Dumb question... I'm installing an second transmission cooler on my 2009 2.3 ranger with a 5r44e. I originally was going to just bypass and plug the factory cooler built into the radiator but was told with the cold temps here it's a good idea to leave it.

so my question is simply, which cooler line do I cut to tie in the rubber lines for the aftermarket cooler? Top or bottom line on the radiator? I have no idea which one is the inlet and which is the return.

 

It doesn't matter, the 2nd cooler is there to provide XX degrees cooling, so when it cools is not important, that it cools is.

I.e.
200degF fluid-----------2nd trans cooler(-10deg)----------factory cooler(-10deg)-----180deg fluid

200degF fluid-----------factory cooler(-10deg)------------2nd trans cooler(-10deg)-----180deg fluid

You could install a temp gauge for fluid, and if trans is running too cool in cold weather you could wrap the 2nd cooler until spring, but automatic transmissions warm up faster than radiators/engine do.

Rad trans cooler runs at 170degF after engine is fully warmed up