I'm installing a trans cooler on my 03 f 150 and would like to avoid cutting the wrong line. It has always been my belief that the upper line on the radiator is the input and lower is the output. Like the engine coolant the hot goes in the top and the cooled fluids come out the bottom(heat rises so the fluid should be cooler at the bottom of the rad.) Can somebody confirm this I don't want to do the cut the line and start the motor method to identify the correct line. thank paul

 

When the truck is cold start it up and put in gear for like 30 seconds. The line out of the trans will be hot.

 

That was a great idea. I got the cooler installed today everythings completed a back together. Thanks for the reply.