The transmission have 2 cooling lines, up and down.

Which one is IN and which one is OUT?

 

 

You don't state which automatic transmission is in question; 700R4, C4, Aisin, Torqueflite....

Assuming a 700R4, the bottom is the out flow, the top is the return.

 

Yes, it a 700R4, thank you.

 

Bottom outflow into the bottom of the passenger side radiator tank. Top of the radiator coming out of the tank is the return to the trans.

 

This is what I want to do:

I don't want to use the radiator, I may even replace it with a radiator with out trans cooling.

 

It's best to leave the radiator in the loop. Trans to the radiator to the external cooler then if you want to the filter back to the trans. In cool weather it helps warm fluid to operating temp. Hot weather it helps by pre cool fluid b4 the external cooler.

 

IMO -

FWIW, I recall reading in a trade journal (Transmission Digest ??) or some generic hot rod magazine that you should keep the radiator cooler. Theory is that there's a risk of overheating the transmission when running just the auxiliary cooler and having no airflow across it (as in stop-and-go traffic).