Hard Shifting/"Thunk" from behind?

Offspring22 · May 12, 2022

So I have a 2017 Sierra 1500, with the 6.2L and 8spd transmission. For the last couple of years, I've had an issue where there are hard shifts (mostly 1st to 2nd) or occasionally while towing on the highway as well. More chronic is when I'm driving, usually the first 2 or 3 stops in the morning I feel like I'm getting bumped from behind. A thunk with the truck being knocked forward. That part has been getting worse where it's not just the first 2 or 3, but more sporadic now as well.

I've brought it in a couple times and been told that they don't notice the issue, or hooked it up to something and everything is "within spec". They did change the tranny valve body or something a while back as well, but no fix, and getting worse.

Anyone had something similar? Any fixes advice? Warranty is up in Sept so would love to get it solved before then. I got it going in for an oil change on Monday, so bringing it up with them again.

Thanks!

steelerdude15 · May 12, 2022

Do you know if you had the Mobil One transmission fluid put into your transmission? That has taken care of the hard shifting for some.

JimCost2014 · May 12, 2022

TSB Bulletin #18-NA-355

With this: Blue Label.

Edited May 12, 2022 by JimCost2014

Offspring22 · May 26, 2022

So just to follow up, I brought my truck in for a oil change and again brought up the issue. They agreed there was an issue this time, and I dropped it off on tuesday. Got it back today, and they said they tore into the transmission and replaced the "backing plate retaining rings" as well as the valve body again. Also used "Dextron HP ATF Mobil Synth fluid" So we'll see if it fixed anything....

Offspring22 · July 10, 2022

May as well update again. The last service didn't fix anything, so I went back in and spoke with the service manager who said he'd take a look at it personally. Long story short, they're ordering me a new transmission under "customer satisfaction", but it's back ordered and who knows how long that may take.

Hopefully it fixes it....

2017gmcsierraslt · August 25, 2022

Hi, I also have the 6.2L and the 8l90 with max tow.

My tranny has clunked from the day it arrived from factory special order.

Wondering if you got the new transmission and if you are happy with it or if it's doing the same thing.

Blbstack · August 29, 2022

How many miles are on your truck, Offspring22? And did they make any mention of the torque converter?

GMCustomerService · September 2, 2022

On 8/25/2022 at 3:47 PM, 2017gmcsierraslt said:

Hi, I also have the 6.2L and the 8l90 with max tow.

My tranny has clunked from the day it arrived from factory special order.

Wondering if you got the new transmission and if you are happy with it or if it's doing the same thing.

This is certainly not what we like to hear, 2017gmcsierraslt. Please know, customer satisfaction is extremely important to us. Our team would be more than happy to take a closer look into the concerns with your 2017 GMC Sierra. To do so, please send an email to [email protected] with ATTN: 2017gmcsierraslt/GM Trucks in the subject line.

Please include your contact information, VIN, a brief description of your concern, and the name of your preferred dealership. We look forward to hearing from you. To learn more about GM’s privacy policy, please visit https://www.gm.com/privacy-statement.html.

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