Understanding the modern auto transmission

[displacedtexan]

I trust the opinions of the old guys working in transmission shops, and that’s cooler is better.

That's the same mindset that keeps people running 15w40 in modern diesels and destroying their valvetrain.

 

[SDHNTR]

It matters to me because I am an analytical person. I like to know the “why” of things. Especially things that go against conventional wisdom.

 

[SDHNTR]

I trust the opinions of the old guys working in transmission shops, and that’s cooler is better.

Right, and I previously thought I did too, hence this post.

 

[displacedtexan]

Fun fact, running a transmission below intended operating temperature is also hard on it. Wears it out faster.

 

[Fowl Play]

An engineer myself, but of course not a part of either team so can only make assumptions about the why. Each transmission cooler is sized to keep the specific fluid within a set temperature band so that it has the correct viscosity to lubricate the needed parts and seals. Toyotas typically use Dextron III or IV (not positive for 2024) while the Chevy likely uses the newer Dextron VI. Dextron VI is thinner so does not require as high of a temperature to hit that correct viscosity band. Toyota’s approach is likely “if it’s not broke, don’t fix it” so they haven’t redesigned anything to use the newer fluids — which have some benefits to lubricating faster at startup and fuel efficiency. As long as both are operating in the correct temperature band, they effectively do the same thing. Toyota just needs the higher temp as it has a thicker transmission fluid.

 

[Foot756]

Well I have a 2008 Titan 4x4 with 5.6 V8 in it. I generally keep and eye on my trans temp gauge. I do fine at times the temp climbing. I'll take it out of overdrive, that has taken care of it. That's with a trailer or I am lugging the engine on bad roads. I find that lugging the trans it's seem not to like. I keep my trans oill changed. My cousin is the head mecx. of a dealership that sell Titan's. He as seen Titan 4x4 come in with 750 to 900 ths miles on them and the engine finally let loose. The trans hadn't been changed. Now the 2x2's are different story. Presently I have about 300 ths miles on the pickup without any major problems. 1starter. radiator, front wheel bearing, rearend bearing. Master cylinder, on 3rd of plugs, rotors, and front calipers, brake pads several times over the years. It will probable last longe than me. What does make me mad is they or most have drop the V8 engines now.

 

[Shell587]

I have a Tacoma and landcruiser so I'm not anti Toyota when I say this, but 225 is not normal temp. Get a transmission cooler.

It's more about the fluid breakdown. There charts you can check out. 195 is operating temp. That's where you want to be ideally. Most of the chevys have a cooler. The 3rd gen's prior to 2021 had a cooler.

Toyota only needs to guarantee the transmission last the life of it. That's 60k or 5 years. That's also why they are "sealed" now, even though you can change the fluid in the pan.

 

[displacedtexan]

I have a Tacoma and landcruiser so I'm not anti Toyota when I say this, but 225 is not normal temp. Get a transmission cooler.

It's more about the fluid breakdown. There charts you can check out. 195 is operating temp. That's where you want to be ideally. Most of the chevys have a cooler. The 3rd gen's prior to 2021 had a cooler.

Toyota only needs to guarantee the transmission last the life of it. That's 60k or 5 years. That's also why they are "sealed" now, even though you can change the fluid in the pan.

You say 225 isn't normal...

It wasn't.

It is now.

My 6r140 isn't supposed to sustain over 240. It's 13 years old with 270k miles. Lots of towing, truck itself is way heavier than it left the factory, and running a hot tune.

It's 2025, not 1985.

 

[RichardSkinner]

Hot fluid is thinner. Thinner fluid doesn’t lubricate as well, but gets better gas mileage. Same thing with super thin engine oil. In no way is that engineered for longevity.

*given the same fluid
Which this is not.

It is very simplistic, like changing oil more often than recommended is also a good idea. I wouldn’t tow anything without a cooler in any truck. I look at the cost of a modern transmission vs the effort and expense to install a quality cooler and that seems like cheap insurance. With so many parts designed to fail my trust that engineers have our best interest in mind is quite low.

A Quick look at the 2024 Tundra and There is a heat exchanger in the radiator so the tranny has to operate at engine temp at a minimum.

So with that logic why don't you add engine oil coolers to your truck to get it down to ~165F?

Adding coolers needs to be thoroughly researched, not just done off an intuitive decision.

 

[RichardSkinner]

An engineer myself, but of course not a part of either team so can only make assumptions about the why. Each transmission cooler is sized to keep the specific fluid within a set temperature band so that it has the correct viscosity to lubricate the needed parts and seals. Toyotas typically use Dextron III or IV (not positive for 2024) while the Chevy likely uses the newer Dextron VI. Dextron VI is thinner so does not require as high of a temperature to hit that correct viscosity band. Toyota’s approach is likely “if it’s not broke, don’t fix it” so they haven’t redesigned anything to use the newer fluids — which have some benefits to lubricating faster at startup and fuel efficiency. As long as both are operating in the correct temperature band, they effectively do the same thing. Toyota just needs the higher temp as it has a thicker transmission fluid.

Engineer as well -

Most Toyotas use Toyota ATF WS (world standard)

The 2025 Tundra is spec'd for Toyota ATF WS.
The 2024 Chevy Trail Boss is spec'd for Dexron ULV. This has a lower viscosity than Dexron III and IV.

These are completely different fluids from each other and are also different from Dexron III and IV.

 

[SDHNTR]

Engineer as well -

Most Toyotas use Toyota ATF WS (world standard)

The 2025 Tundra is spec'd for Toyota ATF WS.
The 2024 Chevy Trail Boss is spec'd for Dexron ULV. This has a lower viscosity than Dexron III and IV.

These are completely different fluids from each other and are also different from Dexron III and IV.

Interesting. Thanks for this info! Are you aware of a Toyota produced spec sheet where normal and severe temperature ranges are actually listed officially? Google says up to 225 but who knows where that source comes from.

Also, that’s 225 in the pan. So if that’s the case, is it running at 260 in the torque converter? My OBD scanner only reads from the pan.

 

[TaperPin]

To see what upper limit on trans temp for any vehicle manufacturer simply look at what temp the computer kicks in limp mode. That’s the absolute maximum to prevent trashing the trans that day right then. At least for 4 runners that’s around 260 degrees. Just googled 2024 gm trucks and about the same number comes up. The closer you run to that temp the sooner the trans craps out. Toyota trucks have used ATF WS since the mid 2000s. It’s viscosity is thin like dextron 6. Unfortunately, I don’t see any evidence of super hero abilities.

 

[TaperPin]

Having said all that, I know newer Tacos and 4 Runners have numerous trans over temp posts - pages and pages, but Tundra owners don’t. It could be the heat exchanger in the Tacoma is better at dealing with trans temps and keeping them down.

 

[TaperPin]

So with that logic why don't you add engine oil coolers to your truck to get it down to ~165F?

Because anyone other than an idiot knows engines run better at a higher temperature range. If someone doesn’t have good judgement then yes don’t touch a factory vehicle.

Adding coolers needs to be thoroughly researched, not just done off an intuitive decision.

You’re an engineer, wouldn’t you agree that all automatic transmissions used for towing will have, need to have, must have some way of shedding excess heat from the factory? That’s not in dispute by anyone. Right?

What you’re saying is an auxiliary cooler is sometimes a bad idea when towing? It’s adding to the BTU capacity of the factory system, retaining the factory bypass system, factory sensors, etc. so maybe you can point to the point on the chart where 20 degrees cooler operating temp is a bad idea?

Maybe I’m not getting it and it’s some other factor you’re worried about? What I’ve known to work universally in towing vehicles has worked so far across the years and models, but may cease to be a valid rule of thumb at some point. I just don’t see that point being 2024.

 

[RichardSkinner]

Interesting. Thanks for this info! Are you aware of a Toyota produced spec sheet where normal and severe temperature ranges are actually listed officially? Google says up to 225 but who knows where that source comes from.

Also, that’s 225 in the pan. So if that’s the case, is it running at 260 in the torque converter? My OBD scanner only reads from the pan.

I believe Toyota states ATF WS upper temp limit of 248F and that is pan temperature, but you may research that more.

That's a fair point to consider TC temp over pan temp. I do believe it's 20-50F higher in the TC than the pan. I may put my truck in drive with the parking brake on and let it sit building heat for a while and compare compare the OBD value to an IR temp gun on the cooler feed line just as it leaves the transmission to compare.

 

[mtnbound]

So on a hunt in CO this last week, I had the opportunity to haul a loaded quad trailer up the exact same mountain Road, with the exact same load, in both my 2024 Toyota tundra, and my buddies 2024 Chevy trail boss. Everything kept constant, I monitored transmission temperatures.

The transmission temperatures in my Toyota have always been alarming to me, but nothing is wrong, I have had it checked out multiple times. By design, Toyota trannies supposedly run hot and there is zero cause for concern. There is no auxiliary cooler. Toyota states that normal temperatures are 195 to 225. Toyota obviously has their reasons and that’s what I’m trying to understand. I always thought the one universal truth to auto transmissions is that heat is always bad, and cooler is always better.

So as I pulled up this grade, my transmission, as measured in the pan, showed temps from about 205 - 226.

My buddy’s Chevy ran from 160 - 178.

Help me understand this. They obviously have two different designs (yet both are 10 speed). Probably very different fluids too. But how or why is there a world in which a transmission is designed to run hotter is considered good or better than the alternative.

Toyota went away from a trans cooler, and I think that was stupid. Heat is bad; I would install a cooler if I were doing much towing.

 

[RichardSkinner]

Toyota went away from a trans cooler, and I think that was stupid. Heat is bad; I would install a cooler if I were doing much towing.

I thought the Tundra (could be wrong here) has a liquid to liquid trans cooler, no? So it's going to sit just above coolant temp or so.

Looking at the temps OP stated for the Chevy, it must be an air cooler.

 

[RichardSkinner]

Because anyone other than an idiot knows engines run better at a higher temperature range. If someone doesn’t have good judgement then yes don’t touch a factory vehicle.


You’re an engineer, wouldn’t you agree that all automatic transmissions used for towing will have, need to have, must have some way of shedding excess heat from the factory? That’s not in dispute by anyone. Right?

What you’re saying is an auxiliary cooler is sometimes a bad idea when towing? It’s adding to the BTU capacity of the factory system, retaining the factory bypass system, factory sensors, etc. so maybe you can point to the point on the chart where 20 degrees cooler operating temp is a bad idea?

Maybe I’m not getting it and it’s some other factor you’re worried about? What I’ve known to work universally in towing vehicles has worked so far across the years and models, but may cease to be a valid rule of thumb at some point. I just don’t see that point being 2024.

My comment was mainly because you mentioned engine oil in there above. No big deal. I agree that in general trans fluid running down towards 165-190 is going to be much better for longevity.

 

[TaperPin]

Fun fact, running a transmission below intended operating temperature is also hard on it. Wears it out faster.

Don’t modern transmissions have thermostatic bypass that don’t circulate oil through coolers until it comes up to operating temp? And diesel oil has what to do with transmissions?

 

[TaperPin]

My comment was mainly because you mentioned engine oil in there above. No big deal. I agree that in general trans fluid running down towards 165-190 is going to be much better for longevity.

???
I never mentioned engine oil.

No biggie.