Yup, the in-tank cooler is not worth much for heat exchange....
I just changed an OE Honda radiator in my 1998 Accord. Below is the new Koyo radiator, with the hose barbs for the ATF lines coming from and returning to the A/T.
Now, the bottom tank removed from the old radiator. The side facing us is the side that faces the motor. You can see the connection that returns coolant back to the engine on the right side.
A different shot to show the orientation of the ATF in-tank cooler.
Now, below, is the cooler removed from the tank, next to a 7/8"-15/16" combo wrench.
The cooler upended to show it is nothing more than a pair of concentric tubes sealed at both ends to form a jacket that the ATF can run through, with the center of the smaller tube left open on both ends. The coolant runs around the bigger concentric tube, and through the center of the smaller concentric tube. I cannot comment on its efficiency as a liquid-to-liquid heat exchanger, but it does have a good amount of surface area along with larger cross-sectional area to slow the speed of the ATF (but not the overal volume/second) so that the ATF moving through it can hang around a little longer in the cooler.
One thing I can say, efficiency aside...from gathering data via my 4-channel thermocouple thermometer, these radiator in-tank ATF coolers leave a lot to be desired. As noted by
gregersonke, if your goal is to pull a lot of heat from your ATF, you need something a little more effective added to the mix to move more BTU's.
Now, below is a shot of the coolers through the years...the OEM items that Honda tells you to add if you install a hitch. 2007 got a pretty nice ATF cooler, while 1999-2006 (the years that could've used a better ATF cooler) ended up with a finned drinking straw. At least Honda finned both legs of the U-tube for 2005-2006.
OF