Chevy and GMC Duramax Diesel Forum banner

Allison 10 speed shudder on letting off accelerator in top gear--towing

269 Views 13 Replies 6 Participants Last post by  blueberv
Greetings Ya'll
Truck is a 2021 Silverado 1500 LTZ Duramax CC medium box.
Couldn't find any discussion on something similar to my experience and am hoping someone here has insights. I mostly haul a 7,000lb trailer and the shudder started to appear ~24,000 miles, now at 30,236; of these miles 25,000 or more were towing trailer. GM service always needed to queue diesel diagnoses, but now finally have downtime so it is now at service, but they say they don't have a way to simulate a load and they say no codes were thrown. Without load there is very minor/or no shudder. With load in 10th gear shudder is more apparent when gently letting off 'gas' pedal than quick off. Towing always in 'trailering' mode.

Could shudder be caused by ATF degradation, in which case replacing fluid will fix it? For severe driving the diesel manual says ATF fluid+filter change should be done at 45,000 miles but there is still some time yet, and I'd rather get a firm diagnosis.

On a maybe (un)related note, when towing, gear change from 1-2-3 seems like it misses 2nd, engine revs to ~2500 before it settles in gear, assuming 3rd. Again, no codes thrown, so 2nd must be a wash gear.
Thanks for any suggestions!
1 - 14 of 14 Posts
It sounds like it could be a TCC (torque converter clutch) issue. The dealership should be able to test drive with the diagnostic computer connected to identify slippage. Even without a load it might still have some imperceptible, excessive slippage that the test drive would identify. I wouldn't hurt to flush the ATF. FYI- I am not a transmission expert, but I have run this test on vehicles while working at a dealership, smaller vehicles. You might need a new TC (torque converter).
Thanks for your quick insight.
A service tech took the truck home for an 'extended road test', hope he also took the diagnostic equipment for a live feed. Otherwise will ask them to do the road test again.
  • Like
Reactions: 1
If it wasn't such a new vehicle and likely under warranty still, I'd tell you to flush the tranny, and put some "Instant Shudder Fix" in it. I've done it to 2 neighbors trannies, 1 which was "sealed" and it got rid of all or most of the shudder. But in your case, if it is still under warranty, I'd push the dealer hard to fix it.
Just a thought, you might get more answers if you remove 'Allison' from your thread title. You don't have an Allison 10L1000 in the 1500 you mention, you have the 10L80.

Has the transmission been serviced and if so, what fluid was used? Places that throw Dex6 or a 'universal' fluid in this is sometimes the result. I know the HD 10speeds definitely do not play well with the wrong fluid, I'd assume the 1500's 10 speed wouldn't either.
  • Like
Reactions: 1
I mostly haul a 7,000lb trailer
Had an issue somewhat similar in my old Dodge, dealer couldn't duplicate. I arranged a time with them and brought the truck and trailer in together, there tech rode shotgun while connected to the truck and I duplicated the issue for them.

If you have the trailer you may ask them for this arrangement.
  • Helpful
Reactions: 1
Appreciate all your input!
@Big Weezy--will need to look into the Shudder Fix but am hoping to convince dealer to repair. And it new as you said .....

It has not been serviced previously.
Yeah, actually spoke to the service manager because my service agent was not returning calls and messages as he was on vacation, but nobody else knew that at service when asked! How's that for working together as a team. Or caring to check!

Let's see what mgr says after his overnight jaunt with my truck, maybe he now likes it enough to change his mind about riding shotgun. He wasn't receptive to idea of me bringing trailer so a tech can ride along and said they can't do that, but it sounded like they 'don't' want to investigate farther. What's the deal with trying to diagnose and repair because it would be covered under warranty? Does GM encourage dealers to be lax on diagnosis for warranty repairs?
Does GM encourage dealers to be lax on diagnosis for warranty repairs?
Warranty claims pay out at a lower rate then the dealer normally charges. So naturally the dealer rather focus time on non-warranty work. It's also getting harder from what I understand to get warranty claims approved without any codes. Essentially they don't want mechanics, they want parts replacers to replace the part the computer says to replace.

The important thing is getting it documented so if it pukes after warranty there's a record of you bringing it in for the issue, rather it makes a diff or not is another story.

Quite sad really, and yes, the dealer can do what you're requesting, they are just choosing not too. May be best to get a second opinion from another dealer.
  • Like
Reactions: 1
Thank you, @Satar! Going to shop for another opinion too, this dealer just bugs me a touch.

Didn't know about warranty being 'less profitable' for dealers. And glad I didn't see your bit about not wanting to repair without a code or I would not have pushed on this point when I got their 'test drive' report.

They said there were no codes stored, or new codes, but that is quite peculiar to me. But also that without documentation GM would not approve the repair. Though that brings up a couple questions -- 1) does the cpu/programming not think shuddering is a fault, or a prelude to fault; 2) is it possible codes are not being retained? On 1), the weird gear 2-3 slip-shift >>should<< also throw a code. They also claim that riding shotgun with trailer attached will not change being able to document because the tech could experience the shudder & gear 2-3 shifts but if no codes appear then it would be a bust (for them).

Hate to wait to 'see what happens' by postponing a firm diagnosis because we are full timing and will be out somewhere else if something cracks, might be on an isolated country road. Have also experienced irresponsible dealers on travels, maybe rationale is I will not be around to make noise. So would really like to get this repaired on current down time.

Another question about shudder, like to take an opinion poll on severity--surely it can't be normal, and doesn't the shudder/knocking introduce repetitive wear? Just don't see how this is not a programmed fault, but anyone experience or is aware of longer term issues with shudder?

Much appreciate all your input!
See less See more
On another thought, could pulling the 7000+ trailer this much (~25,000+ miles) contribute to such an early tranny problem in a 10-speed? One could say probably, but I imagine they (1/2 ton trucks) are designed to work, though perhaps not as hard as 3/4 and up.
They are rated to tow a certain weight, there is no mention of how often or what percentage of miles can be towing. Even if towing 100% of the time the truck should be able to handle it and warranty will cover if there are any problems. Have you ever actually weighed your trailer? Most tend to overestimate the weight of a trailer and it’s likely lighter than you think.
A little comforting about warranty coverage, if I can ever get it done soon :(

Yes, was careful about weights because rated 9000lb tow & 1521 lb loading gets used up quickly. We are at ~82% of tow capacity at fully loaded trailer, last weigh under 7400. Truck is packed quite a bit at capacity. Am trying to unload some, but law of nature dictates loading to be always more than unloading.
Not saying it's your problem but just sayin':
We had something similar,, it was loose flex plate bolts. Not to be tightened, any time they are a problem they have to be replaced. One-time use bolt.
Covered under warranty.

Taking the truck to a dealer with lots of truck sales and a busy service dept means they've seen a lot of mulligans and went right to the issue.
^^ thanks for that tip! Suggesting any potential cause it what it's all about, appreciated.

That would totally make sense because cpu may be blind to it and not tag the event(s). Definitely worth a check.

Another dealership service advisor says about shudder "I have never heard of anything like this on a transmission." Not encouraging but will talk to other advisors.
1 - 14 of 14 Posts