I'm trying to install a tranny cooler and I am to tie into the return line of the existing radiator tranny coolant lines. Which one is the return line, top or bottom line? thanks! Bob

the bottom line is pressure, the top is return...if they are hooked up correctly. bottom line on trans=bottom line on rad=pressure.

i have been told that unless you tow, or live in an extremely warm climate, the cooler should be in the pressure line, so the fluid is brought back up to operating temp as it goes thru the radiator on it's way back to the transmission.

Block off the tranny intake lines to the radiator. The two lines for the trans cooler run directly from the trans to the cooler without tapping into any other line. Essentially, if you have a cooler, there is only two lines (3/8") that run to trans to cooler.

Line out from trans to intake of cooler.
Line out from cooler to trans.

No more, no less.

If in doubt, there should be an instruction sheet with the cooler.