Whats the deal with the 8 Speed transmission problem?

mnm99 · October 6, 2019

I have a 2017 Sierra . Figured I'd throw it out there for a tech. Have any of you been experiencing hard downshift clunks like I have when coming to a stop? Every time I bring it to the dealer for something un-related , I ask for them to check it out. It always comes back as normal. Just wanted your feedback what you thought about the transmission.

mjm-1957 · October 6, 2019

You will be living with those problems until you get rid of the truck. I am the transmission guy at a GMC buick dealer and you can't fix the unfixable. That transmission was horrible from the get go in 2014 and still keeps me plenty busy. Go drive a 10 speed and decide if you want to live with the 8 speed after the road test. Night and day difference between the two transmissions.Too bad you couldn't slip a 4l80e in all the problem trucks and be done with it.

Snowcamo · October 6, 2019

Its fixable just not through gm. Most any auto repair shop that knows these transmissions can fix you up. A few guys have resoved the issue by swapping the tq converter to aftermarket and reprograming others full rebuild and reprograming.

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jay webb · October 6, 2019

i am a gm tech as well but i dont do trans however seeing all the issues with these when i bought my 2018 sierra i made sure to get the 6speed

Thomcat · October 6, 2019

"The deal" is whether or not you have an 8 speed manufactured before 3/1/19. There's a TSB 19-NA-121 out on this which techs should be aware. Original OEM fluid is hydroscopic causing shifting problems. Later production uses Mobil 1 Synthetic LV ATF HP requiring a flush and fluid replacement to cure the problem.

TSB Silverado new fluid.PDF

elcamino · October 6, 2019

I wonder how many problems are the result of dealers using old inventory or low cost local vendor ATF instead of following the TSB? Some dealers have staff that are not well trained, seen it here. The SM does NOT even drive a GM vehicle. I seen him at local restaurant one weekend and he had a new Jeep Cherokee and it had a Jeep dealers name (out of town).

26,500 miles and my 8sp is fine. I get rid of the factory fluids early on, don't trust the bean counters and this trans fluid snafu is a prime example of 'good enough' lubes.

Edited October 6, 2019 by elcamino

Thomcat · October 6, 2019

I'd say it can happen. Had tranny fluid done at a Chrysler dealer specified ATF-4 synthetic, then checked with their parts department to buy an extra quart and was told they don't stock it.....sure enough they stuffed ATF-3 in there........when I complained they said they'd warranty the tranny for 100K.....that's when I switched back to GM.

My 8 speed was mfrd. 3/19 with the new fluid and shifts smooth and reliable. Didn't get it just for the number of speeds, but for the DFM. They've been running that kludged up AFM since my '07 Avalanche and making constant changes to that screwed up system....never made much sense running 4 dedicated part time cylinders with a LOMA manifoild running oil to separate control valves for these cylinders and dumping the bypass into the sump where it caused increased wear on the colder running AFM cylinders. DFM dumped the LOMA manifold and runs directly to separate oil control valves for each cylinder and by rotating firing can maintain constant environment for each cylinder. A simpler mechanical arrangement but requires a more complex electronic control arrangement. Anyway more speeds and engine control translates to more power felt compared to my '16sAFM 6 speed.

Havasubum · October 6, 2019

I just got my 2017 Denali with the 8 speed trans back from the dealer. They changed the valve body and installed synthetic ATF. I hadn't been experiencing the shudder (torque converter) but the 1-2 shifts both up and down were enough to piss off the Pope. I'll let you know how it does after a put a few miles on it. BTW, I had to do a full court press with the dealer (who I've bought several cars from) to get them to do anything. The service advisor said other customers were satisfied with the fix. We'll see. I wonder if the new C8 8-speed will have similar problems. Good luck to all of you.

Snowcamo · October 6, 2019

"The deal" is whether or not you have an 8 speed manufactured before 3/1/19. There's a TSB 19-NA-121 out on this which techs should be aware. Original OEM fluid is hydroscopic causing shifting problems. Later production uses Mobil 1 Synthetic LV ATF HP requiring a flush and fluid replacement to cure the problem.
TSB Silverado new fluid.PDF

Actually members have come back after miles of drive time stating the shudder and vibrations came back. Most traded... That fuid tsb is bull.

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mjm-1957 · October 6, 2019

To all the experts on here, the fluid flush does nothing to help shifting. It is only to remedy the shudder. I have had 2 2019 trucks already with the same problems as all the others. Both built after 3/1/19. The 2-1 downshifts will knock your fillings out, happens randomly. Good luck trying to get it fixed. GM ties our hands and doesn't want to pay for anything lately. I also made sure I bought a 6 speed in my 2017.

mnm99 · October 6, 2019

Wonderful...Thanks guys. Any other GM techs with info in this. So pretty much its unfixable? I have more of a downshift and upshift clunk. Like it doesnt know what gear in.

huntr1117 · October 6, 2019

The dealer flushed and added the new fluid to mine and it helped but it still wasn't great. Had the trans tuned and it's great now. 5k miles so far and it shifts the way it should have from the factory.

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Doug_Scott · October 7, 2019

Have you ever asked yourself how did we ever get along before the internet was invented. Answer is we got along just fine. Take any advice you get with a grain of salt (no idea what that means, I'm old, that is stuff we said before internet) and get the warranty work done. Just because a couple of people did not see an improvement does not mean it won't work for you. You have no idea what is driving their complaint, and you have no idea what their driving habits are. You also need to remember that some people are easily led. Back when I was the transmission mechanic (this was before "techs" were invented, but after grease monkeys were gone) at a Chrysler dealership I got to see what it was like when new technology came out. Chrysler went to lockup torque converters in 77. Between the shuddering converters and the results of putting Teflon rings with overlapping ends instead of the cast iron rings with interlocking tabs I made a good income due to those 2 issues, 4 years at the first place, then another 2 at the next place. Those Teflon rings were maybe 3/16 square ring, they would wear 2 grooves in the reaction shaft support (part #4202089, part number that paid me 8hrs warranty time to replace, could do 2 and a half cars a day). I purchased my first brand new car in 78, and it wasn't a Chrysler product. When management asked me why didn't I buy a Chrysler product, I said all I ever saw were broken ones.

TXGREEK · October 7, 2019

Have you ever asked yourself how did we ever get along before the internet was invented. Answer is we got along just fine. Take any advice you get with a grain of salt (no idea what that means, I'm old, that is stuff we said before internet) and get the warranty work done. Just because a couple of people did not see an improvement does not mean it won't work for you. You have no idea what is driving their complaint, and you have no idea what their driving habits are. You also need to remember that some people are easily led. Back when I was the transmission mechanic (this was before "techs" were invented, but after grease monkeys were gone) at a Chrysler dealership I got to see what it was like when new technology came out. Chrysler went to lockup torque converters in 77. Between the shuddering converters and the results of putting Teflon rings with overlapping ends instead of the cast iron rings with interlocking tabs I made a good income due to those 2 issues, 4 years at the first place, then another 2 at the next place. Those Teflon rings were maybe 3/16 square ring, they would wear 2 grooves in the reaction shaft support (part #4202089, part number that paid me 8hrs warranty time to replace, could do 2 and a half cars a day). I purchased my first brand new car in 78, and it wasn't a Chrysler product. When management asked me why didn't I buy a Chrysler product, I said all I ever saw were broken ones.

You’re correct, there are more trucks on the road today with zero issues than there are on this forum complaining about one thing or another. I’ve got an 18 6.2 with an 8 speed transmission, I LOVE MY TRUCK, will I ever have problems? I’m sure I will as I’ve had with my past Dodges and Fords. There’s not one automotive manufacturer out there that doesn’t have any issues with one thing or another. Buy what you want, don’t base your opinion on whether to buy or not based on complete strangers website comments.

I’ve owned a 14, 16 and now an 18 K2’s and have never had an issue that couldn’t be fixed.

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