The main reason transmissions overheat is because of overworked transmission fluid. The fluid that has outlived its usefulness or has to work in high heat is less efficient at providing the many functions it was intended for, such as lubrication, fluid pressure, and cooling. Heat and age are mainly what causes transmission fluid to degrade, and this only leads to more overheating. Any three methods (Test Port, Cooler line, Oil Pan) will provide you a temperature reading. Many people will argue what method “is best” however, in our research all we have found all three locations are all within degrees of each other.