OEM AT Cooler Bypass?

[llm]

Considering your trans temps are hitting 245F, they are a good 50F hotter than the coolant in the radiator and 150F+ hotter than ambient temp on even a hot day. Being that heat flows from hot to cold, and that a fluid-to-fluid heat exchanger (e.g., the radiator-integrated trans cooler) is more efficient than fluid-to-air (i.e., and external trans cooler), this pretty clearly indicates that the integrated trans cooler in the OEM Lexus radiator is severely under-sized. Were it sized correctly, your trans would shed nearly all of the excess heat to the coolant in the radiator and your fluid temp would equilibrate to the ~195F coolant temp.

I saw this in my GX470. Unlocking the TCC (such as when making a pass on a 2-lane highway) would cause an immediate spike in the trans temps. As would long, slow off-road climbs where engine speeds were low (limited airflow though the radiator). When I upgraded the radiator this went away. The temps more or less don't spike any more and the trans fluid stays a very consistent temperature. I think the CSF must have more rows/loops inside it than the OEM Lexus radiator does (although, one would need to cut open both radiators to verify).

you are the biggest advocate for CSF! I'm planning to add it in the next 10k miles. yep, this street near me is like an off road climb, but paved.

 

[Rednexus]

you are the biggest advocate for CSF! I'm planning to add it in the next 10k miles. yep, this street near me is like an off road climb, but paved.

I wish they'd give me a kick back for every one I've recommended!

To be honest it's a shame that Toyota put such crappy radiators in these rigs. GX460 and GX470. Not only are they prone to breaking and overheating the engine, but they skimped on the integrated trans cooler piece as well.

 

[Rednexus]

IMO.. one should monitor TC lockup as well as temperature. If you can keep in lockup it will definitely help keep temps down. Not always possible though..I know.

It's probably worth contacting Wholesale Automatics or their US distrubitor (whichw as Cruiser Brothers in Stockton, CA but I understand may now be someone else) to see if one of these will work:

Wholesale Automatic Transmissions

Australia's Leading 4WD Automatic Transmission workshop specialising in Upgrades, Accessories and Conversions

automatictransmission.com.au

I have an older model in my GX470. I mounted the switch in the center console, and it will lockup the trans in any gear. However I only really use it now when towing and climbing hills/passes in 3rd gear or below. It's very, very useful for that. Before I upgraded the radiator I used it much more.

 

[r2m]

IMO.. one should monitor TC lockup as well as temperature. If you can keep in lockup it will definitely help keep temps down. Not always possible though..I know.

Hence why I like manual shifting up/down steep hills.

 

[BC11SVT]

On my rig, pinning makes a 5-10deg difference on the low end. No difference on the high end. Instead of cruising at 205, mine is closer to 195 on flat ground with no load. Not much difference I can detect after warmup when pulling my trailer.

Do you have an auxiliary transmission cooler?

Here's my AT pan temp ranges.
Hayden 678, pinned transmission thermostat

 

[Rednexus]

Hence why I like manual shifting up/down steep hills.

Even with manual shifting, the TCC usually won't lock up in 3rd or below (I recall reading they will lock in a GX460 if the trans reaches something like 300F, at which point damage has probably already been done to your transmission).

Manual shifting to a lower gear will, however, make the engine spin faster, thereby pulling more air though the radiator, which can help with cooling relative to running with the TCC unlocked in a higher gear. But it's still a band-aid for the under-sized OEM radiator/trans cooler combo.

 

[xychromosome]

I have found the OEM radiator plus OEM trans cooler along with pinned thermostat more than sufficient for my needs. Sure the OEM cooler is small compared to other options, but after running it for 2+ years, I haven't seen temps over 220F under heavy load (deep sand, long uphill, or offroad crawling), and typically maintain 170-190 during everyday driving. My original radiator lasted 105k and 8 years.

 

[Timujin]

This is good news @xychromosome thanks.

Ordered the factory setup from Bell Lexus yesterday. If that isn’t sufficient then will consider the Hayden. Our 460 is under factory warranty and I don’t want to go through any potential warranty denial issues.

 

[r2m]

Even with manual shifting, the TCC usually won't lock up in 3rd or below (I recall reading they will lock in a GX460 if the trans reaches something like 300F, at which point damage has probably already been done to your transmission).

Manual shifting to a lower gear will, however, make the engine spin faster, thereby pulling more air though the radiator, which can help with cooling relative to running with the TCC unlocked in a higher gear. But it's still a band-aid for the under-sized OEM radiator/trans cooler combo.

An example of one of the grades I drop down to 4th gear in is called Kingsbury Grade (Route 207), running from Gardnerville, NV up and into Lake Tahoe basin.
It's a long, winding, steep grade that I just drop down into 4th the whole way up and usually 5th gear on the way down to save the brakes.

 

[Rednexus]

An example of one of the grades I drop down to 4th gear in is called Kingsbury Grade (Route 207), running from Gardnerville, NV up and into Lake Tahoe basin.
It's a long, winding, steep grade that I just drop down into 4th the whole way up and usually 5th gear on the way down to save the brakes.

In 4th gear you are probably locked and not generating much heat. I think you are also re-geared? Since upgrading to 4.56 gears my TCC unlocks a lot less. I only have 5 gears but it usually pulls hills in 5th gear or 4th gear locked (with 3.73s it was a constant 5th-4th-3rd-4th-5th). Sometimes it will unlock for <30 seconds in 5th on a hill that's not quite steep enough for 4th gear, but I just let it do that. If I'm using 3rd to get up a hill on the highway I'm either going <45 mph on a twisty road or I'm towing something. FWIW, our hills are quite steep around here, although not that long.

All of the GXs are under-geared when 33" or larger tires are added, which causes the TCC to unlock more frequently to bump the engine up a few hundred RPM and make up for the effective gear ratio being too high. The unlocking generates more heat and further exacerbates the transmission cooling shortcomings .

 

[r2m]

In 4th gear you are probably locked and not generating much heat. I think you are also re-geared? Since upgrading to 4.56 gears my TCC unlocks a lot less. I only have 5 gears but it usually pulls hills in 5th gear or 4th gear locked (with 3.73s it was a constant 5th-4th-3rd-4th-5th). Sometimes it will unlock for <30 seconds in 5th on a hill that's not quite steep enough for 4th gear, but I just let it do that. If I'm using 3rd to get up a hill on the highway I'm either going <45 mph on a twisty road or I'm towing something. FWIW, our hills are quite steep around here, although not that long.

All of the GXs are under-geared when 33" or larger tires are added, which causes the TCC to unlock more frequently to bump the engine up a few hundred RPM and make up for the effective gear ratio being too high. The unlocking generates more heat and further exacerbates the transmission cooling shortcomings .

Yeah, when I did the front and rear lockers, I regeared with 4.56. With 34" - 35" tires. I don't get much gear hunting on most of the passes we have down here.
Most the passes are going north to get out of L.A. basin, like the Grapevine, or Cajon Pass. Although I'll drop down to 5th gear on the down hills to keep my speed in check.

 

[WTITW]

Do you have an auxiliary transmission cooler?

Hayden. Pulling 6k# trailer most of the time when Im monitoring.

Is there a key to your plot? All blue… this looks like a plot of ambient air vs idle at temp. You’re top line is 170. Thats the warm up temp .

 

[Rednexus]

Do you have an auxiliary transmission cooler?

Here's my AT pan temp ranges.
Hayden 678, pinned transmission thermostat

Are you an engineer or scientist?

 

[jeremy j]

Thanks for the suggestion, all. Im going to try to pin the thermostat and see what temperatures I get. Also thanks @thescreensavers for that part number.

I have mine pinned with a Hayden cooler and see similar temps to you in the summer so I’ve definitely considered bypassing the oem cooler. I’m interested to see how it works if you do it.

 

[BlkGX2011]

I have mine pinned with a Hayden cooler and see similar temps to you in the summer so I’ve definitely considered bypassing the oem cooler. I’m interested to see how it works if you do it.

Yeah, i am seeing 10-30°F cooler temperatures after pinning my thermostat.
Im curious what the temps will be once summer comes around here in AZ.

 

[BC11SVT]

Hayden. Pulling 6k# trailer most of the time when Im monitoring.

Is there a key to your plot? All blue… this looks like a plot of ambient air vs idle at temp. You’re top line is 170. Thats the warm up temp .

The chart is just A/T temp 1 plotted against ambient temps.
All recorded after the transmission temp has fully stabilized, usually after 20+ miles of driving.

I only have one towing temp recorded in there so far:
Towing ~3000 lbs running 70-75 mph, ~35 degree ambient, 158 degree pan temp.

Basically it works out to around:
130s in the 20s
140s in the 30s
140s in the 40s
150s in the 50s
150s in the 60s
160s in the 70s
160s in the 80s
170s in the 90s

2018 GX460
Hayden 678, OEM radiator, pinned thermostat
Aisin ATF-0WS ATF
Falken Rubitrek 265/70R17s

 

[BC11SVT]

Are you an engineer or scientist?

Armchair only, lol

 

[Rednexus]

Armchair only, lol

That's level of Excel detail/formatting I usually only see by engineers, and occasionally scientists! And that's coming from an engineer who has spent thousands of hours buried in Excel spreadsheets .