Starting in 2nd gear when towing?

alkaspeltzar · May 24, 2020

2019 Silverado 1500 z71 with 8K miles, 3.23 rear, 8 speed.

Today I have towed about 8000lbs of cattle and trailer. I prior towed an old farmall tractor back.

Both times when towing over 6000lbs, I had the transmission in L6, tow haul mode. Sometimes when taking off from a stop, it almost felt like the truck started in 2nd, not dropping down the first. Would it do that? Or is the torque converter just being floated as RPM's rise, and possibly I am not feeling the shift from 1 to 2. This transmission is crazy smooth compared to my 6 speed.

Truck drives great otherwise. Ran it to town empty later, no issues. Even ran it thru the Low mode gears, and it starts in first normally. Thought maybe this was towhaul mode trying to avoid unnecessary shifting if still rolling. Transmission running cool, never over 180 even with that load at 60-70 mph.

Thanks for sharing your experiences.

Jav_eee · May 24, 2020

Just curious as to why you wouldn’t just put in in drive with tow haul?

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flyingfool · May 24, 2020

you should data log the trans next time to confirm this is happening, i know HPtuners has all the PIDS to show you whats happening durring a logged capture of the operation. 180 F is a bit warm unless your going up grades. but if its a lease truck no worries

Edited May 24, 2020 by flyingfool

Wintersun · May 24, 2020

180F degrees is on the low side even if not towing a heavy trailer. Tow/haul mode is recommended when the trailer and truck weight exceeds 75% of the trucks rated GCWR. Tow/haul mode will change shift points and also keep the torque converter locked longer on upshifts which is intended to minimize heat buildup with the transmission fluid. At 180 degrees this is hardly a concern. By the way the flash point with even cheap ATF is over 350 degrees. Using Range Selection mode disables Grade Braking and is not advisable with a heavy trailer.

alkaspeltzar · May 25, 2020

22 hours ago, Jav_eee said:

Just curious as to why you wouldn’t just put in in drive with tow haul?

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Becuase id rather keep it in 5th when towing heavy loads. Sometimes it bounces back and forth between 5-6.

alkaspeltzar · May 25, 2020

3 hours ago, Wintersun said:

180F degrees is on the low side even if not towing a heavy trailer. Tow/haul mode is recommended when the trailer and truck weight exceeds 75% of the trucks rated GCWR. Tow/haul mode will change shift points and also keep the torque converter locked longer on upshifts which is intended to minimize heat buildup with the transmission fluid. At 180 degrees this is hardly a concern. By the way the flash point with even cheap ATF is over 350 degrees. Using Range Selection mode disables Grade Braking and is not advisable with a heavy trailer.

Didn't say temp was issue, just commenting truck stayed very cool compared to my 2010 or 2014

alkaspeltzar · May 25, 2020

I see if can pull it from Drive to Low mode when driving. If I take off next time when towing, I might try that as I'll be able to see what gear it's in.

Also I notice while driving it empty it hoops from 1-2 so quick and smoothly, and being the ratios are very close, I don't notice it. This is most likely the case when towing

Jav_eee · May 25, 2020

Becuase id rather keep it in 5th when towing heavy loads. Sometimes it bounces back and forth between 5-6.

Tow/haul is supposed to prevent that.

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sldhead700 · May 25, 2020

Tow/haul is supposed to prevent that.

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It doesn't always.... I've had to do it also

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flyingfool · May 25, 2020

17 hours ago, Wintersun said:

180F degrees is on the low side even if not towing a heavy trailer. Tow/haul mode is recommended when the trailer and truck weight exceeds 75% of the trucks rated GCWR. Tow/haul mode will change shift points and also keep the torque converter locked longer on upshifts which is intended to minimize heat buildup with the transmission fluid. At 180 degrees this is hardly a concern. By the way the flash point with even cheap ATF is over 350 degrees. Using Range Selection mode disables Grade Braking and is not advisable with a heavy trailer.

at 180F its not the oil having problems, its the TECM components, plastics and all the delicate rubber seals inside that are effected by this temperature by oxidation and loss of plasticity in those components... tow haul just disconnects the clutches in the Torque converter and might boost line pressure to clutch packs while raising shift points

Edited May 25, 2020 by flyingfool

ullose272 · May 26, 2020

at 180F its not the oil having problems, its the TECM components, plastics and all the delicate rubber seals inside that are effected by this temperature by oxidation and loss of plasticity in those components... tow haul just disconnects the clutches in the Torque converter and might boost line pressure to clutch packs while raising shift points

206-215 is considered normal operation temp, modern transmissions usually have thermostats built into the system to keep them at this temperature

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flyingfool · May 27, 2020

yup! and longevity goes south

Edited May 27, 2020 by flyingfool

Traperkeeper80 · April 17, 2023

My 2023 2.7 is absolutely doing this while towing 6,000 lb camper. It even does it launching on the equivalent of a boat ramp incline, from a dead stop.
Who is the EPA idiot at GM that thinks a 2nd gear launch with 6K is the right programing? You can easily tell this is hard as hell on the drive line components. I have to manually shift to low, then force it to 1st gear to take off.

Gangly · April 17, 2023

On 5/23/2020 at 9:53 PM, flyingfool said:

you should data log the trans next time to confirm this is happening, i know HPtuners has all the PIDS to show you whats happening durring a logged capture of the operation. 180 F is a bit warm unless your going up grades. but if its a lease truck no worries

180 is low, especially for towing with the 8 spd. The transmission normally runs at 185 all day long with no load. That transmission temperature is a good thing to see.

Gangly · April 17, 2023

On 5/25/2020 at 10:21 AM, flyingfool said:

at 180F its not the oil having problems, its the TECM components, plastics and all the delicate rubber seals inside that are effected by this temperature by oxidation and loss of plasticity in those components... tow haul just disconnects the clutches in the Torque converter and might boost line pressure to clutch packs while raising shift points

These transmissions are designed to run at this temperature during daily driving duty. There is a video of the GM engineer who helped design the transmission discussing operating temperatures and the design process of the transmission, and ideal temperatures when towing were low 200's with 185-190 being ideal operating temperatures during normal use. Degradation to seals and increased wear to components didn't occur until prolonged exposure to the mid/high 200's

Edited April 17, 2023 by Gangly