This viscous coupling transmits engine torque to the cooling fan through a controlled fluid shear mechanism. It is mounted on the fan drive hub between the engine crankshaft pulley and the fan blade assembly. The silicone fluid inside the coupling changes viscosity with temperature, allowing the fan speed to vary in response to cooling demand. A worn coupling that fails to engage fully will cause the fan to spin at a reduced speed, leading to elevated coolant temperatures under load. The original manufacturer's unit is machined to the same hub pilot diameter and bolt-hole pattern as the assembly it replaces, so no adapter or shimming is required during installation.