From the size of the "cooler" already in place, I would lay odds that it is more for bringing the tranny up to operating temp quicker and minor cooling at summer temps.

An external cooler is definitely a good idea, and a must for towing in any kind of heat, especially for a transmission with such a small fluid capacity. An extra electric fan would probably be a good investment also.

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Sam

84 300SD 350K+ miles ( Blue Belle )