6L90E (gas truck) 2-1 downshift CLUNK

[Blatant]

^^^^ QTF.

Even this GM forum is budget...fck.

[AmericanPie]

My 2020 GMC Sierra 2500HD has 94K on it, and I'm having the same intermittent downshift clunk and jerk from 2 to 1 upon slowing to a stop.  

 

I had it at the dealer for other engine work, and of course they couldn't duplicate the problem. 

 

Has anyone else with this problem found a reason and a fix?

 

 

[Grumpy Bear]

18 hours ago, AmericanPie said:

My 2020 GMC Sierra 2500HD has 94K on it, and I'm having the same intermittent downshift clunk and jerk from 2 to 1 upon slowing to a stop.  

 

I had it at the dealer for other engine work, and of course they couldn't duplicate the problem. 

 

Has anyone else with this problem found a reason and a fix?

 

 

 

Red this: 

 

On 9/3/2022 at 1:12 PM, newdude said:

The 6Ls have have had the same clunks and bumps since 2007.  I had a 2012 (6L80), a 2015 (6L90), 2016 (6L50) and a 2019 (6L80), they all exhibited the same exact 2-3 flares, 2-1 downshift bumps and coasting clunks.  

 

They are normal.  

 

 

[diyer2]

My 2012 does not have this problem. Maybe my doing more frequent fluid changes helped.

[Dunn]

Had a 2020, 2022 and now a 2023 silverado 2500....all gas 6.6l

 

My 2020 not a peep, my 2022 just a lil bit, soft clunk and rare

 

2023 hard jolt usually going up slight hill and coasting into 1st..  it seems to be smoothing out.....the first time was like jeez f#$% what just happened

 

Programming issue makes a ton of sense....

 

Are the 10spds doing it too I wonder?

[SoCal Angler]

Interesting....I had the same 2-1 clunk on my last truck, '03 Ford F350 V10 Gas with 4 speed 4R100 auto.  Truck had 82K miles. It ended up being the driveshaft slip yoke at trans end. 

 

Fix...pulled drive shaft, removed boot, cleaned shaft splines (also cleaned inside trans area where yoke enters), coated with Ford OEM slip yoke shaft grease (blue teflon), installed shaft with new boot......shifted as sooth as butter!  Never another 2-1 clunk.

 

Some Ford forum members claim...there was never sufficient grease even when these trucks were new. 

[GMCustomerService]

On 8/25/2023 at 12:38 AM, Dunn said:

Had a 2020, 2022 and now a 2023 silverado 2500....all gas 6.6l

 

My 2020 not a peep, my 2022 just a lil bit, soft clunk and rare

 

2023 hard jolt usually going up slight hill and coasting into 1st..  it seems to be smoothing out.....the first time was like jeez f#$% what just happened

 

Programming issue makes a ton of sense....

 

Are the 10spds doing it too I wonder?

This is certainly not what we like to hear, Dunn.  Please know, customer satisfaction is extremely important to us. Our team would be more than happy to take a closer look into the concerns with your 2023 Chevrolet Silverado having a downshift clunk. To do so, please send an email to [email protected] with ATTN: Dunn/GM Trucks in the subject line.

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[AmericanPie]

On 8/7/2023 at 5:44 AM, diyer2 said:

My 2012 does not have this problem. Maybe my doing more frequent fluid changes helped.

I believe you are correct sir!  As I'm the second owner of this truck, and not sure of the maintenance performed on the trans, I recently I did a complete fluid flush.  I based my decision to do a complete flush on the condition of the old fluid, and not the mileage, because the overall condition of the trans was very good and the pan was very clean.  Many people warn not to flush a high mileage trans.  However, high mileage trans failures following a complete flush are usually associated with mechanical problems that cannot be fixed by a simple fluid exchange. 

 

I looked for a posting from someone with a 2020 and newer 6L90 trans, but no luck.  So I tried to follow the 2016 to 2019 info that was available.   WRONG!

 

If you're doing a complete flush, on a 2020 L8T - 6L90 combo, then you can follow the steps that I took.  Please read to the end for an afterthought. 

 

1.  Jack up your truck and set it level on the big 6 ton jack stands.  Yes check level, with a bubble level or equivalent, sise to side and front to back on frame rails!

 

2.  Remove the LH front wheel and tire,  then the fiber wheel house, exposing the lower LH trans cooling return line coming out of the radiator. 

 

3.  With the trans cool, pull the fluid level check plug in the pan, and drain the cold trans fluid.  This is the o-ringed plug on the upper horizontal step of the pan.  I think it's a 15 mm socket.

 

4.  Then drop the pan and drain the balance of the cold fluid.  Clean the pan and magnet.   This will give you an idea of the actual condition of the trans. 

 

5.  Remove and replace the trans filter with a new one.  As I don't have the tool to remove the replaceable seal, I reused the original with no issues.

 

6.  Replace the cleaned pan.  You can reuse the pan gasket if it's not damaged.

 

7.  Pull the fill plug (on the RH side of the case (about where a traditional dipstick tube would be), and insert a 2 foot long, 3/8" rubber hose with a funnel attached on top. 

 

8.  Add the correct DEXOS 6 rated full synthetic trans fluid,  in the same amount that you drained.

 

9.  Remove the lower LH trans return line from the radiator and insert a piece of 1/2" ID hard plastic tubing into the radiator and run it into a graduated 2 gallon container.  These are available at Menards in the paint dept.

 

I used a short piece of translucent hard plastic tube into the trans and added clear tubing, so I could see the condition of the fluid.

 

You can add a piece of rubber hose to the actual return line to catch residual fluid.  Place both in your graduated bucket.

 

10.  Have a helper start the engine while you watch the old trans fluid flow through the clear tube into the graduated bucket.  Run it until it stops full flowing.  Add more new trans fluid to equal the total removed, and continue this process until the fluid runs cherry red.

 

I used 3-1/2 gallons (14 to 16 quarts) to do this.  I ran the trans thru all 6 gears (while up on jack stands) to be sure we cleared all old fluid, and warmed the fluid above 160 degrees.

 

11.  With the trans at operating temperature, in park with engine idling, I removed the fluid level check plug from the pan and let fluid run out until it stopped flowing strong.

 

12. Finally we took it on a road trip and the trans operated perfectly (albeit rather hot at 200 plus degrees while towing).   In a short time the hard downshift and lurch were no longer evident. 

 

Afterthoughts...because the 6L90 trans has a transmission Thermostat Bypass Valve (TBV), some of the old fluid kept recirculating in the trans, while I was flushing it.  It would have been advantageous to set it up so fluid would not recirculate, rather go directly into and out of the radiator trans cooler.   This could have been accomplished by flipping the pin in the original TBV, or have the trans at operating temp for the entire time (which is near impossible, unless you have the updated 160 degree TBV (which is available at GM dealers under a Service Bulletin number 21-NA-199).

 

BTW, because we had a hard downshift and clunk reported on a dealer repair order, and the dealer couldn't duplicate the problem,  GM has since replaced the original 190 degree TBV, with a new 160 degree TBV.  WORKS GREAT!

 

 

 

[Pryme]

5 hours ago, AmericanPie said:

I believe you are correct sir!  As I'm the second owner of this truck, and not sure of the maintenance performed on the trans, I recently I did a complete fluid flush.  I based my decision to do a complete flush on the condition of the old fluid, and not the mileage, because the overall condition of the trans was very good and the pan was very clean.  Many people warn not to flush a high mileage trans.  However, high mileage trans failures following a complete flush are usually associated with mechanical problems that cannot be fixed by a simple fluid exchange. 

 

I looked for a posting from someone with a 2020 and newer 6L90 trans, but no luck.  So I tried to follow the 2016 to 2019 info that was available.   WRONG!

 

If you're doing a complete flush, on a 2020 L8T - 6L90 combo, then you can follow the steps that I took.  Please read to the end for an afterthought. 

 

1.  Jack up your truck and set it level on the big 6 ton jack stands.  Yes check level, with a bubble level or equivalent, sise to side and front to back on frame rails!

 

2.  Remove the LH front wheel and tire,  then the fiber wheel house, exposing the lower LH trans cooling return line coming out of the radiator. 

 

3.  With the trans cool, pull the fluid level check plug in the pan, and drain the cold trans fluid.  This is the o-ringed plug on the upper horizontal step of the pan.  I think it's a 15 mm socket.

 

4.  Then drop the pan and drain the balance of the cold fluid.  Clean the pan and magnet.   This will give you an idea of the actual condition of the trans. 

 

5.  Remove and replace the trans filter with a new one.  As I don't have the tool to remove the replaceable seal, I reused the original with no issues.

 

6.  Replace the cleaned pan.  You can reuse the pan gasket if it's not damaged.

 

7.  Pull the fill plug (on the RH side of the case (about where a traditional dipstick tube would be), and insert a 2 foot long, 3/8" rubber hose with a funnel attached on top. 

 

8.  Add the correct DEXOS 6 rated full synthetic trans fluid,  in the same amount that you drained.

 

9.  Remove the lower LH trans return line from the radiator and insert a piece of 1/2" ID hard plastic tubing into the radiator and run it into a graduated 2 gallon container.  These are available at Menards in the paint dept.

 

I used a short piece of translucent hard plastic tube into the trans and added clear tubing, so I could see the condition of the fluid.

 

You can add a piece of rubber hose to the actual return line to catch residual fluid.  Place both in your graduated bucket.

 

10.  Have a helper start the engine while you watch the old trans fluid flow through the clear tube into the graduated bucket.  Run it until it stops full flowing.  Add more new trans fluid to equal the total removed, and continue this process until the fluid runs cherry red.

 

I used 3-1/2 gallons (14 to 16 quarts) to do this.  I ran the trans thru all 6 gears (while up on jack stands) to be sure we cleared all old fluid, and warmed the fluid above 160 degrees.

 

11.  With the trans at operating temperature, in park with engine idling, I removed the fluid level check plug from the pan and let fluid run out until it stopped flowing strong.

 

12. Finally we took it on a road trip and the trans operated perfectly (albeit rather hot at 200 plus degrees while towing).   In a short time the hard downshift and lurch were no longer evident. 

 

Afterthoughts...because the 6L90 trans has a transmission Thermostat Bypass Valve (TBV), some of the old fluid kept recirculating in the trans, while I was flushing it.  It would have been advantageous to set it up so fluid would not recirculate, rather go directly into and out of the radiator trans cooler.   This could have been accomplished by flipping the pin in the original TBV, or have the trans at operating temp for the entire time (which is near impossible, unless you have the updated 160 degree TBV (which is available at GM dealers under a Service Bulletin number 21-NA-199).

 

BTW, because we had a hard downshift and clunk reported on a dealer repair order, and the dealer couldn't duplicate the problem,  GM has since replaced the original 190 degree TBV, with a new 160 degree TBV.  WORKS GREAT!

 

 

 

That’s a nice write up but I prefer the following steps. 
 

1. pull trans dipstick and insert clean clear tube to the bottom of the pan from the comfort of sitting up top. 
 

2. Pump old fluid up out of pan via dipstick tube and measure amount you took out. 
 

3. Put small funnel in dipstick tube and pour new fluid in. 
 

4. Walk by jack stands saying you will use them one day, but not today. 

[AmericanPie]

3 hours ago, Pryme said:

That’s a nice write up but I prefer the following steps. 
 

1. pull trans dipstick and insert clean clear tube to the bottom of the pan from the comfort of sitting up top. 
 

2. Pump old fluid up out of pan via dipstick tube and measure amount you took out. 
 

3. Put small funnel in dipstick tube and pour new fluid in. 
 

4. Walk by jack stands saying you will use them one day, but not today. 

I only wish that it would be that easy on a 2020, L8T - 6L90.  It's a sealed transmission, so no dipstick tube, and no dipstick (thanks GM).  Plus, due to the poor condition of the old trans fluid, I determined that it needed a full fluid flush, not just a pan drop. 

[Pryme]

6 hours ago, AmericanPie said:

I only wish that it would be that easy on a 2020, L8T - 6L90.  It's a sealed transmission, so no dipstick tube, and no dipstick (thanks GM).  Plus, due to the poor condition of the old trans fluid, I determined that it needed a full fluid flush, not just a pan drop. 

Easily doable mod. There is a sticky at the top of the mod page for adding it with factory gm parts. A lot of us already added it to our 20-23 trucks. 

[IowaTexan]

My 2019 1500 5.3L just started doing this recently. Currently have 65k miles on it and have already had 1 transmission flush done it. Feels like it’s shifting down from 3 to 1 and jolts the whole truck. You can hear a clanking up the truck when I have the windows downs too. Gonna take it to dealership this morning and see what they have to say.

[GO-N-KR8Z]

Well Sorry for the late question to this thread but, was the outcome with this issue?

I own a 2022 that has 20k on it and it has been back to the dealer twice. They keep saying this is normal.

2-1 down shift wants to rip the rear-end out sometimes.. My passengers think I am brake checking them.

anyway, just trying to figure out a direction. Thanks

[Justin Keller]

Mine has the same hard downshift.... lock it up on the snow.  It's lovely

[GMCustomerService]

On 3/16/2025 at 9:52 PM, Justin Keller said:

Mine has the same hard downshift.... lock it up on the snow.  It's lovely

We agree this must be an annoying problem. Our team would like to learn more, and get you into a dealership who can assist. Please send us an email at [email protected] so we can look into this further. By sending us a message, you consent to the information you provide being monitored and recorded by GM or those acting on GM’s behalf, subject to the GM Privacy Statement: https://www.gm.com/privacy-statement.