8 speed downshift clunk to 1st gear

11bravoZ71 · December 19, 2020

Anybody else having this exact problem? It’s entirely random and usually does it at full operating temperature. Not when cold. When I come to a stop or red light as it down shifts to 1st gear theirs a clunk unless I hold the brakes tight sometimes it’ll avoid it. Happens 50% of the time. Never does it up shifting also shifts like butter other then that. Usually after a long freeway run Could be the torque converter? Idk. I’ve already drain and filled tranny to blue label new mobil 1 ATF and changed filter to proper level. Dealer is slammed busy right now but going to take it in eventually. But was wondering if it’s just me. I have the new body style 2019 z71 5.3 8 speed crew thought they fixed the 8 speed on these ones.

SPASierra · December 20, 2020

I think there are other posts about that here. Essentially the computer tries to anticipate that you are/are not coming to a stop, and ends up doing a 3-1 downshift. It was a bit annoying at first, but I rarely notice it anymore. I have an extended warranty, so I'm not terribly worried about it.

CamaroVetteGuy · December 20, 2020

I feel mine shift and downshift from 1st to second some. Maybe I would call it a mild clunk. Rest of the gears I barely feel. I think it’s normal.

RackAtak · December 20, 2020

Had nothing but problems with my 2016 8 speed until I took it in for this service bulletin.

https://gmauthority.com/blog/2019/08/tsb-seems-to-fix-8-speed-automatic-gm-transmission-problems/comment-page-4/

Edited December 20, 2020 by RackAtak

Vicarod · December 20, 2020

Mine does it. I wouldn't say 50% of the time. I do notice that it happens a lot less after the computer learns your driving habits, if that is such a thing. Dealer did a transmission software update when I took it in for the recalls in September.

After that it seems as if I was starting all over again with the transmission. It got better after a while.

I have no other issues with the transmission. Very smooth shifting.

But I do notice the downshift occasionally.

It's these weird shifting characteristics that put people off of the 8spd. Personally I don't blame them. I don't know the failure rates / reasons for these transmissions so don't know if this is an indicator of impending failure.

OP welcome to the forum. Hope your dealer can help you get to the bottom of this issue and keep us posted.

Also, what is the mileage on the truck and do you know the build date?

ejdziurka · December 21, 2020

Traded mine in for a 2021 10 speed because of it. Couldn't stand constantly feeling like I was getting rear ended. Noticed it mostly when the weather was hotter. Was outside the timeframe for the newer transmission fluid (already had it), and the TSB provided above is only up to 2018 Silverado, so I never took it in. I did notice if you let up from braking for split second, and starting braking again it didn't do it nearly as much. I had 19,000 miles on my truck when I traded it in.

Zach-91 · December 23, 2020

Mines been doing it a lot more the last week or 2. Happens at least half the time when at operating temp. Only 9000 miles. 2019 rst 2wd 8 speed.

I have done 12 quarts of the new fluid earlier this year.

I came across this post today and hope to try it soon.

11bravoZ71 · December 27, 2020

On 12/20/2020 at 5:02 AM, Vicarod said:

Mine does it. I wouldn't say 50% of the time. I do notice that it happens a lot less after the computer learns your driving habits, if that is such a thing. Dealer did a transmission software update when I took it in for the recalls in September.

After that it seems as if I was starting all over again with the transmission. It got better after a while.

I have no other issues with the transmission. Very smooth shifting.

But I do notice the downshift occasionally.

It's these weird shifting characteristics that put people off of the 8spd. Personally I don't blame them. I don't know the failure rates / reasons for these transmissions so don't know if this is an indicator of impending failure.

OP welcome to the forum. Hope your dealer can help you get to the bottom of this issue and keep us posted.

Also, what is the mileage on the truck and do you know the build date?

Thanks for the welcome. The truck has about 31k as of this post. Was purchased new November of 2019 But I’m not sure of the build date. I’m gonna try what the bottom post suggested see if it does anything. Meanwhile I’ll Be MIA soon for about a month. When I get back ill take it to the dealer and if I get a update I’ll be sure to post it. These cursed 8 speeds should have been fixed years ago.

11bravoZ71 · December 27, 2020

I agree. On the above comment. When approaching a red light or stop sign. If you manipulate your Breaking just right by letting off and then re engaging firmly , you can avoid the clunk to 1st almost every time. This truck didn’t do any of this the first 25k miles it only started this recently at about 27k miles.

bluemonstercali · January 27, 2021

Any update to the guys that have done the c1 or c4 relearn?

Swofford · January 27, 2021

I don't mean to jump into the middle of an existing discussion, however I purchased a 2020 LT Z71 with 5.3 and 8-speed earlier this month. The truck was 'used'/Certified Pre-owned.. if you can really call a truck with 1294 miles on the clock... Anyway, the truck had a hard (almost like being bumped from the rear) shift as I would come to a stop at a light /stop sign, within 2 miles of me taking the truck from the dealership. Called them totally upset and took it back the next morning to them. They took over a week to analyze. It turns out that a valve body on the transmission had been replaced (for some reason) when the vehicle was sold/traded to them in early December.. however through the process, there was to be a resynch/software update of the ECM/TCM which hadn't been done. Once they performed this, they test drove the truck for a few days to see if if had been resolved and to make sure that it wasn't a first startup or temperature issue... After they were satisfied, it was returned to me....and since then (about 2+ weeks), it has not returned and seems resolved. Maybe your dealership could look at this?

11bravoZ71 · May 25, 2021

Late update to this original post it turned out it needed a update from the dealer. After they updated it the 2-1 tranny clunk never came back...

the specific wording they used on the work order was “software anomaly”

Dunn · May 27, 2021

On 12/27/2020 at 8:01 AM, 11bravoZ71 said:

I agree. On the above comment. When approaching a red light or stop sign. If you manipulate your Breaking just right by letting off and then re engaging firmly , you can avoid the clunk to 1st almost every time. This truck didn’t do any of this the first 25k miles it only started this recently at about 27k miles.

I drive this way normally.....i always release the brakes to make the stop " seamless".....which means applying pressure and releasing couple of times for every stop.....just a habit ive always done.....my 19 RST had the clunk to first gear even still, though my 21 Elevation ( 8 spd) has been flawless so far......

Z45 · May 27, 2021

You have to live with it.......

[quote]The lawsuit, filed in the U.S. District Court for the Eastern District of Michigan, alleges GM's eight-speed automatic transmissions may shift violently into gear. ... GM claims the problems with the transmission do not fall under the factory warranty, as it is a design defect and not a manufacturing defect.[/quote]

onebadss · May 30, 2021

On my ‘18 with the 5.3 & 8-speed, it definitely had its share of strange shifting habits and clunks. I will say it seemed to drive smoother when I was using the Range AFM for a short period of time. I ended up trading it on a 6.2 10-speed pretty quickly, so I never had the dealer look at it.