Troubleshoot most transmission cooling system problems by looking for leaks along the cooler lines and, if the vehicle does not have a separate cooler, transmission fluid in the cooling system or coolant in the transmission fluid. If the vehicle has a separate transmission cooler, look for drips from it. When the leak allows the fluid level to drop, the transmission is more likely to overheat. Other common transmission overheating is due to towing a trailer beyond the vehicle's capacity.
The work the transmission does produces heat. Engineers design vehicles so the air passing by the metal transmission housing helps to maintain the fluid at a temperature range between 160 and 200 degrees Fahrenheit. They increase cooling efficiency by directing a tube from the transmission through the radiator. Some vehicles have a separate small radiator, which is a transmission cooler, for the fluid, either as standard equipment from the manufacturer or as an aftermarket addition by the owner.
Exceeding the temperature range for transmission fluid causes it to burn. Burning the fluid lowers the amount in the reservoir and increases overheating. Towing a trailer or otherwise overloading the vehicle is a common reason for the initial overheating. While a separate transmission cooler can lower the temperature significantly, the solution to an overloading problem is finding a vehicle with a larger towing capacity.