OEM AT Cooler Bypass?

[BlkGX2011]

Im thinking about bypassing the oem transmission cooler in my 2011 Gx460 but wanted to see if anyone has done it before and have gotten better results in cooling the transmission fluid.

Background: I wanted to keep the transmission temperatures around 175°F. I live in AZ and know that it will get hotter throughout the year. I already had installed a hayden 677 (6"×11"×3/4"D) and the temps were ~190°F at steady state. I then replaced it with a different cooler that is 10"×10"×2"D and the steady-state temperature also remained at ~190°F.

So I was thinking that if I bypass the OEM cooler, which takes 180-200°F engine coolant to "cool" hot AT fluid, and let my aftermarket cooler handle the cooling entirely.

I know our A760F has a built in thermostat and dedicated coolant heater lines that warm up the fluid initially so I dont think there would be an issue of making the fluid too "cold".

What do yall think?
Hayden cooler:

Type 111 cooler by Shaman Equipment:

 

[Acrad]

Have you tried pinning the AT thermostat open first?

 

[iwashmycar]

Yeah the OEM trans thermostat thing doesnt even open until it reads 190 from what I gather. Gotta pin it if you have not.

 

[thescreensavers]

+1 just pin the thermostat

 

[Acrad]

Mine has been pinned since 2019 or so.. no issues even in colder weather.

 

[Timujin]

Pinned with external ATF cooler?

 

[BlkGX2011]

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.

 

[r2m]

There's also a lot of discussion on trans coolers if you do a search. Everything from performance to different brands, and how to do it.

 

[WTITW]

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.

 

[Acrad]

Complete bypass is likely what you would need to do to achieve consistent 175... I only recall a few over the years doing this, I don't recall temp reports offhand.

 

[Rednexus]

I'd upgrade the radiator instead. The OEM radiators in these have pretty crappy trans cooling in them. Coolers help, but they are just a band-aid for the radiator. I have a GX470 (not a 460), but with a CSF aluminum radiator and Hayden 698 I'm typically running static temps plus 80 degrees, and very rarely if ever get above 200F. In the summer that equates to about 170F on a 90 degree day under cruising conditions. With the better radiator you get improved trans cooling AND engine cooling.

 

[Hiluxforever]

Upgrade to a heavy duty radiator either way. Then maybe consider running additional cooling if you are not happy with it.

I have a CSF and so far am very happy with it.

granted I get up over 12k feet. Why I need additional capacity, with your high summer temps it would help give you back some cooling headroom lost by being in a hot climate

 

[thescreensavers]

CSF would be my go too when I need to replace my OEM radiator.

 

[Hiluxforever]

They were on 6 week backorder when I finally pulled the trigger.

 

[r2m]

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.

Correct me if I'm wrong, but when the transmission's thermostat is pinned, it just keeps the temperature lower during normal driving, but when climbing long grades, heavy loads/trailers, I don't believe it helps any more than if it were not pinned. The reasoning because the thermostat would already be opened via the thermal spring inside when it became hot.
No??

 

[iwashmycar]

Correct me if I'm wrong, but when the transmission's thermostat is pinned, it just keeps the temperature lower during normal driving, but when climbing long grades, heavy loads/trailers, I don't believe it helps any more than if it were not pinned. The reasoning because the thermostat would already be opened via the thermal spring inside when it became hot.
No??

That is my assumption as well, yep. Its more of a fluid warmer, and nothing is going to physically help cool more unless you have additional cooling hardware installed.

 

[thescreensavers]

Thermostat doesn't open till a certain temp, so it sets the floor or min temp before sending fluid to the cooler. It does not impact the max temp.

So by pinning the thermostat it just sends fluid all the time to the cooler. Which now can cool below the Min temp set by the thermostat.

 

[llm]

Correct me if I'm wrong, but when the transmission's thermostat is pinned, it just keeps the temperature lower during normal driving, but when climbing long grades, heavy loads/trailers, I don't believe it helps any more than if it were not pinned. The reasoning because the thermostat would already be opened via the thermal spring inside when it became hot.
No??

if you do long, sustained climbs, pin/no pin wont make a difference. eventually you will reach equilibrium, which hopefully is within safe limits.

If you do sudden surges of high demand, it could help because you are starting at a lower temperature to start. you dont reach the equilibrium point and temp is still increasing. the final temp could be lower than what it would have been had the tstat pinned. For example, there is a residential street near me where my TC would go above 245F with t stat pinned- so it's starting at around 195-200. With it pinned, I'm starting at 180-185 and TC temps "only" go to 230F. If this was a longer climb, of course it would also hit 245 at some point, then continue increasing.

 

[Rednexus]

if you do long, sustained climbs, pin/no pin wont make a difference. eventually you will reach equilibrium, which hopefully is within safe limits.

If you do sudden surges of high demand, it could help because you are starting at a lower temperature to start. you dont reach the equilibrium point and temp is still increasing. the final temp could be lower than what it would have been had the tstat pinned. For example, there is a residential street near me where my TC would go above 245F with t stat pinned- so it's starting at around 195-200. With it pinned, I'm starting at 180-185 and TC temps "only" go to 230F. If this was a longer climb, of course it would also hit 245 at some point, then continue increasing.

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).

 

[Acrad]

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.