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Discussion Starter #1 (Edited)
My 2018 XLE AWD has a transmission cooler, that I'm pretty sure of. At first glance it appeared to be a traditional "liquid to liquid heat exchanger in the radiator tank" arrangement, but as I dug deeper, things got more complicated, or at least confusing for me.

1. There is a junction of 4 lines/hoses between the transmission and the radiator. I figured this was an external thermostat, which is what it appears to be. So far, so good.
2. [according to the part drawings] There is another part with 4 inlet/outlet ports closer to the transmission. It looks like a oil cooler of some sort. It is labeled as a "transmission cooler" in the drawings.

This is where I'm confused If there is a transmission cooler in the radiator tank, why a second one near the transmission? Is it just a 2 stage cooling system (and why?), or is this some sort of "one or the other, but not both" arrangement (FWIW the web page says that both items "fit my car") And
if both are present (I have only seen the thermostat and radiator trans cooler on my car at this point), what type of fluid flows from the trans cooler (labeled 1 in the pictures) to the thermostat (labeled 2)? Is it transmission fluid? Coolant? Or am I just missing something altogether?

thermostat
trans cooler


I'm interested in all of this because I am considering adding an auxiliary cooler to my car for towing purposes. I have monitored the temperatures and did get fairly high at times...and as the summer temperatures (and mountain grades) increase, I think I might want more cooling...so I need to understand what is going on with my car currently.

Any help is appreciated, as always.


-Steve
 

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It's probably easier to understand on http://www.toyodiy.com since one can hover the mouse pointer over the parts shown in the diagrams to see what they are. When I do that I can see that this second little transmission cooler has inlets and outlets for both transmission oil and coolant.

I wonder if the second transmission cooler is really that or if it is instead used by the engine control ECU to alter shifting based on coolant temperature. Whatever the purpose is, it must be important enough to justify all this additional hardware.
 

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Discussion Starter #3
Thanks for the link, Geezer...looks like it could be a great resource, unfortunately I'm haven't seen any diagrams yet...just parts lists. (see attached) Any sugggestions?

I agree that it seems important...I couldn't believe all the extra parts that were involved. I sure wish I had the cell phone number of the engineer who made that decision. I guess it will just be a nagging mystery for me.

-Steve
 

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.....used by the engine control ECU to alter shifting based on coolant temperature. .......
Geezer1 is correct that it is a controller interlinked with the ECU. A lot of modern vehicles with multi gear transmissions have this device the so called "transmission cooler". From my understanding the Toyota unit will cut back engine power if the transmission is running hot so as not to overheat it. When we set up our 2017 van to tow the RV place that has set up all of our vehicles including 2 previous gen Siennas' stated that the '17+ models (8 speed trans) do not need an external trans cooler because of this. The Lexus RX350 has the same transmission and set up as well. We have towed with the van for 2 years now climbing some steep grades without ever running a high trans temperature.
 

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Discussion Starter #5 (Edited)
Well, that's interesting. I was vacillating on the need for a transmission cooler, and I ordered one that will be here for the weekend...but if I could be convinced that it wasn't necessary, I wouldn't put the trans cooler on. I don't really like the idea of adding more things to break...but when towing up 6% grade in ~75 deg weather, I hit 220 degrees...so a taller hill on a warmer day might be an issue.

How high have you seen your temps climb?
 

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In the diagram #2. It has 3 lines going to it. I wonder if Toyota has a wax pellet type system in that unit. They had it years ago. More so on the throttle body. Yes it looks like a thermostat.


The other pic is just a exchange unit. Coolant/oil from what I understand.


I guess the ECU can shut things down due to temp. Guess their is no code if this system fails.
 

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Thanks for the link, Geezer...looks like it could be a great resource, unfortunately I'm haven't seen any diagrams yet...just parts lists. (see attached) Any sugggestions?
Only registered users can view the diagrams on http://www.toyodiy.com/

I assume they are accepting new users.
 

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Hi guys I installed today On my 2017 xle sienna trans cooler from hayden 679 rapid-cool plate unfortunately i never checked the tft before the instal but after the instal i went for a drive and my temps were 188 running down the highway which i think are a little high with the install I tapped in from underneath the radiator to the oem cooler and mounted it to the condenser in the front any ideas what is going on people in the reviews section report temps 150 and under thanks for any help
 

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I recently invested in a more complete service manual for my kid's Toyota RAV4, and there was a section on the thermostatically controlled little heat exchanger. I just quickly scanned the text, but it's designed in part for faster warm-up of the tranny on a cold start by using 'warmed engine coolant'.

Remember that the radiator is stone cold and kept that way for the first few minutes by the closed engine thermostat. So the main tranny heat exchanger in the engine coolant radiator is actually 'cooling' the tranny at first, not warming it up. The new little unit uses the more rapidly warming close circulating engine coolant to bring up the transmission temp.

I'll go back and see if it has more details, but I suspect that the optimum operating temperature for the transmission might be higher than we think it should be. If true, then adding another external tranny cooler without also defeating the new little unit won't buy you much. They could end up fighting each other depending on how 'smart' the unit is.
 
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