A transmission oil cooler works first through the transmission, engine or power steering pump heating the fluid that flows to the cooler lines. Air moving across the fins of the cooler cools the fluid, then routes it back to the transmission, engine or power steering pump in a constant cycle through the return line.
There are two main types of transmission cooler lines: a tube and fin-style cooler and a plate and fin-style cooler. The tube and fin-style cooler works through a tube that transfers liquid to the cooler. Aluminum fins are attached to the outside of the tube to dissipate heat, allowing the heat from the fluid to be absorbed by the aluminum, move out to the fins and to then be carried away from the cooler by the air flowing through the fins on the outside of the cooler.
The plate and fin-style cooler are more efficient versions of the tube and fin coolers. These particular coolers move the liquid through much smaller plates that cause movement of the fluid. The fluid here is cooled better before leaving the cooler because the smaller, flatter plates force more contact with the aluminum surface inside the cooler, creating a more efficient cooling device.