Jump to content

8l90 downshift characteristics - light throttle at 70mph

Miguel Olea

By Miguel Olea
in Troubleshooting & Recalls

 Share

Recommended Posts

Miguel Olea Enthusiast

Miguel Olea

Posted
Posted

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.  

Miguel Olea Enthusiast

Miguel Olea

Posted
  • Author
Posted

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 Senior Enthusiast

CamGTP

Posted
Posted

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 Enthusiast

Miguel Olea

Posted
  • Author
Posted

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 Senior Enthusiast

JimCost2014

Posted
Posted
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 Enthusiast

Miguel Olea

Posted
  • Author
Posted

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 Senior Enthusiast

JimCost2014

Posted
Posted
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 Enthusiast

Miguel Olea

Posted
  • Author
Posted

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 Senior Enthusiast

JimCost2014

Posted
Posted
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 Enthusiast

Miguel Olea

Posted
  • Author
Posted

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 Enthusiast

Miguel Olea

Posted
  • Author
Posted

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 Enthusiast

Miguel Olea

Posted
  • Author
Posted

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.

  • 7 months later...
Miguel Olea Enthusiast

Miguel Olea

Posted
  • Author
Posted

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.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.

  • Forum Statistics

    250.4k
    Total Topics
    2.7m
    Total Posts

  • Member Statistics

    342,759
    Total Members
    8,960
    Most Online
    DM22
    Newest Member
    DM22
    Joined

  • Who's Online   0 Members, 0 Anonymous, 2,212 Guests (See full list)

    • There are no registered users currently online

  • Latest Articles

  • Posts

    • B14Sierra
      Cam chain tensioner question

      By B14Sierra · Posted

      So I'm in the middle of a DOD AFM delete on my 6.2 L86 Sierra. I had a pick holding the tensioner but after I got the Cam phaser gear off I was cleaning off all the rtv and apparently I didn't have the pick seated far enough in. I bumped it and the tensioner sprang forward.  I think everything is good but I want a second opinion. The top of the tensioner is just a looped piece of plastic that rides in that channel right? There is no spring or anything is there? I got the gear and chain back on and it seems tight and everything looks right. I'm hoping nothing fell out.
    • dna9656
      No low range 4 x 4

      By dna9656 · Posted

      My 2015 1500 LTZ Silverado suddenly won't go into 4 x 4 low. It will go into 4 x 4 high.
    • asilverblazer
      Transmission Woes :-(

      By asilverblazer · Posted

      Yep, just a quick reference point.    My main point being I’d do a thousand other things before I’d pay 10k for a transmission.    Speaking in ignorance cause I don’t look at these trucks, what is it worth? 20k?
    • asilverblazer
      Intermittent no-code issues on 2022.5+ V8 trucks — what did the shop fail to capture?

      By asilverblazer · Posted

      I think users are going to want to pick their monitored parameters, which parameters they want to see first.    It should probably start with baseline at a minimum and adjust to learned, but be able to overlay baseline for comparison.   A simple severity level would be able to determine what type of alerting is appropriate or user selectable.    Why not use the OBD port though?   I think a phone connection would be a good idea, push notifications type deal.   Number 1 issue is having data is useless if you don’t know what the data should be under normal conditions. 
    • Chuck FB
      summer road trips in the 2026 2500HD

      By Chuck FB · Posted

      I thought I would use your thread and add to it as I just did my first longer drive with my truck in the last couple of days. I drove from the Grande Prairie area of Alberta down to Edmonton and most of where I drove in the city was the ring road so fairly free flowing but a bit of stop and go as well in the city. Stayed the night and returned home and not too many stops along the way each way but every restart and certainly every cold start sets it back for fuel mileage. Why I say that is I see some people will cherry pick a fuel mileage leg after the vehicle had been warmed up driveline wise before hand and its a forgiving ( easy rolling drive leg for example ) and call that their fuel mileage which can give a false perception of reality. I was not heavily loaded at all but never the less the flip bak cover, rubber bed mat, various tools etc and extra jerry cans of fuel all way up to a few hundred pounds of dead weight so its not an empty truck. The cold inflation tire pressures are set more near the freezing point so once they are warmed up driving I was showing 45 front and over 40 rear and realize high inflation pressures would help a little in fuel mileage but certainly not the ride on our crap sections of highway. The weather was good so was not raining as that can really drag mileage down, in fact I had a bit of a tail wind on average driving home. Most people on here would never have driven on that freeway to visualize it but its got a fair bit of rolling type of landscape with numerous river valleys. For the most part I had it on cruise set to 62 although kicking it off if I caught it in time before it started down shifting and self braking going down the grades. Most of the more substantial grades its shifting into 7th I believe as 8th just doesn't have it. Total distance round trip was 643 miles and my overall average and I did refuel three times in all, figured out to 17.65 miles per US gallon. My best fuel mileage section refuel within all of this figured out to 18.46 and these are all hand calculated figures. I find if anything that the trucks computer can be over optimistic, sometimes its pretty close but other times its stretching it. On paper persay in theory the truck would have just about made it on fumes for that whole drive without refueling once.    Which made me think of the topic thread of the wonder if these trucks could do 20 mpg and that is a good question, certainly would have to be on an easy going flat highway, no head wind, the right temperature, not packing around a bunch of dead weight and puttering along even slower than I was I would suspect and going steady and not stopping to smell the flowers or take a piss !. It probably is possible but not without effort to attain that with the wind resistance and weight of these trucks. Of course on my drive most people are passing me if they have the power as per loaded highway tractors, never mind a lot of speedy vehicles but the speed limit is 68 and most are at or well over that. 

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...