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Check Operation of the Viscous Coupling Fan

Model: BMW Serie 3 E46 - Years: 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006
Part: Radiator

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BMW technicians designed the cooling fan to be driven by a viscous coupling rather than an electric motor; essentially, the fan is always driven by the auxiliary belt, and its speed is determined by the viscous coupling, which operates based on temperature.

Specifically, the viscous coupling (also known as a Ferguson coupling after its inventor) is a mechanical component that connects two shafts in a non-rigid manner, allowing for small differences in rotational speed: two sets of discs mounted on the ends of the two shafts are placed close together in a housing that contains a special silicone fluid that significantly increases its viscosity with temperature.

When the speeds of the two shafts differ, the fluid heats up and, increasing friction with the discs, limits the relative slippage of the two until it prevents it (with response times on the order of one second). When the tendency for different speeds decreases, the fluid cools down and allows for small differences in rotational speed again.


Updated on 16 February 2026 by ScegliAuto


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