Fan Clutch
A temperature-controlled device that engages or disengages a mechanical cooling fan to save power and control temperature.
A fan clutch is a thermostatic device found on vehicles with belt-driven mechanical cooling fans, common in trucks and SUVs. It is mounted between the water pump pulley and the cooling fan, regulating fan speed based on engine temperature.
The clutch contains a bi-metal coil sensor and a chamber filled with silicone fluid. When the engine is cold, the fluid remains in a reservoir, allowing the clutch to slip. This lets the fan spin slowly, reducing engine drag and allowing the engine to reach operating temperature faster.
When the radiator gets hot, the bi-metal coil expands, opening an internal valve that lets silicone fluid flow into the clutch working chamber. This locks the clutch, forcing the fan to spin at full engine speed to draw maximum air through the radiator.
A fan clutch can fail in two ways: it can seize (causing the fan to run constantly at high speed, generating excessive noise and reducing fuel economy) or it can fail to engage (causing the engine to overheat at idle). Replacing a bad clutch restores proper cooling and efficiency.