8l90 downshift characteristics - light throttle at 70mph

Miguel Olea · February 21, 2022

So I just noticed that if I'm cruising at 70 and I give it a little gas the transmission will do a light down shift (maybe drop one gear) and if I keep my foot on the gas it will up shift back to the gear it was previously in. However, If I lightly press the gas and it downshifts and then I take my foot off the gas completely and coast, it will hold that same gear until I press on the gas again. It seems strange. Doesn't matter if grade braking is on or off. Truck is a 2018 6.2 Sierra.

shakenfake · February 21, 2022

Normal operation

rlcole321 · February 21, 2022

Torque converter "Unlocking - Locking", normal.

Miguel Olea · February 21, 2022

it's definitely a gear change. If I'm at 75 it will be at like 1,500 RPM and it will down shift to like 2,500 RPM upon pressing down on the gas. Obviously if I press down more than 1/4 throttle it will drop down several gears and it will up shift even with my foot off the gas. When it drops down 1 gear it will stay there while costing unless I'm on the gas, then it will upshift. Maybe it is the torque converter? I wasn't aware it was that noticeable and that much of an RPM difference. On my 2006 Yukon I can lightly press on the brake with my left foot while driving and watch the RPM go up and down as the converter locks and unlocks and it is very subtle.

CamGTP · February 22, 2022

It's probably dropping two gears if it goes from 1,500 to 2,500 that quickly. Which to me makes sense because I believe those two "gears" are working together and it's faster to jump down from 8 to 6 instead 8-7 because of how the clutch packs/drums work together.

The more gears a transmission has the more it will always try to be in the optimum torque range, so dropping gears is going to happen more often. I can make my 6 speed transmission hold 6th gear pulling a grade but it doesn't take much for it to want to jump down a gear if I am fighting to maintain a speed or losing speed.

The minor 100-200rpm flutter of the rpm would be the torque converter unlocking and locking again.

Miguel Olea · February 24, 2022

I'm going to test drive my brother in law's truck as it has the same exact drive train - I'll report what I've found.

JimCost2014 · February 24, 2022

2 hours ago, Miguel Olea said:

I'm going to test drive my brother in law's truck as it has the same exact drive train - I'll report what I've found.

Have you had the transmission serviced, with the new Mobil 1 Blue Label fluid put in?

Miguel Olea · February 24, 2022

yup - and that's how it all started. My 8l90 has been smooth and very trouble free. It would shift firm when driven aggressive and smooth when driven normal. I never had the shutter, but at 30K miles I wanted to change the fluid as preventative measure. I do all my own work, but b/c GM was retarded and did not include a drain bolt, dip stick or dip stick tube - I decided to have GM do the complete fluid exchange.

Long story short, they made me feel like I got a deal and only charged me 400.00 dollars including 8 quarts of the new fluid. They claimed that the flow tech machine was nearly full when they started. Nearly full with what? The new blue label fluid, Dexron 6, piss, maple syrup???? Also, as I sort of watched the tech work on my truck, he did the fluid exchange before dropping the pan and changing the filter - big no, no in my book. Since then the truck was a clunky mess for the first 1,000 miles. It appears to have smoothed back out - but the doubt is still in my mind!

JimCost2014 · February 24, 2022

37 minutes ago, Miguel Olea said:

yup - and that's how it all started. My 8l90 has been smooth and very trouble free. It would shift firm when driven aggressive and smooth when driven normal. I never had the shutter, but at 30K miles I wanted to change the fluid as preventative measure. I do all my own work, but b/c GM was retarded and did not include a drain bolt, dip stick or dip stick tube - I decided to have GM do the complete fluid exchange.

Long story short, they made me feel like I got a deal and only charged me 400.00 dollars including 8 quarts of the new fluid. They claimed that the flow tech machine was nearly full when they started. Nearly full with what? The new blue label fluid, Dexron 6, piss, maple syrup???? Also, as I sort of watched the tech work on my truck, he did the fluid exchange before dropping the pan and changing the filter - big no, no in my book. Since then the truck was a clunky mess for the first 1,000 miles. It appears to have smoothed back out - but the doubt is still in my mind!

This is what is done with the fluid exchange, and what you can expect afterwards. When did you have it done?

MC-10163890-9999.pdf

Miguel Olea · February 24, 2022

back in November - as soon as it warms up, I'm going to drop the pan at home and replace the filter and refill with the 7.5 quarts of the new mobil 1 fluid. I will do another service after that in 5- 10K miles just to make sure the correct fluid is in the trans.

So the clunky shifting for the first 1K or so miles is normal after introducing the new fluid?

JimCost2014 · February 24, 2022

2 hours ago, Miguel Olea said:

back in November - as soon as it warms up, I'm going to drop the pan at home and replace the filter and refill with the 7.5 quarts of the new mobil 1 fluid. I will do another service after that in 5- 10K miles just to make sure the correct fluid is in the trans.

So the clunky shifting for the first 1K or so miles is normal after introducing the new fluid?

That is how the TSB describes the first 1000 (relearn period) miles.

The odd thing in your post is the dealer only swapped 8 quarts. It takes 20 quarts to flush and fill.

Miguel Olea · February 24, 2022

right! because they said the flow tech machine was already full! So they made it sound like I got a deal and only paid for 8 quarts instead of 20. How will we ever know if it was the correct mobil 1 blue label in the machine? Of course they said "we only use that machine for 8 speed transmissions so it had the right fluid." Doubt is still in my mind.

Miguel Olea · February 25, 2022

I looked over the TSB and I could not find where it states that the trans needs 1,000 miles to relearn; only the 3-400 miles and to cold starts to eliminate the shudder.

Miguel Olea · February 25, 2022

I drove my brother in law's 2018 Centennial edition 2018 with the 6.2 and it is the exact same thing; so I'm more at ease.

On a side note, my truck feels so much more responsive. I wonder if it is the fact that I have the catch can protecting the valves, ported throttle body, and an airaid Jr. intake. I also shimmed the plastic gas pedal bracket to take the flex out of it when you first step on the gas. Both trucks have the same ratios, engine and transmission.

Miguel Olea · October 7, 2022

Update! I've only driven the truck 5K miles since I had the service done. My buddy and I dropped the pan changed the filter and did a flush with 20 quarts of the new blue label Mobil 1 fluid. I also added 1 10oz bottle of the lubegard platinum to further protect against any shudder although I've never had that problem. The truck drives and shifts so much better which leads me to believe that GM had THE WRONG fluid in that flush machine. It probably had Dexron 6.