Torque Converter Handling of Heavy Loads (1 Viewer)

[TeCKis300]

Got to see the torque converter handling of heavy loads in action on a recent tow trip. Laden with 6 people and a 27' Airstream for a long 10-day trip, CAT scaled at 15,620 lbs. Ambient was 90F+ in areas. Combined with some big grades in the west, I was curious how the torque converter manages heat. For context, I'm on 35s, re-geared with 4.3s, effectively factory gearing.

From discussion in another thread, here's the factory logic. I crossed the 266F ATF and 203F coolant threshold. Note there's two ATF temps, one in pan and one at the torque converter.

Light goes on at 302F, shuts off at 275F. 265F is hot, but apparently Toyota isn't concerned enough to warn you at that temp.

The ECU does know trans temp. If you get above 266F *and* the coolant temp exceeds 203F it will force you into 3rd and will lock up the torque converter in 3rd (which otherwise never happens) until the ATF gets below 230F and the coolant below 203F.

View attachment 3097780

Climbing a long 14-mile grade at ~6% and keeping 60-70MPH speeds. Meaning downshifts from 4 to 3, and even to 2. (5.7L is a beast holding fwy speeds on grades with almost +10k lbs load).
1) Transmission temp at both pan and torque converter when locked at cruise is typically ~195F. Coolant ~198F.
2) Once the torque converter unlocks hitting the grade to get a bit more gearing, the torque converter temp spikes to ~225F
3) On grade torque converter stays unlocked. Torque converter temp will continue a steep climb, to 240, 250, 260F+. While pan temp absorbs the heat climbing steadily. Coolant stays mostly steady and may climb very slowly a few degrees.
4) Trip threshold of 266F torque converter temp and 203F engine coolant
5) Trans ECU now aggressively will lockup torque converter. Locking 3rd, even 2nd. Torque converter temp drops aggressively tracking closer to pan temps. And both will very slowly climb in unison if grade continues. Or can steady up around 230-240F and is manageable if letting up a bit.

Highest temp snip on my OBD Fusion dashboard. Crossed threshold shortly after.

Here's torque converter lockup in second gear at 60MPH (indicated by blue horseshoe around 2)

 

[TeCKis300]

Lessons Learned​

For a factory cooling system, I think it's designed and handles load well. Pulling this kind of load is going to generate heat, that's expected. Factory GCWR is rated at 14,645 lbs for which the cooling system is validated at, and I'm 1k over at 15,620lbs. It's possible the ambient temp could have been hotter in the peak of summer where it further exacerbates temps.

What's interesting is on load and at high RPM, coolant temps barely heat up 10-12*F. That's impressive.

Is there any mods to be done? I'm not sure.

Possible Mods​

1) Larger ATF cooler
- Something I'm tossing around. At cruise and normal operation, the temps stay surprisingly low. There's just as much reason not to over-cool the ATF as there is to manage peak ATF.

2) Torque Converter Lockup Controller
- AUS market has these lockup controllers. By locking up at the start of the grade, would greatly minimize a large source of heat with the unlocked torque converter. I'm not sure about this one as the ATF lockup is very active depending on driving conditions. It also unlocks immediately on coast which I assume helps engine braking and smoothness

3) Ease up on grades
- This is an easy one and temps will temper pretty fast letting up. Not as fun, but sure would save some gas. Cranking up significant hills at 4k+ RPM puts consumption down into the 3MPG

Random thoughts​

- I wonder how diff temps are doing. The Tundra has a larger 10.5" rear diff (to our 9.5" diff) probably for managing large loads and heat. But we are full time 4WD so the load gets spread to the front axle.

- The LX handles this weight level with aplomb. Comfort, confidence, and luxury. Turn-off A/C, no way.

 

[linuxgod]

Been considering a secondary or larger cooler myself as I find the A/T temps were running around 200F before my regear but often run 212-217F now (albeit towing at 75 mph).

I’m interested in a TC lockup option. I reached out to Richards in AUS as they also have a transmission ECU flash but they never replied to me. Can’t blame them I suppose…

 

[bloc]

It also unlocks immediately on coast which I assume helps engine braking and smoothness

Could you expand on this? Smoothness I get, but total engine braking should increase if the engine RPMs are higher, which they should be if the TC is locked under coast. Unless I am misunderstanding how it all plays out.

 

[Artie]

On your possible mod list, I like 1 and 2.

A larger cooler would also allow for more fluid, I see this as a win win.

Switchable lockup seems like a no brainer, I assume it would unlock on its own as your RPM’s fall out of the power band.

Easing up on grades… how slow are we talking? In my experience, I have the same high temp issue while also angering drivers behind me. Instead of 3rd and 4th gear in the 3300-4500 RPM range with speeds of 40-55 I’d be in 2nd? Maybe I don’t follow. The slower speed will also reduce air flow over the cooler.

Thanks for your scientific approach to this, I just camped this weekend and climbed a regular for me pass that always gets my temperature up.

 

[MTKID]

Those are some high temps! I guess I don’t need to be worried about my A/T temps sometimes a little bit over 200 for a short time on hills. And I’m not sure which sensor my scanguage is showing me.

 

[bloc]

And I’m not sure which sensor my scanguage is showing me.

If it climbs quickly while working hard, probably TC. If it gradually climbs and falls, pan.

 

[TeCKis300]

I'm not particularly concerned by the temperatures. I've seen these for on various climbs before but I hadn't watched it as closely until recently to see the lockup logic that @linuxgod pointed out. Given the Tundra even got rid of the cooler in some of the latest model years, there's probably sound engineering behind the ability of modern fluids to handle elevated temps. I may or may not do any mods, depending on input in this thread.

Could you expand on this? Smoothness I get, but total engine braking should increase if the engine RPMs are higher, which they should be if the TC is locked under coast. Unless I am misunderstanding how it all plays out.

The trans ECU seems to use lockup only in steady state cruise within say 20-80% of the torque output. Any other condition, and it's generally unlocked. I would agree that engine braking may actually be higher when locked. But it prefers to be unlocked. Maybe that helps smoothness when transitioning back onto power. A manual car for example could stay locked in gear when throttling in/out, but definitely brings about some lash. So the auto transmission is prioritizing smoothness.

On your possible mod list, I like 1 and 2.

A larger cooler would also allow for more fluid, I see this as a win win.

Switchable lockup seems like a no brainer, I assume it would unlock on its own as your RPM’s fall out of the power band.

Easing up on grades… how slow are we talking? In my experience, I have the same high temp issue while also angering drivers behind me. Instead of 3rd and 4th gear in the 3300-4500 RPM range with speeds of 40-55 I’d be in 2nd? Maybe I don’t follow. The slower speed will also reduce air flow over the cooler.

Thanks for your scientific approach to this, I just camped this weekend and climbed a regular for me pass that always gets my temperature up.

It would be easy enough to upsize the existing trans cooler. Or just keep to shorter ATF change intervals. Or both as my ATF change interval is about up.

In regards to easing up - these elevated temps only occur if really standing on the gas while carrying the loads I am. 80% throttle for 10+ minutes at a time at over 4500RPM. If I drop say even 5MPH, the heat loads come down pretty immediately and level out. I'm on a 6-speed so I have a short list of gears. 4th cruise, with enough margin to climb some mild grades. 3rd is my usual climbing gear for anything up to about 5-6%. More extreme grades will put me in 2nd going up to 4500++ RPM.

 

[borfoo3]

1) Larger ATF cooler
- Something I'm tossing around. At cruise and normal operation, the temps stay surprisingly low. There's just as much reason not to over-cool the ATF as there is to manage peak ATF.

Been down this route on the 4Runner.
As long as you add the aux cooler in-line with factory heat exchanger, you maintain the warming effect after startup. Some aftermarket coolers have built in bypass that only allows the fluid to flow and be cooled once it has reached a certain temp.

 

[linuxgod]

In your scenario I wouldn’t bother adding an aux cooler. I’ve only been considering it because I’ll run 212-217F consistently when towing, ever since I regeared. Maybe I just need to step up to 37s to get my gear ratio spot on…

I'm not particularly concerned by the temperatures. I've seen these for on various climbs before but I hadn't watched it as closely until recently to see the lockup logic that @linuxgod pointed out. Given the Tundra even got rid of the cooler in some of the latest model years, there's probably sound engineering behind the ability of modern fluids to handle elevated temps. I may or may not do any mods, depending on input in this thread.



The trans ECU seems to use lockup only in steady state cruise within say 20-80% of the torque output. Any other condition, and it's generally unlocked. I would agree that engine braking may actually be higher when locked. But it prefers to be unlocked. Maybe that helps smoothness when transitioning back onto power. A manual car for example could stay locked in gear when throttling in/out, but definitely brings about some lash. So the auto transmission is prioritizing smoothness.



It would be easy enough to upsize the existing trans cooler. Or just keep to shorter ATF change intervals. Or both as my ATF change interval is about up.

In regards to easing up - these elevated temps only occur if really standing on the gas while carrying the loads I am. 80% throttle for 10+ minutes at a time at over 4500RPM. If I drop say even 5MPH, the heat loads come down pretty immediately and level out. I'm on a 6-speed so I have a short list of gears. 4th cruise, with enough margin to climb some mild grades. 3rd is my usual climbing gear for anything up to about 5-6%. More extreme grades will put me in 2nd going up to 4500++ RPM.

 

[04UZJ100]

I would really like to see documentation from Toyota indicating the 6 speeds lock up in 2 and 3.

 

[TeCKis300]

Been down this route on the 4Runner.
As long as you add the aux cooler in-line with factory heat exchanger, you maintain the warming effect after startup. Some aftermarket coolers have built in bypass that only allows the fluid to flow and be cooled once it has reached a certain temp.

Can you clarify in-line? Replacing the existing cooler with a larger cooler? Or plumbing an aftermarket unit in series with the existing?

Yeah, the stock setup has a thermostat, so probably can't generally over-cool the tranny.

I would really like to see documentation from Toyota indicating the 6 speeds lock up in 2 and 3.

This was posted above and does state that lockup will happen in 3rd gear when the high temp conditions are met.

I confirmed I saw the conditions (266F ATF with 203F coolant). Third gear lockup did follow and the torque converter heat levels did drop pretty immediately. All this was shown on my OBD-II dashboard. What I was surprised to see is that it also locked up in 2nd gear.

Both 2nd and 3rd gear lockup doesn't happen ever until these temperature conditions are met. I got to see this play out 4 times on my long 3k road-trip.

 

[a200]

I would like to know if the tundra ever came with a larger cooler and does it make a significant difference?

 

[TeCKis300]

I would like to know if the tundra ever came with a larger cooler and does it make a significant difference?

Great question. I haven't done enough research but found the following article which is eye opening. Some owners with a early model year Tundras (with trans cooler), buying a new one (without) and immediately seeing elevated temps.

Mildly elevated temps isn't so much a concern (IMO) but some of these guys towing larger loads are seeing temp warnings come up (305F), and generally really high temps into the 265 and 275+ range when towing heavy. Managing the peak temps from getting into danger zones is more of my concern. From what I'm seeing of my own temps relative to what owners in the article are reporting (look in the comments), the 200-series with its aux cooler is managing that well enough. I think.

Curious case of missing transmission oil cooler on 2019-2020 Toyota Tundra

One of the hallmarks of the Toyota Tundra has been reliability, however, the removal of the transmission oil cooler is causing an uproar.

pickuptrucktalk.com

 

[OregonLC]

Is there any room to mount an aux electric fan in front of the cooler? If so, that could be the best of all worlds. Wetted parts are left untouched eliminating inducing new failure points.

 

[04UZJ100]

I would like to know if the tundra ever came with a larger cooler and does it make a significant difference?

It doesn’t. In fact, they stopped selling the Tundra with a transmission cooler altogether in 2018/19. Even fully loaded with tow packages etc.

 

[TeCKis300]

Is there any room to mount an aux electric fan in front of the cooler? If so, that could be the best of all worlds. Wetted parts are left untouched eliminating inducing new failure points.

There's definitely room for that. It's pretty tall and rectangular so that would be a challenge getting coverage.

At speed, I'm not sure it'll have much impact as most of the cooling is provided by forced airflow. Fans for radiators and other coolers generally do their work at a stop or slow speeds.

 

[OregonLC]

There's definitely room for that. It's pretty tall and rectangular so that would be a challenge getting coverage.

At speed, I'm not sure it'll have much impact as most of the cooling is provided by forced airflow. Fans for radiators and other coolers generally do their work at a stop or slow speeds.

You could always run two square fans to get coverage. I wouldn't discount the effectiveness of force-fed cooling air. I believe the airflow is very much slowed and turbulent once it makes it past the grill. Much of it is negative pressure draw from below as opposed to ram from the front. And we're solving for knocking the top off the peak, so it only needs to be a small incremental increase in cooling ability.

 

[borfoo3]

Can you clarify in-line? Replacing the existing cooler with a larger cooler? Or plumbing an aftermarket unit in series with the existing?

Yeah, the stock setup has a thermostat, so probably can't generally over-cool the tranny.

Sorry for the confusion, basically the stock ATF cooling circuit runs like this:

Transmission --> Engine Radiator (bottom portion) --> front aux cooler --> Transmission

Some people opt to bypass the engine radiator directly to an upgraded front aux cooler for lower ATF temps (since engine coolant runs hot compared to the ATF which causes a warming effect) which I advise NOT to do.

Since ATF does not perform well when running too hot or too cold, you should upgrade/ replace the aux cooler while still keeping it plumbed in-line with the engine radiator as that is the primary way to warm the ATF quickly after a cold startup.

 

[borfoo3]

Is there any room to mount an aux electric fan in front of the cooler? If so, that could be the best of all worlds. Wetted parts are left untouched eliminating inducing new failure points.

In our experience, fans mainly help on low speeds only. In road speeds or the OP's case of heavy towing, the bigger aux cooler was the way to go. Overall I've averaged 20F to 30F gains with an aftermarket aux cooler vs the stock setup.

And yes it did leak the first time, so better get the plumbing done properly.