6 Speed Transmission Clunk Downshift

[16LT4]

While i completely agree with Grumpy, for those who’s aren’t testing their case strength, add a bottle of Libeguard Red to you trans and you’ll see problems go away. That and changing every 30k like ya should be doing. 

[CSI-WALLEYE]

7 hours ago, Grumpy Bear said:

Problems with threads like this. 10 guys who never met each other with 10 different situations assuming 10 different assumptions are creating the exact same issue. Make a guys head hurt....

 

How would one begin to sort that actual defective units for those whose operators are aggravating a normal situation like the one below: 

 

What? You mean you can't abuse a transmission and have it ACT NORMALLY?  

 

Thanks for the reply Grumpy.

 

I have the same suspicions.  There are millions of trucks out there and we are hearing mostly from people who are having problems.  Is this due to design flaws? or is it due to abuse? or is it column A and column B?  It may be that several different types of abuse may all expose a weakness in the transmission that will never be seen with proper care.

 

I am new to truck ownership but from reading sites like this is It seems that if you avoid modifications and perform recommended maintenance you can avoid a lot of trouble.

[Baytrucker]

On 8/12/2019 at 10:16 PM, CSI-WALLEYE said:

 

I have the same suspicions.  There are millions of trucks out there and we are hearing mostly from people who are having problems.  Is this due to design flaws? or is it due to abuse? or is it column A and column B?  It may be that several different types of abuse may all expose a weakness in the transmission that will never be seen with proper care.

 

I tend to think it is quality control. GMs level of QC has taken a pretty big hit since around the time of the bailout. This is inevitably going to lead to more problems like this popping up, not just with one-off issues, but with the core design. Defects that everyone experiences.

 

I remember reading companies like GM and Chrysler have a different engineering philosophy than most other car companies. Traditionally, when an engineer designs a vehicle, they submit a drawing to assembly, and that drawing is the final standard. If a parts manufacturer or assembly line does not conform with the drawing, then they are instructed to do-over, and match the drawing.

 

Conversely, GM and Chrysler have a different philosophy; If a part manufacturer designs a part that isn't exactly to specification, GM and Chrysler won't necessarily send it back. If there are no immediately discernible problems with the non-spec part, they will amend the drawing to accommodate.

 

This is just something I regurgitated from memory so take it with a grain of salt. I can't remember where I read it, but I've seen if a few times. If it was true, it sure would make a whole lot of sense, and explain why a lot of the issues we see with GM and Chrysler are shared by so many of their customers, and why the problems with other companies (Toyota's) seem to be mostly one-offs.

 

 

 

 

 

[NorthEast]

Yeah my issue was present from the first or second month of ownership.  Not from beating on the truck.  I had a Nissan Frontier that I drove the shit out of, on and off road, that only started to show a slight tranamission issue at 125,000 miles.  Even at that mileage, it still performed with more refinement than this GM 6 speed.  

[jerry g]

Hey guys, mine does the same thing hard shift at times. And when it goes to 3rd  makes a clicking noise under the car.

[Fullback]

I have a 2018 Silverado LT with a six speed tranny.  Cruising at any speed, I will experience a random clunk for no apparent reason!  Almost like it can’t make up its mind whether to shift up or down. Just a random clunk with a slight slipping feeling.. 

anyone out there with the same thing?  Only 10,000 miles on this vehicle!  Did I buy a lemon?  

Edited January 18, 2020 by Fullback
Misspelling and added info

[CamaroVetteGuy]

Mine will occasionally down shift hard. I don’t know if it’s a big deal or not? Tends to happen if I’m on the freeway and step on it. It slams violently. Otherwise no shake or other issues. The majority of the time it’s very smooth.

[Fullback]

Yes mine will do that occasionally as well,  I agree, I don’t think it’s that big of a deal.   I can set my cruise at any speed on miles of flat highway and it will randomly clunk about every 60 seconds. While doing so the RPMs will fluctuate plus and minus 200  or so.   I just fear the dealership is going to tell me that they couldn’t find anything or that it’s normal.   

[Clark2002]

I recently purchased a used 2014 Silverado and it has a hard downshift once the transmission warms up. I’ve been driving it for a few weeks and just noticed it the other day. One time is was quite a hard downshift. Not sure if there is a fix exactly. I keep hearing “oh they do that”. This is the first Chevy I’ve owned in a long time. 

[Winter63]

On 3/30/2019 at 5:50 PM, 6.2_GMC_DEN said:

It took hours any many revisions to fix the driveline clunk issue I experienced. It is a combination of upshift and downshift scheduling problems, torque converter lock up and unlock at the wrong times,  desired output torque factor issues, torque adder issues, stall torque management, tip-in torque management, limit torque management, torque reduction, and more. Basically the stock tune is commanding too much of everything at the wrong times.

 

Mine shifts like butter at part throttle and is nice and crisp at WOT. The way it should of been from the factory.

 

 

Its a long shot but do you have your tune i could compare mine with?

 

[superdave160]

I have a 2017 Silverado 1500 WT with the 5.3L and 6 speed transmission, and I have learned quite a bit about the transmission lately.  When the truck was bone stock, I had an engine and transmission tune from BlackBear Performance that I was really happy with.  Before the transmission tune, there were clunks and jerks and a lot of the issues described by others in previous posts.  From the factory, the torque converter clutch locks up in all 6 gears, when upshifting and down shifting (probably in reverse as well).  The BlackBear tune prevents the torque converter from locking up in gears 1-3.  That eliminated the issues when decelerating and in the lower gears.  That is #1 on my list of recommendations.

 

I also changed the 90 degree C. thermostat for the 70 degree C. thermostat.  So now the transmission runs much cooler under normal conditions (#2).

 

In January, 2023, I had the DOD delete done on my engine.  Long story made short, I had LOTS of issues with the engine.  In the process of getting my engine tuned, the transmission tune was changed as well.  The shop that fixed my engine tune, changed the transmission tune so that it didn't shift gears until the engine RPM was a lot higher than before.  I'm talking in the range of 3500 RPM for normal street driving and even higher when harder on the throttle.  Well, around town fuel mileage went way down because my engine was revved all the time.  I pulled the tune to look at it on my computer and I found that the main parameters that control when the transmission upshifts and downshifts are throttle position and speed.  There are other modifiers, but those are the main parameters.  I played with the shift speed until I had the transmission shifting when I thought that it should.  Another issue that I had was that the transmission was very reluctant to downshift.  If I was running 75 MPH on the interstate and started up a big hill, the speed would drop off and the engine timing would drop into the single digits (I have an OBDII monitor so that I can monitor spark advance, LTFT, STFT, gear, torque converter lock, etc.).  When it did downshift it usually dropped 2 gears.  It appeared to me that the upshift values were modified without modifying the downshift values to match.  I adjusted those values as well so that it downshifts when it should.  My current part-throttle shift table is copied below.  Notice how the upshift values are slightly larger than the downshift values so that it won't toggle between two gears.

 

 

Another issue is the torque converter lockup.  Throttle position and speed also control torque converter lockup.  If the gears are changing and the torque converter is locking and unlocking you will definitely feel it in the seat.  As previously stated, I set the torque converter so that it will only lock up in gears 4-6.  I set the torque converter lockup a few miles per hour higher than the 4th gear shift speed so that the transmission shifts into 4th gear and then speeds up a few more miles per hour before the torque converter clutch locks.  I set the 5th and 6th gear shift speed and torque converter lockup speed to be the same.  Essentially, once the torque converter locks up in 4th gear, it will remain locked in 5th and 6th gear.  I tried giving 5th and 6th gear about 5 MPH of torque converter unlock speed and found that the transmission temperature got WAY hotter.  I found that torque converter unlocked = hot fluid; torque converter locked = cooler fluid.  So I attempt to keep the converter locked as often as possible, without constantly locking and unlocking.

 

 

Understanding the effect that throttle position has on gear selection and torque converter lockup, if you are driving and reduce throttle input, then the transmission will likely upshift.  If you then increase throttle, then it will likely downshift.  And a quick decrease/increase on the throttle will likely cause the transmission to upshift/downshift very quickly, which makes it appear that the transmission is confused, but it is the throttle input that caused it.

 

I tow a 5th wheel camper with my truck.  I have found that the transmission shifts much harder after I have unhooked the camper.  I use tow/haul mode when towing, and turn it off when not towing.  Still, the transmission shifts harder after I unhook the camper.  I have found that this condition will go away if a few days.  I suspect that the transmission is "learning" the driving style when towing the camper, and it takes a little time to "unlearn" that data.

 

I do not know if all GM 6 speed transmission have the same transmission tuning file or if they are different based upon gear ratio, tire size, or other parameters.  I suspect that a quality transmission tune would help alleviate a lot of the shifting issues experienced by some owners.