Towing Heavy Transmission Temps

[Foggy]

We have the same transmission cooler, I believe (in my 2012 XL Denali, not my suburban 3500). I usually run in 4th on the highway, pulling 7500 lbs at 65 but my transmission runs 15-20° cooler in mid 90’s ambient temps than what you’re describing, except when pulling up a long grade, but it comes back down to 198-200 pretty quickly after cresting the hill. Similar engine temps though, except mine holds 210° except when climbing. Can’t tell if that’s what you’re experiencing or not.

A couple of things come to mind. One is I’ve read that shifting has a significant impact on transmission temperatures. I read a study on it that showed as much as a 5° increase in fluid temperature for every shift under towing loads. I used to let mine shift between 4th and 5th, but I keep it in 4th now unless I’m on a long gentle descent.

The second thing is related to what John said about oil temperatures and the wise old tuner saying run the engine for cooler oil temps, fuel be damned. In gasoline engines, the amount of heat being sent into the cooling system (and oil) from each combustion event is highest when the AFR is very slightly rich because it results in significantly higher peak internal cylinder pressures. Go richer still and the power output is the same, but the combustion event generates less of a pressure spike, sending less heat into the cooling system. Go on the other side to a leaner burn, and EGTs go up and power output decreases slightly, but it will also result in lower peak cylinder pressures and also generate less heat into the cooling system. I could show you this happening in real time in a piston-powered aircraft with controllable mixture and the effect on cylinder head temperatures is dramatic. It’s related to the speed of flame front propagation relative to piston position near TDC. It could be that enriching the AFR just a little under the typical RPM and throttle settings you would see while towing at 62-65 in 4th, would generate considerably less heat.

Because the radiator and transmission cooling system are linked in our trucks, more heat in the coolant means more heat in the transmission fluid. It’s a wild-ass theory but plausible also. My Derale is good, but significantly less effective than the trucool 40K in my ‘07 XL Denali at keeping trans temps down when towing. That said, its position in front of the radiator send engine temps to the roof when towing (thanks, 6.2), so it’s not a viable solution for us.

What kind of MPG's to you see when towing your camper in the Summer Dave ???

 

[Geotrash]

What kind of MPG's to you see when towing your camper in the Summer Dave ???

8-9 mpg

 

[Foggy]

8-9 mpg

OK... When I can keep it in 5th I get about 9-10... In 4th gear it's 7-9 mph
But way better than when I was supercharged.. that was 4.5-5.5 mpg

 

[Geotrash]

OK... When I can keep it in 5th I get about 9-10... In 4th gear it's 7-9 mph
But way better than when I was supercharged.. that was 4.5-5.5 mpg

Wow...no wonder you went back to NA.

I would wager that a switch to 3.73 would keep you in 5th more. And 4.10's, fuhgeddaboudit - you'd hardly ever see 4th. The Suburban 3500HD is interesting to have alongside the other two to compare. The 6.0/6L90 spends 95+% of its time in 5th pulling the same camper.

 

[strutaeng]

...I was gonna ask if everyone here is talking about the 6LXX transmissions? Seems like it?

 

[mikez71]

I've got my PE coming in at 60 kpa. And it's a bit lower multiple than the stock settings.
I've watched my LT fuel trims and WideBand in real time while towing. ..It's hard to
closely watch both sometimes

There is a dedicated channel that is PE active or inactive that should show up...

I always thought the PE was like an accelerator pump, but watching one of these youtube tuning videos, apparently it is for cylinder cooling when you are heavy on throtte/load.

Or the HOT table for PE, which seems to be set at 87% throttle from factory. Perhaps you're over 87% in 5th, but under in 4th.

...I was gonna ask if everyone here is talking about the 6LXX transmissions? Seems like it?

I hope so! (they are going into 5th gear...)

 

[Geotrash]

...I was gonna ask if everyone here is talking about the 6LXX transmissions? Seems like it?

Yes, 6L80 for the XL Denali's.

 

[strutaeng]

Gotcha @Geotrash & @mikez71

@Foggy I think the transmission is running a bit warmer on 4th gear simply because it's running at higher RPMs. Higher RPMs I'm pretty sure simply means more friction (= more heat). Probably not a linear relationship because you have higher ATF flow, and different clutchpacks engaged/holding and other factors. Maybe we can look at the range reference chart...

Also, the engine IS making more power at the higher RPM as well, so the more power, the higher the heat generated.

But TBH, those transmission temperatures don't seem concerning to me at all.

I'm not too familiar with the 6LXX, and not sure what the temperature difference between the 6L80 vs 6L90 would make. I'm thinking the 90 would run slightly lower temperature since it has the larger output planetary gearset, if both were pulling the same exact gross combined weight, same speed, same power, etc.? But usually on the heavier trucks there's also differences in driveline losses due to larger axles, etc.

Also, maybe @NickTransmissions can offer some insights from an geartrain perspective?

 

[Geotrash]

Gotcha @Geotrash & @mikez71

@Foggy I think the transmission is running a bit warmer on 4th gear simply because it's running at higher RPMs. Higher RPMs I'm pretty sure simply means more friction (= more heat). Probably not a linear relationship because you have higher ATF flow, and different clutchpacks engaged/holding and other factors. Maybe we can look at the range reference chart...

Also, the engine IS making more power at the higher RPM as well, so the more power, the higher the heat generated.

But TBH, those transmission temperatures don't seem concerning to me at all.

I'm not too familiar with the 6LXX, and not sure what the temperature difference between the 6L80 vs 6L90 would make. I'm thinking the 90 would run slightly lower temperature since it has the larger output planetary gearset, if both were pulling the same exact gross combined weight, same speed, same power, etc.? But usually on the heavier trucks there's also differences in driveline losses due to larger axles, etc.

All good thinking. Some observations to add that seem relevant: There is only around 1K difference in weight between our campers. Mine is also 32' long and they likely have a similar 'sail' areas, which generates most of the drag that the driveline needs to overcome.

I have no data for this, but I would estimate that the additional power needed to tow our respective campers on an average highway drive would be less than 5% and probably much less. And if I'm understanding Tom's notes right, his transmission temperatures are averaging 5-10% higher than mine. Not a linear relationship of course, but for an otherwise nearly identical drivetrain and cooling setup, both spending most of the time in 4th, I still suspect something else is going on.

I say nearly because I have a new Denso factory radiator and he's running a larger Mishimoto. Yet he's still seeing coolant temperatures slightly higher than mine as well. I was running a Cold Case 2-row radiator for 3 years before it started leaking, and mine ran even cooler still. The coolant temps rarely came off the 210 mark, and only during sustained climbs. With the new factory radiator, I see temperature rises earlier in the climb on hot days.

Long story short, I suspect it's his engine that's actually generating the additional heat that's finding its way into the transmission fluid as well. But, it's just a theory. It could very well be both.

 

[donjetman]

When ambient temps are under 85* it's fine... but when gets over that my highway trans
temps creep up to 210-218*(highest I've seen) and my coolant temp creeps from about
200* also up to about 215* (not that concerning imo, on coolant temp)...

Just some thoughts I had:

I've owned a few 40K to 50K # motorhomes with 11L & 12.7L diesel engines and Allison 6 speed autos. The rule to keep them from getting hot is "keep the rpm up. Don't lug em." I don't know if the same applies to gasoline engines.

For many yrs I've been doing used ATF analysis with some of my vehicles. My ATF samples I send to JG Lubricant Services because a President/owner, x GM fluid engineer Tom Johnson, is at ph 877-971-7799 ext#2 to answer question(s). I've talked to him several times. Give him a call. Nice man.

IIRC, Tom says your Dexron6 atf temps aren't hurting a thing. Prolonged temp greater than 235 and the fluid starts to degrade.

If your worried about Dexron6 viscosity thinning out, you might consider running an Allison approved TES668 atf. They have a higher viscosity. Some Corvette folks that "track" their car do it.

You might consider ordering some kits too so you can see what is what.

https://www.jglubricantservices.com/docs/Technical Information Sheet.pdf

https://www.allisontransmission.com/parts-service/approved-fluids/on-highway-fluids