This viscous fan clutch controls the rotational speed of the engine cooling fan based on temperature. It is mounted between the fan and the engine-driven pulley. The clutch contains a silicone fluid that changes viscosity with heat; when the air temperature behind the radiator rises, the fluid thickens and engages the fan at full speed. When the air is cooler, the fluid remains thin and allows the fan to slip, reducing parasitic drag on the engine. A failed clutch that no longer locks up will cause the fan to spin freely, leading to insufficient airflow through the radiator and potential engine overheating. Conversely, a clutch that remains locked continuously increases fuel consumption and noise. The original assembly matches the thermal response curve designed for the host engine, ensuring the fan engages at the correct coolant temperature threshold.