The fan clutch is a device to keep engines at a normal temperature. A car is cooled by a radiator. The radiator is a grid of metal tubes at the front of the car. A pump sends fluid past the engine, through the radiator and back again. The fluid draws heat away from the engine. When the fluid reaches the radiator, air flowing into the front of the engine compartment from outside cools it off. A fan behind the radiator turns on when the engine gets too hot, drawing extra air in past the radiator to provide more "cooling power." The fan clutch automatically controls the speed of the fan to keep the engine from getting too hot.
The fan clutch is driven by a belt or pulley directly from the engine's crankshaft. Inside the fan clutch are two metal plates and a special fluid. One plate is driven by the crankshaft, while the other drives the fan. The plates do not directly touch each other.
The fluid inside the fan clutch thickens as it gets hotter. The plate attached to the engine is constantly spinning, causing the fluid to spin and turn the plate attached to the fan. When the fan clutch is cool, the thin fluid only pulls weakly on the plate attached to the fan, causing it to spin slowly and not use very much power. As the clutch heats up with the rest of the engine compartment, the fluid thickens. The thicker fluid acts like glue, pulling on the fan plate and making the fan spin almost as fast as the crankshaft. When the engine cools down again, the fluid thins and lets the fan slow down.
