Abstract: A viscous coupling, or clutch system, is either coupled to a water pump or combined with a water pump and is used for controlling the coolant flow rate in a cooling system. At engine idle or low speeds, wherein the water pump is driven at very close speeds to the input speed, the viscous coupling would have little effect on the speed of the pump. However, due to the presence of the viscous coupling, a larger water pump may be used, resulting in good coolant flow at engine idle or lower speeds. As engine speeds are increased, the viscous coupling slips, resulting in lower input speeds for the water pump, thereby reducing the risk of pump cavitation. By increasing the water pump speed at lower engine speeds and decreasing the water pump speed at higher engine speeds, the engine likely will operate at ideal temperatures, and thus fuel economy may be improved and emissions minimized.