 16
From: Monroe Township New Jersey
St. Jude Donor '05-'06-'07-'08-'09-'10-'11-'12-'13-'14-'15-'16
St. Jude donor in memory of jpee '14
From the RPM descriptions it could also be a very slight drone from the exhaust at those RPMs. When you change RPMs by shifting the noise goes away. Quite common in V8 engines with aftermarket exhausts but, for a finely tuned set of ears, may even be audible with stock exhausts.
If the car is equipped with the NPP option and it is set to open all the time it is possible. There are times when my 14 does drone until the npp valves close. On my C6 (npp) with the vacuum lines blocked off it droned at 1500 rpm.
Premium Supporting Vendor
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From: Providing the most proven supercharger kits for your C5/6/7 609-752-0321
Consider a Diablo In Tune to shut of the AFM and give a performance upgrade at the same time without voiding the warranty. We have plenty in stock if anyone is interested. Sounds like that's your issue, but of course without seeing it... GLWI
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Thread Starter
8th Gear
Joined: Feb 2015
Posts: 8
Likes: 0