Towing stability

[Geotrash]

www.tsbsearch.com/Chevrolet/21-NA-199

If gm went through the trouble to replace the original 19x degree thermostat in the trans with a cooler one, one would think any temp over the 194 or 195 would not be good for the transmission.

The problem with the original thermostat was that by the time the fluid gets to the point of opening the thermostat, it may be well on its way to spiking much higher under some circumstances - e.g. stop and go traffic on a hot day, climbing a mountain with a full load, towing, etc. Opening the thermostat 10,20,30 degrees cooler gives the coolers a chance to take more heat out of the system earlier, creating more thermal headroom, thereby reducing the spikes where fluid breakdown happen at a much faster rate, significantly.

 

[tagexpcom]

Last Aug 2023 we purchased a used 2021 Yukon Denali w/6.2L and it did not have towing package or enhanced cooling that we could find. It's the magnetic (but no air) ride, 22" wheels, middle level Denali customization level.

Before purchase, the dealer let us tow our 5300lb 7 x 14 cargo trailer -> camper up our local mountain (Mt Ashland, Southern Oregon) at 107F ambient / middle of afternoon. It's a ~10mile, 6% grade. We pulled it at 60mph all the way.

The transmission reached 200F on the car gauge which I understand is perfectly OK for short duration (15min), maybe 3 times a year. So we were sold - doesn't seem we need extra cooling.

Since then we've towed ~4,000miles over and around Southern Oregon / North-Middle CA mountains in more normal temps (80F hi) and I've not seen it go over 180F transmission temp and even then this was short duration - just when going up these 5-10 mile 6% grades.

Maybe a 2021 Denali with 6.2L/10-speed naturally has good cooling or maybe this is standard for Yukons?

 

[Geotrash]

“GM's in-house towing team expert provided RV Clinic with this statement:
The max allowable automatic transmission fluid temperature is dictated by the ATF itself.
The ATF begins to degrade significantly above 270F, so we design vehicles so that in all but the most extreme conditions, the ATF temperature in its sump stays below 270F.

We allow for up to 285 degrees F in extreme conditions (i.e. towing a trailer with combination loaded at GCWR in Death Valley). But for customer usage anywhere else in the country, even at GCWR, transmission sump temperature should stay well below 270 degrees F. Above this point, certain internal components, such as seals, begin to disintegrate rather quickly. Although newer synthetic fluids can withstand higher temperatures we still recommend 270F as a maximum temperature.”

1st: ATF in the sump is NEVER the hottest ATF in the transmission, especially under heavy duty use.
2nd: he mentions that ATF begins to degrade significantly above 270F,
meaning that any ATF in the transmission over 270F is beginning to degrade 'significantly'
3rd: he mentions that in Death Valley, GM allows for up to 285F.
4th: he then mentions that internal components, such as seals, begin to disintegrate rather quickly above 270F.

To oversimplify, I recommend keeping the ATF under 221F, anywhere in the world.
I don't see how my recommendation hurts any end user of ANY ATF.

Agreed. But it's still helpful guidance. The hottest fluid temp will be where the fluid exits the TC, of course. GM's guidance for that is 350º absolute max with a 300ºF redline with Dexron VI. All of those trans fluid temp charts of doom on the interwebz are for older fluids. My personal max (as in, pull over and let it idle until the temp comes down), is 235ºF.

With the current setup on my '12 Yukon XL towing 7500 lbs, my trans temp stays around 196º on the highway in the summer and rises to 210ºF in stop and go traffic. I have seen very rare spikes to 235ºF on very hot days at the top of a mountain pass (both here in the east and out west), and I'm okay with that. I change my fluid every 20K on the towing rig. I'm running a Derale 13960 cooler now that's mounted below the bumper, but the numbers were very similar with the factory cooler. The reason I moved the cooler was because having it in front of the radiator was causing problems for me in keeping the engine cool. Mine puts out about 50hp more than stock, which was just enough extra heat to create coolant temp anxiety. Now, I have no more issues. It stays nice and cool.

 

[Geotrash]

Last Aug 2023 we purchased a used 2021 Yukon Denali w/6.2L and it did not have towing package or enhanced cooling that we could find. It's the magnetic (but no air) ride, 22" wheels, middle level Denali customization level.

Before purchase, the dealer let us tow our 5300lb 7 x 14 cargo trailer -> camper up our local mountain (Mt Ashland, Southern Oregon) at 107F ambient / middle of afternoon. It's a ~10mile, 6% grade. We pulled it at 60mph all the way.

The transmission reached 200F on the car gauge which I understand is perfectly OK for short duration (15min), maybe 3 times a year. So we were sold - doesn't seem we need extra cooling.

Since then we've towed ~4,000miles over and around Southern Oregon / North-Middle CA mountains in more normal temps (80F hi) and I've not seen it go over 180F transmission temp and even then this was short duration - just when going up these 5-10 mile 6% grades.

Maybe a 2021 Denali with 6.2L/10-speed naturally has good cooling or maybe this is standard for Yukons?

Those are about 20ºF cooler than the temps I saw under similar circumstances with the stock setup on the GMT900 platform. Seems that GM is running them cooler than before, which is a good thing. It may be partly due to the 10-speed being able to stay in the optimal zone more of the time, too.

 

[Marky Dissod]

Agreed. But it's still helpful guidance. The hottest fluid temp will be where the fluid exits the TC, of course.
GM's guidance for that is 350º absolute max with a 300ºF redline with Dexron VI.
All of those trans fluid temp charts of doom on the interwebz are for older fluids.
My personal max (as in, pull over and let it idle until the temp comes down), is 235ºF.

Applying the ATF temp charts of doom for pre-Dexron6, to Dexron6, cannot possibly hurt, and might help.
Given GM's track record regarding what they're willing to get away with in terms of 6L80 / 6L90 / 8L90 longevity / durability,
there's nothing wrong with using pessimism to provide additional margins.

Still concerned about 6L80 / 6L90 in cars ...

 

[89Suburban]

And here’s ours, *with* the WDH properly set up. This was taken after it had been parked for a few hours and the factory rear air bags weren’t contributing much, if anything to the leveling at this point.

View attachment 421941

Need to pick me up and take me to a NASCAR race in the thing homie. Get a roof deck on it so we can see over the fence.

 

[Geotrash]

Need to pick me up and take me to a NASCAR race in the thing homie. Get a roof deck on it so we can see over the fence.

Let’s do it! Richmond raceway is right down the road from me. Literally less than 10 minutes from my front door.

 

[Grady_Wilson]

Tongue weight is very important to prevent trailer sway.
The others that have mentioned having plenty of weight on the tongue, but not too much, are spot on.
I have a Class A CDL but I have not used it for work for over 2 decades, I keep my CDL current - just in case. And with the antique cars I own, I don't ever want to be pulled over and told I am overweight for my license when towing in an enclosed trailer.
I've towed a lot of huge loads with various large trucks in the past so I have plenty of towing experience and I know what to expect with cross winds, passing semis and whatnot.

Almost two years ago I towed a car for one of my cousins from So Cal to WY.
He purchased a trailer specifically to tow his new car home and he wanted to make sure the trailer would be large enough to haul antique cars.
I used his Ford F350 as the tow vehicle.
When I first loaded the car, I wanted to get the bulk of the weight over the trailer axles but have enough tongue weight.
But, I completely forgot that it was a mid engine car and I loaded it like a front engine car.
After about 20 minutes of towing I pulled over, climbed back inside the car and moved it up almost 2 feet.
Zero sway or dancing around after I moved the car up.
You can't really see the F350 in this pic but you can see the trailer and the car I loaded up.
We ended up using race ramps from the dealer because the front of the car didn't like the trailer ramp being that steep.

 

[donjetman]

I have a 2014 Tahoe with the tow package. It has 157k miles on it. I just installed new Gabriel HD shocks (coil overs) on the rear. I purchased a Keystone trailer with a dry weight of 5400 lbs thinking the Tahoe could easily handle towing it because the book says max tow capacity is 8200 lbs. I have the proper sway bars and hitch set up, but it still gets uncomfortable (sway) at 45-50 mph. New tires are on the trailer. The hitch drops approx 2.5 inches when hooked up, so I believe I have enough tongue weight. The sway bars are tight. They pull the hitch up approx 1.5 inch. The drop without the sway bars is 4 inches. The drop with the is 2.5 inches. I'm not sure what to try next.

I do a lot of towing with our motorhome. I use a https://www.weigh-safe.com/ hitch that has a gauge in it that reads tongue weight so I can get the tongue wt correct.

 

[Grady_Wilson]

I do a lot of towing with our motorhome. I use a https://www.weigh-safe.com/ hitch that has a gauge in it that reads tongue weight so I can get the tongue wt correct.
View attachment 422598

How accurate is the scale indicator and how long do they generally last?
That looks like a real handy hitch.