I want to install a trans oil cooler and the directions say hook up the aux trans cooler in series to the return line from the radiator. I cannot resolve which oil line on the radiator is the return line and which is the feed.The directions said start the car and wait two minutes and feel which line was cooler and that should be the return line, of course they both felt the same. Did a google search and some guys say the top (of the radiator) is the return and others say the bottom is the return. Rock Auto calls both lines inlet hoses. The top connector is 5/8 threaded, the bottom is a 3/8 quick disconnect. Much easier to install the new aux hoses on the top (the bottom radiator hose is in the way of the bottom connector). The guys that swear the bottom inlet is the return line also say that if you hook it up using the top inlet your doing more harm than good. Anybody have a factory service manual?

 

The top is more than likely the return. The fluid should be pushed up through the cooler.

 

The bottom is the return line and the top is the feed line so gravity will push the fluid down through the cooler, I have installed a trans cooler

 

That's incorrect.

 

No because if u take off the top line from the radiator and start the jeep trans fluid will spray everywhere, I have just done a trans cooler

 

I'm saying letting it flow by gravity is incorrect. Anyway, Jeep changed it some year.. Idk when. To be sure, the line coming out of the front of the tranny is the feed line, the one coming out of the rear is the return.

 

I know gravity doesn't move it what I was getting at is that's the way fluid would go if so,

Think of it this way the top of the radiator is hotter then the lower same ans the trans cooler

 

Uh. What? A radiator doesn't work the same way a trans cooler does.

 

Uh, you guys aren't helping much.

 

What year is your jeep?

 

Instructions should have come with your cooler.

 

Its a 2001 XJ and I just checked the front line from the trans is going to the bottom of the radiator. So are we sure that the front line is the feed line?

 

Yes.

 

This!

 

I have a '98 and I just installed mine. Mine was the bottom line as well. It went to the front of the transmission as stated earlier.