I did the procedure of checking which factory cooler line was the coolest after 2 minutes idling and also tried to eye which line is longer. I figured it was the bottom line as the return line. My cooler instructions say to hook the auxillary cooler hoses up to the return line of the factory cooler. I was told by someone at Ford, my local speed shop, and a transmission shop the top line is the return line on 98 Rangers. I have my auxillary cooler hoses tied in with the bottom line. The Ford manual says the bottom line is the inlet line. Also I was told by the guy at my local speed shop and the guy at the transmission shop that it doesn't matter which line you splice into. Is this right, do I need to swap them back around?

 

My Ford manual show the bottom as the inlet.

 

I am also putting an aux. transmission cooler in my '97 4.0L. My Chilton's manual does not indicate which transmission line is the inlet and which is the outlet at the radiator. Anyone have specifics from a Ford manual?

 

The diagrams and torque settings on pages 6-7 of the following PDF file can probably be of help to you:

FORD 4R/5R55E Transmission Fluid, Filter & Gasket Change

It appears from the diagram on page 6 that the upper line coming off the tranny is the "Fluid Cooler Inlet Tube". That would have to mean that the "outlet tube" (from the tranny to the radiator) is the lower line. So if you follow the lower line from the tranny to the radiator you should therefore come to the radiator inlet. Make sense?
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Thanks, Rockledge. That's somewhat helpful, though my recent transmission fluid flush only adds to the confusion.

I removed the upper line at the radiator and attached my drain tube to it. With the engine running, ATF was generously pumped out of the upper radiator line and into my catch bucket. As a result, I didn't expect to see anything come out of the upper female fitting on the radiator. Here's the kicker. Much to my surprise, ATF also shot out of the upper female fitting with significant pressure.

So...how is it that I can have significant flow out of the upper tube AND out of the upper radiator fitting at the same time?? Sure makes it difficult to figure out which way the fluid is flowing.