1998 XJ 4.0, AW4. At the radiator, how are the transmission cooler supply lines routed? This diagram from Dorman shows the supply from the trans entering the bottom, and the return exiting from the top, on its way back to the tranny. Is this correct?

 

That's the way my '99 is. I'm currently in the process of replacing the supply line due to it being crushed almost closed along the metal tube section. Then I'm going to splice into the return line to add an aux cooler.

 

That's how it works on my '98 as well. In from the bottom and out the top. I verified this by starting the engine and watching the fluid shoot out the disconnected lines when I did my aux cooler install.

 

I have heard it said that the flow should be in from the bottom, out from the top. I don't know how valid it is, but if Jeep did it that way, they have engineers way smarter than me. I'll probably plumb my aux cooler the same way.

 

It's the farthest point from the rad inlet which has the hottest coolant. You could also look at it as the closest to the rad outlet that would have to coolest coolant.

 

I found another thread which seems to confirm that the supply line does indeed go in the bottom fitting on the radiator. Some people have stated that the FSM is wrong, and uses an image from 1996 and back. I'm glad the Dorman picture is correct, because that's the orientation that I used to install an auxiliary cooler. I'm getting ready to do a transmission flush on a 1997, and this will help.

 

Yep. My '96 is in at the top and out at the bottom.

 

Yeah, that seems normal to me. But, Jeep.... Mopar.... Engineering....

 

The proper way is to fill from the bottom so that the buoyancy of air bubbles helps bleed them from any enclosed spaces. With the plain ol' piece of tube used in most every built-in transmission "cooler", it's not an issue; the fluid flow will easily push any air down and out the bottom port.

If you consider a plate-style aftermarket cooler however, if you mount one with the ports facing down, or to the side and fill it from the top, it will almost certainly form a large air pocket in the cooler since the fluid can easily bypass the pocket through the lower rows of passages.