Infamous 8 Speed Transmission Clunk - Dealer/GM Current Position on the Issue?

YukonXL04 · January 21, 2021

I have had the random 1-2 cold shift issue from 5k-30k miles. The shudder started at around 25k. I took it to the dealership, the did the fluid change and some harness. The shudder is gone. The stupid 1-2 is there still. Doesn't seem as common right now. And I can live with it. Its the first shift of the day, and it doesn't happen every day. And I absolutely love the truck otherwise so it is what it is. I have a 6.2 and 8spd and I love how it drives and tows. My mileage suffers, but its fun to drive so oh well lol.

flyingfool · January 21, 2021

i have a feeling the 8 speeds issues are software related. i suggest purchasing the HPTuners MPVI and the tuning schools GM transmission booklet

bbwb · January 23, 2021

I found this YouTube video out there where he said GM is arguing that they do not have to fix the transmission under warranty in court because it was a design flaw, not a material or labor issue.

flyingfool · January 23, 2021

the 8 speeds are designed well. The transmissions software is designed to be flawed because our Gov. and EPA forces GM to meet federal emmisions mandates for higher MPG ratings.... thus why the lock up converters fail. they are over using the TCC to gain MPG by reducing torque converter slippage, the shudder is the TCC lugging the engine to maintain lower rpm's while driving.

when i shut off my TCC lock up from 1-4th gear i lost 2-3 MPG city driving. yet the truck runs so much better. it feels like a hot rod now. I did activate the power enrichment tables on the stock software. so i am sucking more fuel because of this

Edited January 23, 2021 by flyingfool

OkieDavid · January 27, 2021

Not to beat on a dead topic, but I took my truck in today (65K miles) for the tsb to be done. I don't mind paying for the transmission fluid to be changed because I almost see that as a wear and tear item, but what really pissed me off was paying for a software update from GM. You would think with all the know issues with this transmission GM would do what they can for free to fix it.

flyingfool · January 27, 2021

1 hour ago, OkieDavid said:

Not to beat on a dead topic, but I took my truck in today (65K miles) for the tsb to be done. I don't mind paying for the transmission fluid to be changed because I almost see that as a wear and tear item, but what really pissed me off was paying for a software update from GM. You would think with all the know issues with this transmission GM would do what they can for free to fix it.

your dealer is just gougeing you, call GM's consumer support number and complain, if your under warrantee i bet they'll side with you and credit it back

sgiangregorio · January 29, 2021

My 2018 Sierra SLT is going to the dealer Tuesday for the back window recall, and I told them to check out the transmission. I didn't think I had any issues until I read this thread. I have some shudder I'd say. Doesn't really bother me much, but if there's a TSB or something that can improve it, why not. It's still under warranty. I didn't see the TSB in the thread though, but could have missed it. I skimmed the 9 pages. Could someone post the number so I can read it? Thank you!

Edit: Ugh, just found it, I think. There are 2 TSBs mentioned, 18-NA-355 and 16-NA-412. I'll read up on them both. Are there others?

Also, my bumper to bumper warranty is up in 1000 miles, but I assume this would be part of the 100k powertrain warranty if it persists or got worse?

Edited January 29, 2021 by sgiangregorio

JimCost2014 · January 31, 2021

On 1/29/2021 at 10:37 AM, sgiangregorio said:

My 2018 Sierra SLT is going to the dealer Tuesday for the back window recall, and I told them to check out the transmission. I didn't think I had any issues until I read this thread. I have some shudder I'd say. Doesn't really bother me much, but if there's a TSB or something that can improve it, why not. It's still under warranty. I didn't see the TSB in the thread though, but could have missed it. I skimmed the 9 pages. Could someone post the number so I can read it? Thank you!

Edit: Ugh, just found it, I think. There are 2 TSBs mentioned, 18-NA-355 and 16-NA-412. I'll read up on them both. Are there others?

Also, my bumper to bumper warranty is up in 1000 miles, but I assume this would be part of the 100k powertrain warranty if it persists or got worse?

Powertrain on the 18's are 5 years/60000 miles. We were the some of the first (maybe model year 16) lucky ones when they started reducing services/warranties.

Edited January 31, 2021 by JimCost2014

sgiangregorio · February 1, 2021

On 1/30/2021 at 9:42 PM, JimCost2014 said:

Powertrain on the 18's are 5 years/60000 miles. We were the some of the first (maybe model year 16) lucky ones when they started reducing services/warranties.

Thanks. I'm bringing the TSB with me, so hopefully they just change the fluid to the Mobil 1 synthetic without too much fuss.

JimCost2014 · February 1, 2021

32 minutes ago, sgiangregorio said:

Thanks. I'm bringing the TSB with me, so hopefully they just change the fluid to the Mobil 1 synthetic without too much fuss.

If they won't, find a dealer who will. Warranty work can be done at any GMC dealer.

flyingfool · February 1, 2021

has anyone attempted to have a tuner like DIablo or Blackbear, disable the TCC system for the first 4-5 gears? this cured the clunk on mine.. i've looked at the tcc parameters on the 8 speed and its engaged right off the line , from accelleration and to a full stop. this is why your converters are failing and get clunks durring downshifts , and between clutch engage/disengagements

mookdoc6 · February 1, 2021

1 hour ago, flyingfool said:

has anyone attempted to have a tuner like DIablo or Blackbear, disable the TCC system for the first 4-5 gears? this cured the clunk on mine.. i've looked at the tcc parameters on the 8 speed and its engaged right off the line , from accelleration and to a full stop. this is why your converters are failing and get clunks durring downshifts , and between clutch engage/disengagements

I do think your onto something with the EPA and GM using the TCC for added mpg's. I told my tunner "GUY" to off that crap too! He told me, "he could, but it's not advisable." He is extremely knowledgeable HP tunner guy and so I took his world the morning 1-2 will never go away it's to warm up the fluid.

Edited February 1, 2021 by mookdoc6

Chad2815 · February 1, 2021

The Amsoil ATF really helped mine alot. Had the tsb done 3 different times and finally pulled the trigger on Amsoil and did a complete flush. I also replace the tc and plan on doing both diffs in the future. My recommendation would be having the dealer do a quick learn as well.

flyingfool · February 1, 2021

1 hour ago, mookdoc6 said:

I do think your onto something with the EPA and GM using the TCC for added mpg's. I told my tunner "GUY" to off that crap too! He told me, "he could, but it's not advisable." He is extremely knowledgeable HP tunner guy and so I took his world the morning 1-2 will never go away it's to warm up the fluid.

if the TCC is active it will actually keep the fluid cooler, it gets warmer with the tcc shut off. so shut of atleast 1-4th gear to aid in warm up, and reduce clunk.

i also found zeroing out the tcc slip tables also made the trans run cooler because ther is no more 10-30 rpm slip difference between the converter and internal clutch.

today i drove 100 miles, at 65 mph, fully locked up in overdrive no slip on the converter, transmission is running at 113 degrees F, ambient air temp was 55-60 degrees out. my engine t-stat is a 175 F from katech, i am thinking about swaping in the cts-v 196 t-stat for more heat. but she runs so good, I averaged 34mpg fuel consumption for 40 mile trip average. this was a flat almost 1-2degree downhill section

in city driving my tcc is off for 1-4th gear so my fluid runs about 145 degrees F. stop and go traffic , hot rodding around town stuff

Edited February 1, 2021 by flyingfool

LunkerHunter · February 2, 2021

has anyone attempted to have a tuner like DIablo or Blackbear, disable the TCC system for the first 4-5 gears? this cured the clunk on mine.. i've looked at the tcc parameters on the 8 speed and its engaged right off the line , from accelleration and to a full stop. this is why your converters are failing and get clunks durring downshifts , and between clutch engage/disengagements

Can disabling the TCC system for the first 4-5 gears be accomplished with the Diablo i3?

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