Understanding the modern auto transmission

[SDHNTR]

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.

 

[bowhuntercoop]

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 good or better than the alternative.

Clutch material and packs for one, some trans are actually made to slip where others are not. The 10 speed aisin in the new tundras is amazing, over built to the gills to say the least and very robust.

I’ve spent 10s of thousands on different builds for my diesels over the years, from mild to wild. The 10 speed in my 2025 tundra is just a joy to drive and puts the power to the ground, aisin knocked it outta the pack with the engine / trans combo in this gen.

 

[Hondo64d]

I have a ‘23 F250 with the 7.3 gas motor and 10R140 transmission. To me, it seems to run hot, up to 221 degrees towing, but a google search tells me this is normal. We’ll see…

John

 

[brca005]

I have a ‘23 F250 with the 7.3 gas motor and 10R140 transmission. To me, it seems to run hot, up to 221 degrees towing, but a google search tells me this is normal. We’ll see…

John

I had a 21 F350 with the same transmission and I monitored temps while towing our 16K fifth wheel and it would get to 207 on up regularly. I also googled it and it said it was normal, I also talked to the mechanic that serviced the transmission and said the fluid is made to run that hot. I never ran into any issues with mine.

 

[SDHNTR]

Ok you are all coming to the same conclusion I have. I know it’s normal. No one is addressing the question tho…

Why would one engineering department design their transmission to purposefully run hotter than another? How can hotter possibly be better?

 

[pacoalcracker]

If it's normal and engineered that way. why do you care if it's engineered differently. My 2 sons are very different, and they were made that way, and I don't care why. I'm pretty sure you have many other more important things to spend time thinking on. If not, go fishing.

 

[stumblinhorse]

Everything is designed to run hotter today than in the past. Mostly for CAFE Standards and the large penalties associated with not meeting the standards. Many transmissions are looped into using coolant systems. So if your coolant is warm so is your trans. Eventually they will be the same temp. 220’s on average for most.

 

[BBob]

How can hotter possibly be better?

More efficient? Less internal power losses at the higher temps? Toyota trucks are notoriously bad with fuel economy so maybe an attempt to help manage fleet fuel economy averages?

On a side note but related. Chrysler Pentastar engines are purposefully run hot at normal temps. This was done for efficiency/fuel economy, not because it's better for the engine. Maybe a similar issue with your transmission?

 

[RS_Non-Res]

Economics. CAFE. Many variables in one equation (thus many different solutions).

 

[Kurts86]

Ok you are all coming to the same conclusion I have. I know it’s normal. No one is addressing the question tho…

Why would one engineering department design their transmission to purposefully run hotter than another? How can hotter possibly be better?

Why does it matter what temp the transmission runs at if the fluids, seals and other materials can handle it and the transmission works.

If the materials support higher temps it means less material, cost and complexity involved in cooling which is a net positive. The direct material and labor content to support it has been beaten to death for years to make those decisions.

 

[JRMiller]

This

 

[TaperPin]

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.

 

[ODB]

Why does it matter what temp the transmission runs at if the fluids, seals and other materials can handle it and the transmission works.

If the materials support higher temps it means less material, cost and complexity involved in cooling which is a net positive. The direct material and labor content to support it has been beaten to death for years to make those decisions.

This is my thought. oil temp is one variable - it says nothing of any other component or metallurgy, etc.

You could flip this on its head and ask why Chevy is afraid of letting its components get hotter?

 

[bpa556]

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.

Sent from my iPhone using Tapatalk

 

[TaperPin]

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

 

[Kurts86]

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.

That’s an extremely simplistic way to look at it. If you have higher operating temperatures you may use a higher weight oil to have the same functional operational viscosity as a lower temp system. Also viscosity is usually only published at 2-3 temperatures as standard with temp to viscosity curves vary a lot by oil.

 

[TaperPin]

That’s an extremely simplistic way to look at it. If you have higher operating temperatures you may use a higher weight oil to have the same functional operational viscosity as a lower temp system. Also viscosity is usually only published at 2-3 temperatures as standard with temp to viscosity curves vary a lot by oil.

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.

 

[IdahoElk]

Ok you are all coming to the same conclusion I have. I know it’s normal. No one is addressing the question tho…

Why would one engineering department design their transmission to purposefully run hotter than another? How can hotter possibly be better?

Do they both have coolers? maybe one is bigger than the other?

 

[displacedtexan]

Ok you are all coming to the same conclusion I have. I know it’s normal. No one is addressing the question tho…

Why would one engineering department design their transmission to purposefully run hotter than another? How can hotter possibly be better?

Clutch material is different, fluid and seal technology is much improved.

Heat in an automatic transmission comes from friction between clutches and steels, there's frequently more than old transmissions had to hold more power.

Pressure generates heat too, line pressures are higher, to hold more power.

So the clutch materials and fluids are engineered for it.

 

[displacedtexan]

Modern synthetic fluids are also more viscously stable across a wider range of temperatures.

A big part of why heat was so hard on older automatics was that the fluid would thin out a little bit, the clutches would slip, and the fluid would cook and burn.