Gas/Diesel and Resulting Transmission Temps

[slarsen]

I have written a few times about the joys of towing with gas, but that isn't my intent this time. It's really to ask a question I haven't seen discussed here that has been on my mind . . .

Others have described the transmission temps they experience on this forum, and with my low-mileage F250 (2012) pulling a trailer about 11K pounds up the side of some mountains, I've seen much higher transmission temps. Even when the ambient temps have been quite pleasant. I've wondered why that might be. Frankly, I wondered if some people were BSing us. But now I have a theory:

So, going up the mountain, my engine is turning maybe 4500 to 5000 rpm's, sometimes a bit more, sometimes for fairly long periods. Everyone says that isn't a problem for a gas engine, and I believe that is so. BUT . . . a diesel will be running roughly half the rpms, which means that input shaft/gears are turning much closer to the speed of output shaft/gears with diesel. There is quite a mismatch in input/output speeds with a gas engine, which means, I should think, more gears and busier gears moving quite rapidly. It would seem to me that could result in significantly higher oil temps.

Does this make sense?

[Sport45]

christopherglenn wrote:
Most (all?) automatics have a cooling loop in the main radiator. Tow packages usually add a oil to air cooler as well. Pulling up a hill, the engine coolant is MUCH hotter, usually hotter then the tranny oil, and is actually heating the oil as it goes through the cooling loop. The aux cooler pulls some of that heat away, but the tranny temps climb. Before you reroute the cooling line to bypass the main radiator, when starting from a stop with a load, or any time the torque converter is unlocked, and you are under power it generates many times more heat then the aux cooler can dissipate, so you NEED the rad cooling loop.

The transmission cooler is in cold(er) tank of the radiator. It is much cooler than the engine coolant readout on the dash. Once the transmission is up to operating temperature the cooler in the radiator will be cooling the transmission fluid, not heating it. If the coolant was heating the transmission fluid then the engine would soon be toast too.

[carringb]

N-Trouble wrote:

carringb wrote:
The newer transmissions are designed to run hotter, as this lowers fluid viscosity which reduces drag and improves efficiency ever so slightly.

Not true for the current Allison. In 2011 they lowered the line pressure which resulted in significantly lower temps. I have yet to see mine even approach 200* pulling hills

Good to know. I guess i should have qualified that to say "newer Ford transmissions". Also related, the newer gas motors also have a higher T-stat opening temp than the older Triton motors. I've had a couple folks ask me about this because they were concerned, but I looked it up in the shop manual, and the new gas motors don't fully open the the thermostat until 211F.

[BenK]

Similar because this is comprised of several sub-systems to the main thermal management
system

Many, many variables and that there are controls that vary the FPM/GPH/PSI/etc
between them.


Another is the computers. Not just the ECU, but the tranny CPU/etc


Tranny CPU has torque management duties and it WILL throttle that back if the
temps reach the safety thresholds (several levels)

[Tystevens]

I kinda like the OP's theory, but interestingly, my trans temps are pretty similar towing the same trailer with diesel and gas. Not apples to apples, of course -- the diesel had the mighty Allison, while the gassers have been 1/2 tons.

The hottest temps I've seen have been going up a steep grade fast at full throttle -- the kind of grades we have here on I80 in Utah and Wyoming, where the road is straight and around 6-7% for a bit. I may have tried to see how fast I could go on a couple of those grades in the past, which caused my temps to rise quickly in both my diesel and my 1/2 ton gassers.

Stop and go traffic causes it to creep up there as well. Interestingly, slower but steeper roads (think 2-lane through the mountains at up to 10% grade and speeds in the 30-40 mph range) don't seem to generate much transmission heat in any of the rigs I've had.

[Tystevens]

N-Trouble wrote:

carringb wrote:
The newer transmissions are designed to run hotter, as this lowers fluid viscosity which reduces drag and improves efficiency ever so slightly.

Not true for the current Allison. In 2011 they lowered the line pressure which resulted in significantly lower temps. I have yet to see mine even approach 200* pulling hills

Interesting. My '06 would get a little hot. I'm struggling to remember the numbers on the dash gauge (I think the middle number was 190*? It has been a few years now ...), but towing, it was usually just to the cool side of the middle hash mark, and going up hills and in traffic, it would often creep just to the hot side of that hash mark. It would get up to about 3/4 if I mashed the throttle up a long high speed ascent like Parleys Canyon, where you have enough straight road to do 70 up a 7% grade. I guess I don't know what the actual numbers were, since I didn't have a digital readout.

[N-Trouble]

carringb wrote:
The newer transmissions are designed to run hotter, as this lowers fluid viscosity which reduces drag and improves efficiency ever so slightly.

Not true for the current Allison. In 2011 they lowered the line pressure which resulted in significantly lower temps. I have yet to see mine even approach 200* pulling hills

[NHIrish]

2011 Duramax with Allison trans;

Running with Valvoline Dex VI, I have hit 190 on one occasion....towing on warm day in mountains. I am amazed at how cool it runs towing a 15,000 lb. fifth wheel. Over 35,000 miles towing this combo and temps usually below 160.

[christopherglen]

Most (all?) automatics have a cooling loop in the main radiator. Tow packages usually add a oil to air cooler as well. Pulling up a hill, the engine coolant is MUCH hotter, usually hotter then the tranny oil, and is actually heating the oil as it goes through the cooling loop. The aux cooler pulls some of that heat away, but the tranny temps climb. Before you reroute the cooling line to bypass the main radiator, when starting from a stop with a load, or any time the torque converter is unlocked, and you are under power it generates many times more heat then the aux cooler can dissipate, so you NEED the rad cooling loop.

[slarsen]

Lots of good information. Thanks for these responses.

[carringb]

The newer transmissions are designed to run hotter, as this lowers fluid viscosity which reduces drag and improves efficiency ever so slightly.

[Denny___Jami]

I'm also pulling with a 6.2 Ford and compared to other Fords I have owned the transmission temp runs higher all the time when using the readout on the information center. It may have to do with were they take the reading but it's normal and it seams to be constant. Mine always stays around 200 and as high as 220 when pulling hard up a grade.

Denny

[intheburbs]

wintersun wrote:
The flash point of ATF is over 300 degrees and that is when you need to be concerned. If I am towing up a grade at elevation I expect to see a higher ATF temperature but as soon as I start down the other side the temperature starts to drop and continues to drop as I drive.

A friend of mine is an engineer for GM. He works in the transmission dyno/test lab.

A few points he tried to drive home with me:
1) Dexron VI starts to break down at 270°. Stay under that number (not 300°), and you'll be fine.
2) No matter what you're doing to your truck, the test guys have already done it, and worse. Loads, grades, temperatures - they've thrown it all at the truck.
3) The factory transmission cooler is designed to keep the transmission within acceptable operating temperatures in a worst-case scenario. Installing an aftermarket cooler is not necessary.

[wintersun]

With the tow package you should get a larger cooler unit for the transmission. You also get tow/haul mode which causes the torque converter to stay locked more of the time and this keeps the ATF running cooler.

The problem with towing up a grade is that the load on the transmission is the greatest and at the same time the speed of the truck is reduced so there is less air flowing over the cooling fins and if at elevation the air is thinner as well. This is even worse with high ambient air temperatures.

The flash point of ATF is over 300 degrees and that is when you need to be concerned. If I am towing up a grade at elevation I expect to see a higher ATF temperature but as soon as I start down the other side the temperature starts to drop and continues to drop as I drive.

If you want to lower the maximum ATF temp when going up a grade you can have a larger oil pan installed and double the amount of ATF in the transmission and have a larger fluid reservoir to absorb the heat. Another option is adding a larger cooler to the transmission in place of the factory one.

[intheburbs]

I think you have several variables in play...

-electric vs belt-driven radiator fan
-velocity of air through the various heat exchangers
-torque converter locking and unlocking
-frequency of transmission shifts

On our last trip, we pulled the TT on I-70 through the Eisenhower tunnel, and did some slow, crawling 4-wheeling in Moab. Ambient temperature on I-70 was 75°, the temperature when we were four wheeling was over 100°. Both times, my trans temp was in the low 230s.