8L45/8L90 1-2 Cold Shift Flare??

[AdmiralYoda]

Wondering if anyone or @clinebarger knows of a fix or help for the classic 8L45/8L90 1-2 and sometimes 2-3 shift flare (slip) when cold.

I've got a 2021 GMC Canyon with the 3.6L engine and 8L45 transmission. I bought it in 2023 with 21k miles on it and it appeared to be in good condition. No obvious signs of hard use or towing. Drove great and shifted great, and still does. It now has 60k on it.

I did a drain/fill and replaced the trans filter at 25k with Mobil LV HP and another drain/fill at 50k for good measure. The old fluid I drained out was in good condition, not burnt/dark and smelled fine.

I don't drive it hard or tow and do 90% highway miles, about 28k miles per year commuting... 45 miles each way to/from work. I've always noticed a bit a shift flare between 1-2 and sometimes 2-3 when I get going after starting the truck up and putting it in drive. It only happens during the first 1-2 shift, after that it doesn't happen again unless the truck sits for awhile and cools down all the way. It happens at any ambient temperature but is much more noticeable when it is cold outside. Even if I warm the truck up for 5 minutes when its cold it still does it.

Usually I take it easy when accelerating for the first time but if I give it more gas it is more noticeable. This morning it was 15 degrees out, the truck had warmed up for 5 minutes and I had to give it a bit more gas out of my driveway than normal as a car was coming up behind me quicker than I expected. It flared a bit between 1-2, maybe 500-750 rpms before it shifted and shifted a bit harsh. After that it shifts great.

The 8L45/8L90 gets a bad wrap for torque converter shudder and other issues but in 2019/2020 they switched from AC Delco trans fluid to Mobil LV HP and that seemed to fix most of the issues. I still read plenty of complaints about "morning sickness" where people notice a funky 1-2 or 2-3 shift after startup.

Is there any known fix for this? Does Sonnax or Transgo have anything to improve this? Is a better trans fluid available that helps? Or is this just the nature of this transmission and it is what it is? Sorry for the long winded post. Thanks guys!

 

[buster]

I looked into it as I have the same transmission in the 22 ZR2. There are a few things about the 8L45 that are consistent across the board - first cold 1-2 shift of the day is one of them. It was explained that it was part of the programming to have it hold the gear longer to help warm up the transmission. It's also been said that these new thinner transmission fluids don't like being cold. The quirky 1-2 cold shift only happens on the first cold shift after truck has sat for day or overnight. It's one and done.

To eliminate it, start truck and once RPMs settle put it into drive and hold the brake pedal down for about 30 seconds. This helps increase pressure and it goes away. If I keep the truck, I have considered trying Amsoil ATF, however, the fluid Mobil developed for this transmission is very specific in that it counters moisture. I don't know if all ATF's have this capability so I'm not sure how Amsoil ATF would do.

Once warm, transmission is very smooth and responsive.

The biggest issue I have is a rough Park to Reverse clunk. Goes away once warm, for the most part, but can occasionally still happen.

 

[Kira]

Try a 10oz. bottle of LubeGard (red bottle)".
Since, "they switched from AC Delco trans fluid to Mobil LV HP and that seemed to fix most of the issues...", there's nothing to fear with trying the LG.

What kind of "AC Delco trans fluid" was it? Dexron VI, I assume?

 

[Cujet]

That sounds like a problem that a typical aftermarket tune would address.

 

[AdmiralYoda]

@Kira I was going to mention that. I did add a bottle of Lubegard Red to each drain/fill. No change. I've always loved their products. Looking through the Colorado/Canyon owners manuals it looks like 2017 GM spec'd DEXRON-VI, I think that was the year the 8L45 was introduced in these trucks. 2018+ they spec the DEXRON-HP for the 8-speed.

@buster The 8L45, at least in the Colorado/Canyon runs COLD! There is no trans fluid bypass thermostat as far as I can tell. 100% of the fluid flow goes through the radiator/trans cooler all the time, it doesn't matter if it is -20F out or 120F. These trans run about 50F above ambient in normal conditions. I have never seen my transmission above 130F in the summer and in the winter it rarely hits 100F, if ever. This morning on the way to work the hottest it got was 70F over a 45 mile commute.

I've been considering adding a transmission cooler line adapter that replaces it with a thermostat assembly. Various options for temperatures. It lets the fluid warm up enough to drive off moisture, but will still be full flow at 145F or 165F depending on the thermostat chosen and remain plenty cool. Improved Racing 8L45/8L90 Thermostat

@Cujet Now that I am out of warranty I have been considering a Trifecta tune. It wakes up the motor a bit but they also drastically improve the shifting characteristics. Not sure if it will do anything to the 1-2 shift though.

I'm hoping a transmission expert like @clinebarger will chime in and tell us a bit more about this problem and if there is anything that can be done about it.

 

[buster]

@Kira I was going to mention that. I did add a bottle of Lubegard Red to each drain/fill. No change. I've always loved their products. Looking through the Colorado/Canyon owners manuals it looks like 2017 GM spec'd DEXRON-VI, I think that was the year the 8L45 was introduced in these trucks. 2018+ they spec the DEXRON-HP for the 8-speed.

@buster The 8L45, at least in the Colorado/Canyon runs COLD! There is no trans fluid bypass thermostat as far as I can tell. 100% of the fluid flow goes through the radiator/trans cooler. These trans run about 50F above ambient in normal conditions. I have never seen my transmission above 130F in the summer and in the winter it rarely hits 100F, if ever. This morning on the way to work the hottest it got was 70F.

I've been considering adding transmission cooler line adapter that replaces it with a thermostat assembly. Various options for temperatures. It lets the fluid warm up enough to drive off moisture, but will still be full flow at 145F or 165F depending on the thermostat chosen. Improved Racing 8L45/8L90 Thermostat

@Cujet Now that I am out of warranty I have been considering a Trifecta tune. It wakes up the motor a bit but they also drastically improve the shifting characteristics. Not sure if it will do anything to the 1-2 shift though.

I'm hoping a transmission expert like @clinebarger will chime in and tell us a bit more about it and if there is anything that can be done about it.

I have to wonder that if these units run too cold to the point that the moisture doesn't get burned off which leads to the shudder. It's probably induced by a lack of heat to evaporate the moisture out of the fluid. The newer fluid is less susceptible to moisture, so it helps, but is a bit of a band-aid.

The 8L45 is the Achillies heal to the Gen 2 Colorados. Engine is solid and reliable. If you can keep the transmission going it's a very reliable truck.

 

[panthermike]

The only things I can think of were already mentioned;

1. Let truck warm up, try what Buster mentioned holding it in drive. Easy to try. Occasionally our CX-9 will have a rough 1st shift but I've noticed that if I let it warm up a little and am real ginger on the throttle, it typically won't do it.

2. An aftermarket tune could adjust shift points/pressures. Not all are created equal, not sure who the better ones are for GM products.

I use LG platinum in my vehicles, but I see you're already using Red, which is good.

 

[Marcozi]

I'd scan the transmission control unit to see if any errors show up.

 

[AdmiralYoda]

@Marcozi I have a bluetooth scan tool and the Torque Pro app. When I scan for codes nothing shows up. I assume it checks more than just ECU codes but I'm not certain. The check engine light has never come on.

 

[Marcozi]

@Marcozi I have a bluetooth scan tool and the Torque Pro app. When I scan for codes nothing shows up. I assume it checks more than just ECU codes but I'm not certain. The check engine light has never come on.

Can Torque Pro scan the transmission computer? I thought it was limited to the engine computer.

 

[AdmiralYoda]

I'm pretty certain no codes are going to show up. The internet is full of people mentioning the weird 1-2 shift for the first shift of the day, made worse by cold temperatures. The consensus is "this is normal" and "GM forgot how to design transmissions".

Transgo, Sonnax and others usually come up with neat fixes for these types of problems or maybe a better trans fluid like HPL or Amsoil might help. Not many answers out there, so I'm hoping someone knows....

 

[LDM]

Had this issue on my 2018 Sierra. Took it in the for the TSB where they exchanged the old Dexron HP LV fluid for the new Mobil 1 Blue label Dexron HP LV fluid as the original fluid was apparently hygroscopic and would pick up water in the fluid. They flush all the fluid out for the TSB, which ends up being something like 20 quarts of fluid total for the job. Mine had around 35K miles on it and I noticed the issue shortly after I purchased the truck.

Haven't had the issues since and I now have around 55K miles on the truck. It was very cold this morning, around 1 degree F and no issues at all. It was really bad when I first got the truck, it would slam hard from 1-2 when I would pull out of my driveway in the morning on a cold day.

I know you said you did a couple of drain and fills but that might not have gotten enough of the fluid out to fully remove all the water from the old fluid. I'd check and see if your truck qualifies for the TSB and take it in to have it done if it does. Even out of warranty it should still be covered by the TSB.

If not, I know some people have gotten transmission tuning done on the trucks to improve the shifting. GM stock tuning just doesn't seem to be all that great when it comes to the 8 speed.

 

[AdmiralYoda]

@LDM Thanks for mentioning the TSB....I had forgotten that! Here is the TSB

It applies to many GM vehicles with the 8L45 but for the Colorado/Canyon it applies to 2017-2022 models. The name of the TSB is "Harsh 1-2 Shift Upon First Start Up/Shift of the Day Under Light Throttle".

GM states the cause of this condition is "... due to the initial clutch fill time of the 2-3-4-6-8 (C4) clutch."

The corrective action??? "Inform the customer this condition is normal and will not impact the designed performance or reliability of the vehicle"

 

[ctechbob]

For anyone interested in some of the issues and design in the GM 8 speeds, there were a bunch of unsealed documents from one of the lawsuits that go into a ton of detail on what is wrong with them, what band aids they tried, and what you can expect out of those fixes. There is a lot of stuff you can skim through in the docs, but there are several spots that are REALLY interesting reading.

The problem you're talking about here is what GM calls a 'garage shift', or the first shift of the day.

https://www.cohenmilstein.com/wp-co...f-Misc.-Exhibits-Document-GM-Litigation_0.pdf

Fluid can only do so much for the problems and everyone should be using the updated fluid in the trans. Sadly, it doesn't look like there was a real fix until the 'Gen 2' transmission in.....2022? I forget the date, been a while since I read through the docs.

https://www.cohenmilstein.com/wp-content/uploads/site/ECF No. 201-8 PX149 unsealed.pdf

 

[clinebarger]

@Kira I was going to mention that. I did add a bottle of Lubegard Red to each drain/fill. No change. I've always loved their products. Looking through the Colorado/Canyon owners manuals it looks like 2017 GM spec'd DEXRON-VI, I think that was the year the 8L45 was introduced in these trucks. 2018+ they spec the DEXRON-HP for the 8-speed.

@buster The 8L45, at least in the Colorado/Canyon runs COLD! There is no trans fluid bypass thermostat as far as I can tell. 100% of the fluid flow goes through the radiator/trans cooler all the time, it doesn't matter if it is -20F out or 120F. These trans run about 50F above ambient in normal conditions. I have never seen my transmission above 130F in the summer and in the winter it rarely hits 100F, if ever. This morning on the way to work the hottest it got was 70F over a 45 mile commute.

I've been considering adding a transmission cooler line adapter that replaces it with a thermostat assembly. Various options for temperatures. It lets the fluid warm up enough to drive off moisture, but will still be full flow at 145F or 165F depending on the thermostat chosen and remain plenty cool. Improved Racing 8L45/8L90 Thermostat

@Cujet Now that I am out of warranty I have been considering a Trifecta tune. It wakes up the motor a bit but they also drastically improve the shifting characteristics. Not sure if it will do anything to the 1-2 shift though.

I'm hoping a transmission expert like @clinebarger will chime in and tell us a bit more about this problem and if there is anything that can be done about it.

The Initial Fill Time/Air Bleed for the 2-3-4-6-8 Clutch calculations are off by a few milliseconds, The only real fix is custom tuning tailored to your specific unit, GM can't get this right for EVERY unit....A slight flare is FAR better than a Tie-Up/Bind where the 1-3-5-6-7, 1-2-7-8-R, & 1-2-3-4-5-R Clutches are on at the same time the 2-3-4-6-8 Clutch is on, Even for just a few milliseconds.

GM would have to start with better quality control far a clutch clearances before developing a calibration to fix it, I don't see GM putting anymore development into 8Lxx units.

2017 & up 8L90 TCM's are locked, Not sure how Trifecta is tuning these....Or IF they're actually changing parameters in the TCM as you can manipulate some things through the ECM.

I have to send '17 & up T87 TCM's to HP-Tuners to be physically unlocked.

 

[AdmiralYoda]

@clinebarger Thanks for that info! It sounds like this is an "it is what it is" situation and there is nothing majorly wrong.

FYI, Trifecta doesn't touch the TCM...but they do say they manipulate every parameter they can transmission related in the ECM. Many users say it makes the transmission shift much better, essentially less slushy. I've seen some people mention that it helped their first of the day 1-2 shift as well. I plan to get this tune in the next few months since my warranty is up.

In the meantime I'm going to keep changing the fluid often with Mobil LB HP, just because.

 

[LDM]

The Initial Fill Time/Air Bleed for the 2-3-4-6-8 Clutch calculations are off by a few milliseconds, The only real fix is custom tuning tailored to your specific unit, GM can't get this right for EVERY unit....A slight flare is FAR better than a Tie-Up/Bind where the 1-3-5-6-7, 1-2-7-8-R, & 1-2-3-4-5-R Clutches are on at the same time the 2-3-4-6-8 Clutch is on, Even for just a few milliseconds.

GM would have to start with better quality control far a clutch clearances before developing a calibration to fix it, I don't see GM putting anymore development into 8Lxx units.

2017 & up 8L90 TCM's are locked, Not sure how Trifecta is tuning these....Or IF they're actually changing parameters in the TCM as you can manipulate some things through the ECM.

I have to send '17 & up T87 TCM's to HP-Tuners to be physically unlocked.

The TCM for the 17+ can be unlocked by HP Tuners, at least for the T87A. You can then tune it like any other TCM with their software, although it costs like 4 tuning credits for each TCM. Not sure if Trifecta is using that or something else but its definitely possible to do.