I would want to use a thermostaic switch in the trans cooler lines or in the tranny somewhere so the fan would only come on at or above 170 F Most tranny don't run hot to start with, that is the reason the factory routes the tranny fluids into the radiator, it helps warm the fluids up ATF needs to be at or above 160 F for the additives to work My old 1998 Dodge Ram 2500 CTD tranny fluids ran at or below 120 F when not towing a big enclosed trailer, that was in the Mojave desert in the summer, it ran colder in the fall, winter and spring It did have the trailer towing package with a small external tranny cooler mounted in front of the radiator, no fan My Dusters tranny fluid will warm up to around 180 F in the summer when cruising round, it has a T/A 8 inch race converter that flashes to 5600 RPM, it has a small three row external tranny cooler mounted in front of the radiator also