Do you have any links? I'm not understanding how one cooler (radiator type) differes from an "exchanger". Is it more or less a heatsink desgin that will also hold more oil?
I think the terminology is getting us mixed up Tony. A radiator is just a specific kind of heat exchanger. In the context of automotive applications, to cool the engine, crankcase oil, ATF, power steering fluid, or whatever you need a fluid medium to transfer the heat away from whatever it is you're trying to cool, and something with a lot of surface area away from the item to be cooled, so that the heat can be exchanged or bled-off into the surrounding air. For automotive applications, that heat exchanger is always something with fins soldered to the fluid tubes, to give it more surface area to shed heat into the atmosphere. For any of these to work, there must be a temperature difference between the item to be cooled and the surrounding air. The greater that difference in temperature is, the more efficient the heat exchanger will be and the more cooling capacity it will have. Fortunately, all the things we want to cool in a truck are usually much warmer than the surrounding air, so the various heat exchangers have enough temperature difference to work.
A radiator uses water (and anti-freeze) as a fluid medium to transfer heat away from the engine, to the radiator, and there to be exchanged with the air. The stock thermostat in the FJC opens at 180 degrees F, so the radiator is open to flow above that engine temperature, and will exchange engine heat with the atmosphere, through the air moving across the fins on the radiator. If the air temperature is relatively low, then the delta-T between the hot engine water and the air is great and the radiator works efficiently. If the air temperature is hot like in Death Valley or the southern Arizona summer, then you have a lot less delta-T to work with and the radiator is not nearly so efficient. That difference in temperature (delta-T) is extremely important in a radiator.
The other common kind of heat exchanger in an automotive application is just some tubes with cooling fins soldered on the tubes, inserted somewhere in the oil/fluid line. These use the fluid being cooled itself as the fluid medium for heat exchange, and exchange heat directly with the atmosphere instead of using water as an intermediate heat transfer step. Examples are power steering fluid coolers and auxiliary engine oil and ATF coolers.
So the reason why an auxiliary ATF cooler will do a better job than the stock cooler in the radiator is just the greater delta-T between the temperature of the hot ATF in the transmission and the air, versus the temperature of the ATF and the radiator. The radiator is already close to 180 degrees to begin with before you ever start trying to shed heat from the ATF into it, while the ambient air temperature is going to be quite a bit less than 180 degrees no matter where you are. The ATF temperature is supposed to be between 115 and 130 degrees F, so if the radiator runs above that temperature, it is not going to take any heat out of the ATF until the ATF gets hotter than the radiator. In fact, it will transfer heat into the ATF if the radiator is hotter than the ATF. That's why I said that the stock ATF "cooling" system in the radiator is more of a temperature regulator than a cooler.
Sorry for the lengthy explanation - does that make more sense now, or am I not understanding your question?
I stopped as soon as it happened, but was surprised to see the light given I was practically dragging the bottom side through the snow.
