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Directly from GM Authority, written by BY JONATHAN LOPEZ — AUG 13, 2019:

We’ve covered the problems of the eight-speed GM transmission on numerous occasions. In fact, we cover a class action lawsuit brought against GM back in April, complaints from owners, and even a recommendation that GM replace the eight-speed altogether with a new unit as quickly as possible. Now, GM Authority has learned that there’s a new fix that seems to be alleviating the eight-speed’s shifting woes outright.

As of July 8th, 2019, GM began using a new type of Mobil 1 automatic transmission fluid for the troublesome transmissions. Customers that complain of shuddering and harsh shifting characteristics in the 8-speed GM transmissions are recommended for a trans fluid flush. As of August 7th, GM dealers should also be flushing all potentially affected new vehicles on the lot.


According to TSB (Technical Service Bulletin) 18-NA-355, the latest update addresses “torque converter clutch (TCC) shudder conditions on 8L45 and 8L90” eight-speed automatic transmissions, RPO codes M5N, M5T, M5U, and M5X. Affected vehicles include Chevrolet, GMC and Cadillac vehicles, as follows.

Chevrolet

2015 – 2018 Chevrolet Silverado 1500
2015 – 2019 Chevrolet Corvette
2016 – 2019 Chevrolet Camaro
2017 – 2019 Chevrolet Colorado
GMC

2015 – 2017 GMC Yukon
2015 – 2018 GMC Sierra 1500
2017 – 2019 GMC Canyon
Cadillac

2015 – 2017 Cadillac Escalade
2016 – 2018 Cadillac CT6
2016 – 2019 Cadillac ATS
2016 – 2019 Cadillac ATS-V
2016 – 2019 Cadillac CTS (excluding V-Sport models)
2016-2019 Cadillac CTS-V

The issues with the 8-speed GM transmission are two-fold. First, some customers report the transmission shuddering frequently. Secondly, the shifts are very harsh between first to second, and then second down to first, especially when accelerating and the transmission is upshifting or decelerating and the transmission is downshifting. These conditions are most commonly observed under light and medium throttle applications, and as often during full throttle applications.

However, a new fluid flush appears to fix the issues, with some customers beginning to report a dramatic change in shift quality after swapping out the old juice. If the flush doesn’t fix the issue, the next step may be to replace the torque converter, which could be glazed.

The issue seems to stem from “moisture content in the transmission fluid.” As such, the old GM transmission liquid is swapped for new Mobil 1 Synthetic LV ATF HP fluid. Driving the vehicle for about a week or so improves shift quality even further as the new fluid cleans things up inside the transmission and the adaptive shift algorithms relearn various parameters.

This is obviously a big deal for owners struggling with their vehicle’s eight-speed. Hopefully the new fluid swap will provide a lasting fix.
 

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Directly from GM Authority, written by BY JONATHAN LOPEZ — AUG 13, 2019:

We’ve covered the problems of the eight-speed GM transmission on numerous occasions. In fact, we cover a class action lawsuit brought against GM back in April, complaints from owners, and even a recommendation that GM replace the eight-speed altogether with a new unit as quickly as possible. Now, GM Authority has learned that there’s a new fix that seems to be alleviating the eight-speed’s shifting woes outright.

As of July 8th, 2019, GM began using a new type of Mobil 1 automatic transmission fluid for the troublesome transmissions. Customers that complain of shuddering and harsh shifting characteristics in the 8-speed GM transmissions are recommended for a trans fluid flush. As of August 7th, GM dealers should also be flushing all potentially affected new vehicles on the lot.


According to TSB (Technical Service Bulletin) 18-NA-355, the latest update addresses “torque converter clutch (TCC) shudder conditions on 8L45 and 8L90” eight-speed automatic transmissions, RPO codes M5N, M5T, M5U, and M5X. Affected vehicles include Chevrolet, GMC and Cadillac vehicles, as follows.

Chevrolet

2015 – 2018 Chevrolet Silverado 1500
2015 – 2019 Chevrolet Corvette
2016 – 2019 Chevrolet Camaro
2017 – 2019 Chevrolet Colorado
GMC

2015 – 2017 GMC Yukon
2015 – 2018 GMC Sierra 1500
2017 – 2019 GMC Canyon
Cadillac

2015 – 2017 Cadillac Escalade
2016 – 2018 Cadillac CT6
2016 – 2019 Cadillac ATS
2016 – 2019 Cadillac ATS-V
2016 – 2019 Cadillac CTS (excluding V-Sport models)
2016-2019 Cadillac CTS-V

The issues with the 8-speed GM transmission are two-fold. First, some customers report the transmission shuddering frequently. Secondly, the shifts are very harsh between first to second, and then second down to first, especially when accelerating and the transmission is upshifting or decelerating and the transmission is downshifting. These conditions are most commonly observed under light and medium throttle applications, and as often during full throttle applications.

However, a new fluid flush appears to fix the issues, with some customers beginning to report a dramatic change in shift quality after swapping out the old juice. If the flush doesn’t fix the issue, the next step may be to replace the torque converter, which could be glazed.

The issue seems to stem from “moisture content in the transmission fluid.” As such, the old GM transmission liquid is swapped for new Mobil 1 Synthetic LV ATF HP fluid. Driving the vehicle for about a week or so improves shift quality even further as the new fluid cleans things up inside the transmission and the adaptive shift algorithms relearn various parameters.

This is obviously a big deal for owners struggling with their vehicle’s eight-speed. Hopefully the new fluid swap will provide a lasting fix.
jsvette, do you know if this actually fixed the problem?
 

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Discussion Starter · #5 ·
I'm looking at getting a 2015 Stingray, but don't want to buy into a problem. Have you experienced the shuddering, etc. problems, and did you have anything done to fix it?

Thanks!
Sorry, RedHot is a 2014 and does not have the 8 speed.
 

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I have a 2015 Stingray. I had some minor shuddering at low speeds and, as d/t warranty was approaching expiration last fall, I had an oil change and asked about the flush. My dealer did it at no cost and I think it made a big difference. It has been silky smooth shifting ever since.
 
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Hi guys; I don't post much anymore but I thought this might be helpful. My A8 (with 20K miles) started shuddering this spring. Last week I had the trans tech at my local dealership perform the fluid change as per the TSB. I noticed improvement immediately and after 200+ miles it's perfect.
 

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Discussion Starter · #8 ·
Hi guys; I don't post much anymore but I thought this might be helpful. My A8 (with 20K miles) started shuddering this spring. Last week I had the trans tech at my local dealership perform the fluid change as per the TSB. I noticed improvement immediately and after 200+ miles it's perfect.
Thanks Paul. Many who have had it done report it greatly improves the transmission.
 

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Hi guys; I don't post much anymore but I thought this might be helpful. My A8 (with 20K miles) started shuddering this spring. Last week I had the trans tech at my local dealership perform the fluid change as per the TSB. I noticed improvement immediately and after 200+ miles it's perfect.
Hi Paul, had mine done recently, running smoother.
Take care up there.
 

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Like Paul, I haven't posted in a while, but I still lurk! Does anyone know if dealers will comply with this TSB if the vehicle is out of warranty? My 2015 Z51 has been shuddering for about a year (don't drive her too much) and just had a friend call me and tell about his '19 GS that has been doing this and the fix. He had it done today and thinks it's a noticeable improvement.
 

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Like Paul, I haven't posted in a while, but I still lurk! Does anyone know if dealers will comply with this TSB if the vehicle is out of warranty? My 2015 Z51 has been shuddering for about a year (don't drive her too much) and just had a friend call me and tell about his '19 GS that has been doing this and the fix. He had it done today and thinks it's a noticeable improvement.
I spoke with dealer, my car is out of warranty and the miracle flush will cost me 611.00. So GM uses a defective fluid but charges its customers 600 to implement corrective actions——- maybe!!
 

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Discussion Starter · #12 ·
Welcome to our forum 37Tanker. What year C7 do you have?
 

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I spoke with dealer, my car is out of warranty and the miracle flush will cost me 611.00. So GM uses a defective fluid but charges its customers 600 to implement corrective actions——- maybe!!
My car was also out of warranty when I had the flush/fillx3 done. Cost me $625.64
 

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I spoke with dealer, my car is out of warranty and the miracle flush will cost me 611.00. So GM uses a defective fluid but charges its customers 600 to implement corrective actions——- maybe!!
I called my servicing dealer and was told since I'm out of warranty, it would be about $700! I asked if there was any other recourse and the service writer said he could check with GM and see if there was any "GOODWILL" relief. He called me back later in the afternoon and said GM would "authorize" the repair but I would have to pay $229.

At least that's moving in the right direction. I'm going to contact the dealer where I purchased it because it came with a Lifetime Limited Powertrain Warranty. Limited being the key word. To be continued...
 
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New to Corvette ownership and glad I found the forum. I purchased a 2019 Z51 2LT on 1/30/21 with 3,322 miles on it. Still under factory warranty. The dealer told me an older gentleman bought it and obviously didn't drive it much. During the short time with "my new girlfriend" as my wife calls it, I have noticed the frequent shuddering and jerking at low RPMs when driving in "D" and occasionally in manual. Also have noticed when in 7th or 8th gear the transmission will not downshift when going uphill as speed reduces which causes more shaking and vibrations. Taking it back to the dealership today to address the first issue but was curious if the transmission should also downshift as needed when going uphill or when speed reduces. Is this a separate issue or could the fluid/TC change fix this as well?
 

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Discussion Starter · #17 ·
Welcome to our forum Uglyman71 and congrats on your Z51. Hopefully your dealer will be able to better diagnose and resolve the issue. Since you mention it also occurs when you are in manual, it could not be related to the V4-V8 shift. It is possible that the transmission needs to be retrained, resetting the adaptive learning. Something to ask your dealer about.
 

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Thank you for the info. I called the dealership yesterday and they are draining the transmission, TC, and replacing the fluid based on the TSB info. Fingers crossed this will resolve the issue. If not, I can take it back. And since it was driven very little in the two years prior to my purchase the adaptive learning could also be part of the issue.
 
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