I've got a 98 F150, need to install a cooler for the tranny, to save me a mess, which stock cooler line at the radiator is the return to the transmission?....the radiator has side tanks with lines on the top and the bottom....thanks to all....Mick

 

To find out, with the truck cold, start it and put in in drive for about 30 seconds. Then shut it off. Feel the lines, the warm one comes from the tranny, the cool one is the return.

 

Bottom line should be the return to transmission.
Warm water is returned to the top of the rad from the engine and cool water sucked into the engine from the bottom of rad.
If that is so then it makes sense that the coolest part of the water column in the rad is on the bottom, and as such, any oil cooler would return to the transmission from this point also.

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