New Sierra Transmission Clunk

Austin718 · March 11, 2018

Just bought a brand been 2018 GMC Sierra All Terrain and the transmission just started clunking!

2700 miles on it and I just threw her into 4wd for the first time. Headed up to the snow last weekend and right after I put her back into 2hi she started this clunks from 1st to 2nd fear.

super bummed I didn’t know about this before I bought it. Dealership told me it’s what GM calls acceptable.

Anyone have any any fixes for this issue?? Or know of any updates coming to fix it?

thanks guys! First post. Love everything about this truck. Except that huge clunking when she starts up

-Austin

Jonty711 · March 12, 2018

Were the front wheels disengaged?

Austin718 · March 12, 2018

After I took her out of 4wd, I did a couple donuts to feel for a crow hop and nothing. So yes the front wheels were disengaged

Crstfr · March 13, 2018

there's a service builtin for the 1-2 clunk.. i just had the computers flashed in my truck just for that... and it fixed the issue.. . mine was loud and would sometimes push you forward...

Austin718 · March 15, 2018

Dang thanks Chris i'll have to try that out. So weird, on Monday I got in a tight spot and had to floor it for a little. SInce then I have not felt it. Even trying to recreate it and nothing. So I'm obvioulsy happy for now but if it shows any sign of coming back Ill be back at the dealer asking about the computer.

Lost Hog · March 20, 2018

Hi all,

My 2017 1500 5.3 shifts much better after the 3 firmware updates, not sure which one it was as it dosen't show on the repair order. I still have a very hard downshift from 3-2 and 2-1 not all the time but when it does it its very obvious and annoying. Coming to a stop or exiting the highway on the ramp. I'm sure as it continues It will get worse as the specs increase from the wear. I pulled the dipstick today just to see if the fluid was still good and when I cleaned the stick it had alot of what appears to be metal or friction disk material. Is this normal? Attached pic. I have also noticed that after the temp goes past 160 or so that's when its more consistant with the hard downshifting.I was curious if anyone having this issue has tried full syn or those high end multi application fluid? My normal op temp is around 190 and I dont think thats too hot to cause issue. Seems like the computer is just mis-calcuating the speed during downshift,

Sorry if this has been covered already, I love the truck excluding the wierd transmission. Forgot to mention its a 6 speed and I have 1,933 miles on it.

nluchau · March 20, 2018

9 hours ago, Lost Hog said:

Hi all,

My 2017 1500 5.3 shifts much better after the 3 firmware updates, not sure which one it was as it dosen't show on the repair order. I still have a very hard downshift from 3-2 and 2-1 not all the time but when it does it its very obvious and annoying. Coming to a stop or exiting the highway on the ramp. I'm sure as it continues It will get worse as the specs increase from the wear. I pulled the dipstick today just to see if the fluid was still good and when I cleaned the stick it had alot of what appears to be metal or friction disk material. Is this normal? Attached pic. I have also noticed that after the temp goes past 160 or so that's when its more consistant with the hard downshifting.I was curious if anyone having this issue has tried full syn or those high end multi application fluid? My normal op temp is around 190 and I dont think thats too hot to cause issue. Seems like the computer is just mis-calcuating the speed during downshift,

Sorry if this has been covered already, I love the truck excluding the wierd transmission. Forgot to mention its a 6 speed and I have 1,933 miles on it.

Search on this forum for hard downshifting and odds are you'll find a few of my posts. I have a 2016 5.3 6 speed (bought it new and currently have 19k miles). I had it in several times for the downshift concern and sadly there is no fix. I've had it drilled into my head that it's a characteristic of the transmission. I even had the valve body with TCM replaced a few weeks ago, this did not fix it. Sadly, the issue is in the programming and GM will do nothing anymore with the 6 speeds. Love the truck, just hate the downshifts when coming to a stop. In town driving is stupid, on highway its awesome. BTW I see you are in Tampa. I love Florida!!! Fargo, ND here.

Crstfr · March 20, 2018

On 3/14/2018 at 8:47 PM, Austin718 said:

Dang thanks Chris i'll have to try that out. So weird, on Monday I got in a tight spot and had to floor it for a little. SInce then I have not felt it. Even trying to recreate it and nothing. So I'm obvioulsy happy for now but if it shows any sign of coming back Ill be back at the dealer asking about the computer.

hope this helps... this was the fix for mine.. with the clunks... and above is what they did.. drives totally different, and my gas range increased 2mph on my daily driving..

Lost Hog · March 20, 2018

Thanks so much for the info guys. I am going to bring it Fri and give them the bulliten info you provided. I will update on my visit there afterwards. So hope this corrects it. For all of our sake

Gary

Austin718 · March 22, 2018

Awesome this is exactly what I was looking for. Ill be headed in this weekend! Did they help you out Gary?

Lost Hog · May 17, 2018

No just alot of talking. The svc mgr showed me the bulletin where the do the flush thing but said that mine wasent bad enough to preform the procedure because GM wouldnt reimburse them for the time and materials. He would do the bulletin at my expense of course. I have a good friend that works at a transmission shop for 25 yrs and Iam thinking of just having them do the flush with the BG fluid exchange. They will warranty the transmission up to 200k with a full replacement as long as I have it serviced every 30k. Thinking its probably worth the $300. From what ive been reading the AC delco fluid just meets the minimum spec's to begin with.

On a similar note I installed the Range AFM delete module and it made the downshifting way worse so off it comes. Ill be returning that thing. It also leads me to believe more and more its the computer prematurely throwing down the gears to early.

Sorry so late responding

Thanks again for the input all!

JacobC1983 · May 27, 2018

Mine is a 6 speed.

i have the same transmission downshift clunk from 3rd to 2nd pretty consistently.

It it depends how fast I am slowing down.

The dealer has reset the learning parameters twice and flushed the transmission fluid once.

nothing has worked.... frustrating!

Lost Hog · June 8, 2018

Just paid for a flush and BG syn fluid upgrade at the advise of trans specialist. Made no difference at all. Still slamming down 3-2 really hard. Hate to think the damage thats going on with drive train. Going to make another appointment at the dealership to make sure I have the latest updates. She just Turned 5k and I have to say I'm less than impressed with GM's response to this issue. Hate to do it but I also may be jumping the fence if this remains unresolved. Although GM says its normal, its not. Love everything else about the truck.