This paper presents a discussion of the modulating fan clutch which was chosen as a fan speed regulating device to improve the overall efficiency of the internal combustion engine. Generally the engine cooling system is oversized for normal operating conditions. With a fan speed regulating device providing the exact amount of airflow to maintain proper engine cooling temperature, the horsepower not required to turn the fan can be utilized for increased speed or fuel economy. This device also serves to reduce noise as the fan is one of the major contributors to this problem. Design goals and types of mechanisms considered are presented along with results of component and system testing of the modulating fan clutch.