That's the best way to achieve maximum cooling. The hottest oil hits the radiator cooler first, where it is still plenty hot enough to give up heat to the (already warm) water in the radiator. Then it goes to the auxiliary cooler, where its warm enough to give off heat to the ambient air, and return to the transmission as cool as possible, having given up heat in both coolers.

Reversing it means that the auxiliary cooler dumps all the heat it can, and then the oil goes to the radiator. Air flowing over the tranny cooler and air flowing over the radiator can lower both fluids to about the same temperature (depending on load, etc) so you're more or less putting fluid from the aux cooler into a radiator-mounted cooler that's immersed in water thats ABOUT THE SAME TEMP as the fluid from the aux cooler- so it isn't doing any added good, and the final temperature of the fluid going back to the trans is considerably hotter than if you run the fluid to the radiator-mounted cooler first.

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Obviously a lot of factors were ignored, including pressure. Some others were the conducitivity of the metal seperating the fluid and the water/air, any imperfections in the flow, etc.

I really eliminated the major variables to make it a simple algebra problem, though the conclusion would be about the same. For example I made the assumption that the metal would be the same in both instances, thus they would cause a negligible difference (effectively cancelling each other out). Most of my assumptions were to simplify the problem and make the math easier.

Short end, would the answer be identical when applying barometric pressure and fluid system pressure? No. Would they reflect a similar result? Definately. Though not a perfect representation, it does get the point across .

hello,

I have heard from several trans guys that in older non-eletronic trans you
cant run the fluid to cool, or the cooler the better. My real world experience
with an 85 d150 318 999a non eletric lock up converter. I had the trans
rebuilt, installed it with only an external cooler and a temp gauge on the
return from the cooler to the trans. On a 98 degree day, pulling about an
800-900# trailer, the tranny temp never went over 115-120 degrees. after
47k of hwy driving (heavy foot) the fluid still looks and smells brand new.

The cooler is about half the size of the rad, mounted to the drivers side, in front of the condenser. The engine temp increased about 5 degres in all conditions. One more reason to bypass the rad cooler is that if your engine overheats, and your trans is run through the rad, it overheats too.

just my $0.02 worth

I was working on my Cherokee mail Jeep with factory trans oil cooler and see that the pressure line out from the trans goes to the cooler in the radiator first, out to the aux cooler and then back to the trans. Looks like the factory engineers came to the same conclusion as some of our esteemed list members.