2020 2500HD Transmission issues

KC Desert · September 20, 2020

Any of you have a situation where you are going to stop and just before it stops something goes thunk in a way that feels like you have been slightly rear-ended? No unusual lights or other behavior but is it the drive shaft or tranny? No idea. Clunk, thunk or Cathunk then full stop and it’s RIGHT before the full stop and not every time but seems to be when coming from speed to a stop not just light to light drive n stop.

Tomato · September 20, 2020

12 hours ago, KC Desert said:

Any of you have a situation where you are going to stop and just before it stops something goes thunk in a way that feels like you have been slightly rear-ended? No unusual lights or other behavior but is it the drive shaft or tranny? No idea. Clunk, thunk or Cathunk then full stop and it’s RIGHT before the full stop and not every time but seems to be when coming from speed to a stop not just light to light drive n stop.

Mine has done it a couple of times, but my 2015 did it regularly. Must be something with the torque converter...

Geoff Hubbard · November 29, 2020

I have a 2020 GMC AT4 2500. (Diesel) Currently at 7040 miles. Problem started 28 Nov 20. Just prior to a full stop the transmission will violently slam into it's lowest gear. I thought at first it was just a hiccup but not the case. I came to three more stops and it kept on doing it. So I have been driving in manual mode until I can get it to the shop tomorrow. Had similar problems with my 2016 GMC 1500. They sort of fixed that problem. Changed fluid, did something with the computer but I was back in again in about 6 weeks. I am somewhat confident that this is a signal problem of sorts. Sense it does not do it in manual I would assume that this is an electrical component when only in drive mode. We will see. Haven't even got my first oil change. How sad.

Geoff Hubbard · December 2, 2020

OK. Took the truck in on 30 Nov 20 and they did a transmission update. Hmmmm OK, so the truck has been running and shifting fine, just like new. So today 2 Dec 20 the transmission started shifting into that first gear violently again. Did it consistently for the next four stops. Called the shop and they actually said that the transmission is still learning and it will for the next 1000 miles. BS. It should not do this at all. So now I cant get in until Monday. Great. Will see what they do this time. So they really expect me to believe that the transmission is still learning.

Tomato · December 4, 2020

On 12/2/2020 at 3:15 PM, Geoff Hubbard said:

OK. Took the truck in on 30 Nov 20 and they did a transmission update. Hmmmm OK, so the truck has been running and shifting fine, just like new. So today 2 Dec 20 the transmission started shifting into that first gear violently again. Did it consistently for the next four stops. Called the shop and they actually said that the transmission is still learning and it will for the next 1000 miles. BS. It should not do this at all. So now I cant get in until Monday. Great. Will see what they do this time. So they really expect me to believe that the transmission is still learning.

Are you in tow/haul? If not, you should be stopping to and starting out in 2nd, right? I would try with tow/haul on and off to see if that makes any difference. Doesn't make it right, but I would be curious.

KuiMonster · December 4, 2020

On 9/19/2020 at 8:48 PM, KC Desert said:

Any of you have a situation where you are going to stop and just before it stops something goes thunk in a way that feels like you have been slightly rear-ended? No unusual lights or other behavior but is it the drive shaft or tranny? No idea. Clunk, thunk or Cathunk then full stop and it’s RIGHT before the full stop and not every time but seems to be when coming from speed to a stop not just light to light drive n stop.

Mine has been doing this lately but i figured it was the case of water i have in the bed... I guess once i get my truck back from the dealer for brake and steering issues ill remove the water and see if it still happens.....

Geoff Hubbard · December 4, 2020

11 hours ago, KuiMonster said:

Mine has been doing this lately but i figured it was the case of water i have in the bed... I guess once i get my truck back from the dealer for brake and steering issues ill remove the water and see if it still happens.....

The problem is just before the stop. It has actually slammed into the lower gear bad enough to make my front tires skid. I have made sure that I was in normal mode. The dealer does recognize that there is a problem. I am taking it back again Monday. I will try tow/hall mode and see if it does it at stops. It does not do it while in manual mode at all. So. Yes its electrical causing a mechanical malfunction to include premature ware on components. Not good. Although they say its learning your driving habits. BS

Eyehunter54915 · December 24, 2020

Less than 500 miles on a 2021 3500 Denali HD, headed down the road and started slipping bad. Engine light, the whole deal. 82k on a truck and didn't make it to 4 digits on the OD without a trip to the dealer. Guess I'll just keep making payments while they beta test these new "Allison's" on the consumer.

Eric Zbytowski · December 28, 2020

@gpraceworks Any update on your truck,?

I just purchased a 2021 GMC Denali 2500 and it did the same thing you described 3 times now. Going to pass punch the throttle and as soon as I pull out it's like the transmission is missing gears and shakes the truck. In all cases I was running around 60~65mph, punched it, and pulled out as it was downshifting and starting to put the power down large shaking and shuddering. I was not pulling or loaded down. Basically just 2 of us in the vehicle.

- The first time I pulled over and couldn't find anything wrong. I tried several times to repeat it that day and could not. (about 1500miles on the truck)

- The second time again but I just back off the throttle, rolled back on, and continued my pass. (about 2500miles on the truck)

- The third time (same day) I started recording my passes with my ODBII scanner and found that it looks like the transmission is slipping in comparison to good passes. It down shifts from 10th gear to 6th gear. Holds that gear but it was slipping on that event. In this case I backed off and pulled back behind the truck I wanted to pass. I tried to repeat it again several times but had no luck.

Other than this issue it up shifts and down shifts pretty well in all other cases. Any others having this problem?

Geoff Hubbard · December 29, 2020

Sounds like you might have a clutch issue. Here's the deal with what I did. Mine, not so much like yours but would slam into gear right as I'm completing a stop. Took it in after it did it 5 times. They unloaded new software for the transmission. Now. Here is the catch. After getting the upload I found out the it was in fact a bulletin that GMC put out for that transmission. Not considered recall. So you won't get it until you have an issue. They uploaded the new software and things were fine for about a day. Then it happened again. So I went on YouTube and watched a guy from allison transmission returned a transmission. The real catch to this is PATIENTS. Now I'm not a patient person when spending money like that but he's right. THE TRANSMISSION MUST LEARN YOU DRIVING HABBITS AGAIN. So I was patient. Its been almost two months and drives perfect. No issues. Ask for the updated bulletin on that transmission. See if it works. We as owners now have a part that we have to do thanks to software changes. Once you get the unloaded driving done you will need to go through it again when you pull a heavy load. Good luck

jrushmaine24 · March 10, 2021

12,000 miles on my truck and I unexpectedly seized 5 gears in my transmission. I was 50 miles in the north Maine woods and crawled into cell phone range with 4WD. No high temperature alarms throughout the whole ordeal. Cherry on top was the towing company peeling the paint off my front bumper. GM gave me a new transmission, but I feel like I’m driving an $80k time bomb. Not happy to say the least. Should have kept my 2009.

Craighurd87 · March 10, 2021

2021 2500 hd diesel . I noticed a leak coming from the transmission inspection cap leak is coming from the bell housing.

duquettecraig · March 31, 2021

2007 2500HD 6.6L with Allison 240k miles. I change the fluid every other summer, Zero engine or Transmission problems. I hope I didn’t just jinx myself.

Scott Wedge · September 9, 2021

On 11/16/2019 at 4:28 AM, gpraceworks said:

I have a 2020 AT4 2500. At 375 miles, I pulled out onto a 3 lane road an went to give it the throttle and the truck jerked an bucked and wouldn't go. It limped home at about 35-40 mph because the transmission wouldn't shift. I got an email from OnStar to have it serviced immediately. I connected my Diagnostic tool and it gave me 4 errors, all transmission related.

I took it to the dealer and they said the transmission fluid was low. They also fixed the Wireless charging pad, (factory didn't plug it in) I drove the truck for another 2 weeks and at around 1100 miles on the odometer the same thing happened. I haven't even towed anything yet!

Took it in again. Dealership said GM is going to replace the transmission. Then called back and said they are not going to replace the transmission, but instead the "valve body". I asked for the part# of what they are replacing and he didn't know. I will get that information when I pick the truck up next week.

This is super frustrating to say the least. I got a text from the salesperson where got the truck, and he said ther rep told him that 4 out of 5 trucks are having this or similar transmission problems.

Has anyone else had this problem?

My 2020 Silverado 2500HD is in the shop for the exact same thing. Chevy will have had my truck for 4 weeks by the time it is repaired! Very disappointing.

Rodney Goins · September 9

2- 2023 duramax trucks both less than 60,000 miles one was in shop for month 2nd one still in shop over a month and waiting. Have driven 6 speeds never touched with milage in the 500,000 miles no problems and better fuel milage time to go back, because ford amd dodge went to more gears doesn't mean you need to match stick to what works