2014 - 2018 5.3 6 speed downshift jerks

nluchau · February 21, 2019

On 2/20/2019 at 11:29 AM, chrispassas21 said:

you have to do this everytime you turn the truck on?

Yes you need to disable grade braking at every turn on of the key and sadly this did nothing for my deceleration downshifting.

mmmikkke · July 15, 2019

I have a 2016 4.3L and 6L80 and it too behaves like a 16 yo kid downshifting a manual transmission (using the clutch properly) 3 to 2 and 2 to 1 as his truck comes to a otherwise casual stop. The gear clutch engagements are about what you'd expect with the 16 yo driving a manual trans and doing the downshifts himself with zero throttle and an engine at idle rpm. There is no roughness beyond what a 16 yo would consider "sport downshifting" and therefore I am not concerned about any damage from this behavior of my 6L80.

But I *would* like to fix this.

When I am coasting to a stop, and my 6L80 downshifts 3-2 and a few seconds later 2-1, I can feel the mechanical coupling of the TCC (torque converter clutch) is energized (the torque converter is locked) during both the 3-2 and 2-1 downshifts and only unlocks when vehicle speed drops to 4 or 5 mph, saving the engine from being dragged below about 600 rpm. If the TCC would unlock sooner, those harsh 3-2 and 2-1 downshifts would be softened by a TCC unlock and transition from a mechanical coupling to an hydraulic coupling.

Is this bad TCC behavior by design? It might be.

I discovered an excellent youtube video on tuning the 6L80 using hptuners at https://www.youtube.com/watch?v=1L006vkWbkQ

At time 1:12 in the youtube video, a trans table for a 4L60/65/70 is shown. I captured the frame and reproduced it below.

Studying it reveals some useful insights about hptune and GM's electronic transmissions shifting strategy:

1. The minimum gear where TCC can be engaged is set in a table. In the 4L60 example below "Min Gear" is set to 3rd gear. Look for it in the frame below. It is in the Torque Converter table.

2. TCC lockup during acceleration is set in a table. In the 4L60 example below, in the 3rd sub-table under Shift / Lockup Data (MPH), TCC lockup is permitted in 3rd gear at or above 32 mph with zero throttle and only at or above 127 mph (!) with 88% to 100% throttle.

3. TCC unlock during deceleration set in a table. In the 4L60 example below in the 4th sub-table under Shift / Lockup Data (MPH), TCC is forced to unlock below 29 mph with 0 throttle, etc.

Presumably these are factory settings.

Look for these values in the frame grab below and you will start to understand what hptune might be able to do to help.

Presumably the corresponding table for a 6L80 would follow a similar protocol. Somewhere along the way I learned that the 6L80 is more aggressive saving every last drop of fuel, so TCC Min Gear is set at 2, while the 12 MPG 6.0L 6L90 is given a pass with its TCC Min Gear set at 3. GM has decided 6L90 users aren't as fuel economy conscious, apparently. (I used to have a 2500HD with 6L90 and TCC lockup waiting until after the 2-3 up-shift made a big improvement in drive-ability.)

Conclusion and Plan: the objectionable 3-2 and 2-1 downshifts could probably be fixed by raising the minimum speed values for TCC unlock by a few mph in the last 2 sub-tables under Shift / Lockup Data (MPH). The tables for the 6L80 would look slightly different, but the general idea is the same. For the 4L60 above; it would be analogous to changing line 3 cell 1 from 32 to 35 mph 0% throttle accelerating (with similar mph adjustments across the table for non-zero throttle positions) and changing 29 to 32 for the last sub-table 0% throttle while coasting (with similar mph adjustments for non-zero throttle positions). Remember, the 6L80's TCC Min Gear is 2, not 3, so lock and unlock speeds will be substantially slower. But I'd also change Min Gear to 3, to get that 6L90 drive-ability improvement.

Your thoughts? Questions?

Edited July 15, 2019 by mmmikkke
Clarification of main premise about the TCC

nluchau · July 15, 2019

9 hours ago, mmmikkke said:

I have a 2016 4.3L and 6L80 and it too behaves like a 16 yo kid downshifting a manual transmission (using the clutch properly) 3 to 2 and 2 to 1 as his truck comes to a otherwise casual stop. The gear clutch engagements are about what you'd expect with the 16 yo driving a manual trans and doing the downshifts himself with zero throttle and an engine at idle rpm. There is no roughness beyond what a 16 yo would consider "sport downshifting" and therefore I am not concerned about any damage from this behavior of my 6L80.

But I *would* like to fix this.

When I am coasting to a stop, and my 6L80 downshifts 3-2 and a few seconds later 2-1, I can feel the mechanical coupling of the TCC (torque converter clutch) is energized (the torque converter is locked) during both the 3-2 and 2-1 downshifts and only unlocks when vehicle speed drops to 4 or 5 mph, saving the engine from being dragged below about 600 rpm. If the TCC would unlock sooner, those harsh 3-2 and 2-1 downshifts would be softened by a TCC unlock and transition from a mechanical coupling to an hydraulic coupling.

Is this bad TCC behavior by design? It might be.

I discovered an excellent youtube video on tuning the 6L80 using hptuners at https://www.youtube.com/watch?v=1L006vkWbkQ

At time 1:12 in the youtube video, a trans table for a 4L60/65/70 is shown. I captured the frame and reproduced it below.

Studying it reveals some useful insights about hptune and GM's electronic transmissions shifting strategy:

1. The minimum gear where TCC can be engaged is set in a table. In the 4L60 example below "Min Gear" is set to 3rd gear. Look for it in the frame below. It is in the Torque Converter table.

2. TCC lockup during acceleration is set in a table. In the 4L60 example below, in the 3rd sub-table under Shift / Lockup Data (MPH), TCC lockup is permitted in 3rd gear at or above 32 mph with zero throttle and only at or above 127 mph (!) with 88% to 100% throttle.

3. TCC unlock during deceleration set in a table. In the 4L60 example below in the 4th sub-table under Shift / Lockup Data (MPH), TCC is forced to unlock below 29 mph with 0 throttle, etc.

Presumably these are factory settings.

Look for these values in the frame grab below and you will start to understand what hptune might be able to do to help.

Presumably the corresponding table for a 6L80 would follow a similar protocol. Somewhere along the way I learned that the 6L80 is more aggressive saving every last drop of fuel, so TCC Min Gear is set at 2, while the 12 MPG 6.0L 6L90 is given a pass with its TCC Min Gear set at 3. GM has decided 6L90 users aren't as fuel economy conscious, apparently. (I used to have a 2500HD with 6L90 and TCC lockup waiting until after the 2-3 up-shift made a big improvement in drive-ability.)

Conclusion and Plan: the objectionable 3-2 and 2-1 downshifts could probably be fixed by raising the minimum speed values for TCC unlock by a few mph in the last 2 sub-tables under Shift / Lockup Data (MPH). The tables for the 6L80 would look slightly different, but the general idea is the same. For the 4L60 above; it would be analogous to changing line 3 cell 1 from 32 to 35 mph 0% throttle accelerating (with similar mph adjustments across the table for non-zero throttle positions) and changing 29 to 32 for the last sub-table 0% throttle while coasting (with similar mph adjustments for non-zero throttle positions). Remember, the 6L80's TCC Min Gear is 2, not 3, so lock and unlock speeds will be substantially slower. But I'd also change Min Gear to 3, to get that 6L90 drive-ability improvement.

Your thoughts? Questions?

Interesting and great info here mmmikkke! Thanks for this write up and yes I think this is the concern with our trucks and downshifting. I had my transmission replaced under bumper to bumper and it still does it. Now it goes into 5th gear at 32mph. I think this is a bit much. Let me know what your outcome is here.

Emjay33 · July 24, 2019

Do the held hand tuners rectify the transmission issues?

thekevin · July 26, 2019

My 18 catches me off guard sometimes with a really harsh 2 - 1 shift that clanks and pushes me forward into my seat belt. Today I pulled out into traffic and accelerated about half throttle for 100ft and then got on the brakes at the stop light. The 3 - 2 downshift was fine, the 2 - 1 downshift I am surprised didn’t chirp my tires. I get the same thing when taking the exit going home most days. The exit is at the top of the hill so I am on the gas pedal up the hill and up the exit and go straight from gas to brakes at the top. All downshifts are fine until the 2 - 1 and sometimes it feels like I hit something.

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nluchau · July 26, 2019

18 hours ago, thekevin said:

My 18 catches me off guard sometimes with a really harsh 2 - 1 shift that clanks and pushes me forward into my seat belt. Today I pulled out into traffic and accelerated about half throttle for 100ft and then got on the brakes at the stop light. The 3 - 2 downshift was fine, the 2 - 1 downshift I am surprised didn’t chirp my tires. I get the same thing when taking the exit going home most days. The exit is at the top of the hill so I am on the gas pedal up the hill and up the exit and go straight from gas to brakes at the top. All downshifts are fine until the 2 - 1 and sometimes it feels like I hit something.

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Funny you mention that: I've noticed if I'm harder on the gas before slowing my downshifts are way more desirable yet almost normal feeling. If I'm easy on the gas the downshifts are harsher when slowing.

dieselfan1 · July 26, 2019

Something I've noticed on both my 2018 Silverado's 5.3 6spd. 3.42.

With the air conditioning on, coasting to a stop sign the 3-2 downshift is a little harsher than the rest of the gears.

Both trucks do it only with the A.C. on.

A.C. off it doesn't do it.

Crew cab has 11,000 miles , Double cab has 25,000 miles.

Anyone else notice this with the A.C. on?

Bob2C · July 26, 2019

Have the same issue. Harsh downshift coasting to stop. Been getting worse. Have 12k miles on my 2018. I’m thinking of taking it in to the dealer. Should I or just drive it until something happens. I have a while on warranty

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dieselfan1 · July 27, 2019

43 minutes ago, Bob2C said:

Have the same issue. Harsh downshift coasting to stop. Been getting worse. Have 12k miles on my 2018. I’m thinking of taking it in to the dealer. Should I or just drive it until something happens. I have a while on warranty

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Does it do it only with the AC on?

I know it's weird.

Bob2C · July 27, 2019

Does it do it only with the AC on?
I know it's weird.

I haven’t noticed. I will switch off ac tomorrow and will report back.

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Bob2C · July 28, 2019

No A/C and it downshifts smooth. What gives?

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Baytrucker · July 30, 2019

On 7/28/2019 at 3:48 PM, Bob2C said:

No A/C and it downshifts smooth. What gives?

I'd like to know this as well.

Grumpy Bear · July 30, 2019

On ‎2‎/‎20‎/‎2019 at 9:18 AM, njk4o5 said:

wheres all the clowns that say there arent any shifting problems with these trucks now?

Right here. 100K miles of silk.

On ‎7‎/‎26‎/‎2019 at 5:47 PM, dieselfan1 said:

Something I've noticed on both my 2018 Silverado's 5.3 6spd. 3.42.

With the air conditioning on, coasting to a stop sign the 3-2 downshift is a little harsher than the rest of the gears.

Both trucks do it only with the A.C. on.

A.C. off it doesn't do it.

Crew cab has 11,000 miles , Double cab has 25,000 miles.

Anyone else notice this with the A.C. on?

What I've noticed is when I have my coffee in my left hand and the radio is on I get a "peaceful easy feeling".

mmmikkke · July 30, 2019

1 hour ago, Grumpy Bear said:

Right here. 100K miles of silk.

What I've noticed is when I have my coffee in my left hand and the radio is on I get a "peaceful easy feeling".

Is it the coffee, the radio, or the 6L80?

Grumpy Bear · July 30, 2019

On ‎10‎/‎4‎/‎2018 at 10:47 PM, nluchau said:

Thanks for this piece of info Grumpy Bear. I do find this helpful info, it really is, but heck this is 2018 we're talking about. Technology is ever so advancing and one would think that a 5.3 with a 6L80 transmission would be tried and trued by now. This isn't the first year of them.

TV was introduced in 1948. I was born in 1953. I would have thought by now it would make my dinner but it doesn't. Technology is a wonderful thing but it can't keep people from the eventuality of death, feed the thousands suffering of hunger and disease. Prevent corruption. Keep itself from laying our data bare I'm pretty sure it will never provide perfection to the millions of people the have entirely different expectations either. It's technology not God.