One of the main reasons I worked with Devon was to get away from any sort of zip tie mounting scheme. I wanted a cooler to be fully supported in a mounting bracket that isolates the cooler itself from mounting stresses, flexing of the radiator support structure and the vibration and shock of off roading. I also wanted a clean install and straightforward hose routing.

One thing I'll throw in is that I am running full (RCI) skid plates so air flow across the transmission housing is restricted by some amount as compared to the more minimal OEM skid plate.

For the following I will defer to anyone who has deep knowledge of the 4Runner transmission: A number of online sources have made statements that most modern transmissions have an internal temperature bypass valve that prevents fluid from flowing to an external cooler until the temp has reached some minimum such as 165 deg F. (Unknown rating, I need a citation) If true for our Toyota transmissions we have no worries about over cooling the ATF by adding an inline cooler to the lines coming out of the OEM cooler located within the engine coolant radiator.

Next, the Hayden 698 seems to perform a bypass function by relying on the viscosity of the ATF versus temperature. Hayden understandably does not share how they designed their cooler but states this "When Automatic Transmission Fluid (ATF) is cold it is viscous. The unique Tru-Cool design allows the colder, thicker ATF to flow more efficiently through two open bypass channels positioned at the top of the cooler. As operating temperatures increase, the ATF becomes hotter and thinner, It's then directed through the core where it is cooled."

Sample curve I found of how ATF viscosity varies with temperature


Colder, denser fluid probably doesn't easily flow through the smaller internal orifices of the 698 (my guess) that lead to the finned areas of the cooler. Given that the ATF has already been pre-warmed to 180-ish degrees by the engine coolant radiator the Hayden 698 is essentially "active" all of the time and incoming ambient air temperatures will affect how much additional cooling the Hayden can impart.

I recently took a five day trip to the Mojave Desert with my 4Runner and offroad trailer. I use a ScanGuage2 for monitoring the trans, engine coolant and torque converter temperatures. In the numbers presented below I show the converter temps as it can run hotter than the transmission. I imagine that whether or not the converter is locked up makes a difference. Off road I use the sport mode and on the freeway I use drive except for steep hills.

Ambient air temp - converter temp - coolant temp - notes

62 - 98 - 184 just idling in the driveway
73 - 150 - 188 starting the drive on city streets
64 - 199 - 191 freeway driving, level in general, at normal traffic speeds
61 - 193 - 195 towing uphill in Cajon Pass, at normal traffic speeds
70 - 96 - 195 idling at camp site before return trip
68 - 118 - 186 in 4WL 3rd gear Sport Mode (SM) in a sandy wash @ 15 MPH
68 - 130 - 186 in 4WH Drive on dirt roads @ 25 MPH
70 - 149 - 190 up a hill, at normal traffic speeds
70 - 147 - 190 freeway driving, level in general, at normal traffic speeds

In the past I had found the converter temps would rise well above 210 degrees when towing up a hill and I would usually switch to SM and reduce my speed. Now I no longer have to do so.

I would recommend this product to anyone who tows or operates in hotter climates and who is capable and confident of performing minor mechanical work to deal with fitment hose routing, etc.