Understanding the modern auto transmission

[SDHNTR]

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.

You are correct. My Toyota has a coolant heat exchanger. Tundras 2021 and newer no longer have an external air cooler. Bean counter influence, apparently. Cost cutting aside, that seems like a poor design choice to me, but I’m not an engineer. I’d love to hear it from Toyota what really happened.

So what you are saying is from an engineering perspective, air cooled tranny coolers run cooler overall? I know a lot of guys are installing auxiliary air coolers after the heat exchanger in these newer Tundras. Yes, I could do that and have in previous vehicles (99, F250 7.3 diesel) I just have a hard time with that as I’d like to think Toyota engineers were smart enough to know what they were doing. Especially when they claim a 12,000 pound towing capacity and I wasn’t even towing half of that. One of my pet peeves is having to improve something in the aftermarket that should’ve been designed properly OEM.

 

[displacedtexan]

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?

Some do. Some use fluid/fluid coolers to bring it up faster and then keep it in the intended range.

Old guys telling you what worked 30 years ago doesn't always work, and can even be destructive to modern stuff.

 

[TaperPin]

Some do. Some use fluid/fluid coolers to bring it up faster and then keep it in the intended range.

Old guys telling you what worked 30 years ago doesn't always work, and can even be destructive to modern stuff.

If my 2025 transmission craps out I trust the old guys who have 30 years experience rebuilding transmissions of all kinds old and new over any young tech at the dealership who can’t wipe his ass without a diagnostic flow chart. You can poo poo experience - I tend to trust it.

 

[RichardSkinner]

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?

They do, although people delete these to try to make their trans run cooler. ie 5th gen Ram 2500/3500 I know has an aftermarket thermostat delete kit available that advertises to run the trans down near 130F. However they don't talk about the fact that it now takes much longer to even get to that temp since it is an open loop when stone cold. That may not be a big deal in Sacramento during most months of the year, but in cold climates, over a lot of cold starts, that *could* be a mistake.

Without actually testing this stuff, or putting more effort into than this quick response, I can't say for certain whether it is significant, but it should be considered. That was all my disclaimer earlier about fully thinking through changing a designed coolant circuit.

 

[TaperPin]

They do, although people delete these to try to make their trans run cooler. ie 5th gen Ram 2500/3500 I know has an aftermarket thermostat delete kit available that advertises to run the trans down near 130F. However they don't talk about the fact that it now takes much longer to even get to that temp since it is an open loop when stone cold. That may not be a big deal in Sacramento during most months of the year, but in cold climates, over a lot of cold starts, that *could* be a mistake.

Without actually testing this stuff, or putting more effort into than this quick response, I can't say for certain whether it is significant, but it should be considered. That was all my disclaimer earlier about fully thinking through changing a designed coolant circuit.

I agree with you, if someone has poor judgement they shouldn’t mess with anything.

 

[displacedtexan]

If my 2025 transmission craps out I trust the old guys who have 30 years experience rebuilding transmissions of all kinds old and new over any young tech at the dealership who can’t wipe his ass without a diagnostic flow chart. You can poo poo experience - I tend to trust it.

I don't trust either of those groups.

Experience should also come with continuing to learn. It sadly frequently doesn't.

Modern transmissions are drastically different.

 

[Fowl Play]

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.

Thanks, google failed me when looking up which fluid they used. Still due to temp requirements vs viscosity though.

 

[mtnbound]

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.

They did come with a cooler but in 2020 +\- Toyota stopped that. My 2021 does not have a cooler where as my previous one did.

 

[IdahoElk]

I just have a hard time with that as I’d like to think Toyota engineers were smart enough to know what they were doing.

Bean counters force engineers to save money, not do what's right, Not having an air cooled trans cooler to me sounds like saving money not engineering.

 

[RichardSkinner]

You are correct. My Toyota has a coolant heat exchanger. Tundras 2021 and newer no longer have an external air cooler. Bean counter influence, apparently. Cost cutting aside, that seems like a poor design choice to me, but I’m not an engineer. I’d love to hear it from Toyota what really happened.

So what you are saying is from an engineering perspective, air cooled tranny coolers run cooler overall? I know a lot of guys are installing auxiliary air coolers after the heat exchanger in these newer Tundras. Yes, I could do that and have in previous vehicles (99, F250 7.3 diesel) I just have a hard time with that as I’d like to think Toyota engineers were smart enough to know what they were doing. Especially when they claim a 12,000 pound towing capacity and I wasn’t even towing half of that. One of my pet peeves is having to improve something in the aftermarket that should’ve been designed properly OEM.

If you find out the answer, let us know. I am curious as well.

Your second paragraph gets complicated.

Air cooled coolers can run cooler than liquid-liquid if they have the capacity since in the liquid to liquid case the radiator is the initial low energy sink and it is operating at 180-205F or so (although the radiator is ultimately transferring the heat to the outside air). The trans fluid is not going to be able to reject all of its heat to the radiator to match the radiators temp (and certainly in a perfect world it's still impossible to cool the trans fluid below the radiator temp), so it will exit the cooler ~10F higher than the radiator or whatever the system cooling capacity is designed to. If the rate of heat generation increases, the exit temp of the trans fluid could climb depending on how effective that liquid-liquid cooler is and what temp the coolant is at.

An air cooler is rejecting heat to ambient temp, so it can cool the fluid down below a radiator's coolant temp if it is designed correctly. Putting one in the circuit after the liquid-liquid cooler certainly could bring your temps down from where they are if you're inclined. You would also need to add more fluid to compensate for the added volume. This will further help cooling some small amount.

 

[Predator_SD]

Why do they run hotter?

planned obsolescence

 

[displacedtexan]

Why do they run hotter?

planned obsolescence

Then why does the 6r140 hold ups so well to towing and significantly more power stock, and way more power on modified trucks than the older units with lower operating temperatures?

Bean counters force engineers to save money, not do what's right, Not having an air cooled trans cooler to me sounds like saving money not engineering.

The predecessors of the 6r140 used liquid/air, it uses liquid/liquid... It's the tougher transmission by a significant margin.

 

[Predator_SD]

Then why does the 6r140 hold ups so well to towing and significantly more power stock, and way more power on modified trucks than the older units with lower operating temperatures?

The predecessors of the 6r140 used liquid/air, it uses liquid/liquid... It's the tougher transmission by a significant margin.

Get back to me at the 10 yr/200,000 mile mark kiddo

 

[fatbacks]

My trusty Allision never ran too hot over the past years - towing or not, but is deciding to crap out on me right after the warranty expired

Interesting read as I chase down a gremlin in my transmission and/or t case. Found out this week that new transmissions are about the price of a pretty nice fully outfitted hunt. FML.

If anyone is well versed on GM drivetrains, I could use a second opinion.

 

[displacedtexan]

Get back to me at the 10 yr/200,000 mile mark kiddo

I'm at 13 years and 270k

Lots of towing, about 150hp over stock.

 

[Predator_SD]

I'm at 13 years and 270k

Lots of towing, about 150hp over stock.

Uh huh, sure ya are, smh.


You’re the greatest ever man!

 

[IdahoElk]

I'm at 13 years and 270k

Lots of towing, about 150hp over stock.

What are you driving and what are you towing? Is this a totally stock motor?

 

[Tmac]

They did come with a cooler but in 2020 +\- Toyota stopped that. My 2021 does not have a cooler where as my previous one did.

They stopped the tranny cooler with 2019, which I have. Has a warming/cooling puck that does not do much. I added a dedicated cooler, made a huge difference. Stays happily between 185-195 99% of the time, high is now 205 ish when towing, not 240-250. I’d even hit 235 on the fwy at 70 mph lightly loaded on a summer day.

 

[SDHNTR]

My trusty Allision never ran too hot over the past years - towing or not, but is deciding to crap out on me right after the warranty expired

Interesting read as I chase down a gremlin in my transmission and/or t case. Found out this week that new transmissions are about the price of a pretty nice fully outfitted hunt. FML.

If anyone is well versed on GM drivetrains, I could use a second opinion.

Sorry about your trouble. If you need it done right, there is none better than Inglewood Transmission in Long Beach, CA.

 

[SDHNTR]

Anyone think there is hope that during the 2026 Tundra mid-cycle refresh Toyota might bring back an air/oil tranny cooler?

I don’t understand the cost cutting nature of these decisions. Anyone willing to buy a $75k+ truck would gladly pay another $1500 for something that would make their transmission more reliable!