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Replacing the Radiator Fan Viscous Coupling

Model: Mercedes Classe M W163 - Years 1997-2005
Part: Radiator

The viscous coupling operates by driving the fan when the engine is very hot, cooling it with high-speed drawn air. The fan also spins at low speed when the engine is cold, but at a reduced rate compared to engine RPM, especially when the engine is accelerated. When the engine overheats, the blades of the coupling detect the heat, and the viscous fluid inside the coupling solidifies, causing the fan blades to connect with the coupling itself, resulting in the fan spinning at the same speed as the coupling (which is much higher) and sending significantly more air to the engine (provided everything is in order and functioning perfectly). The fluid loses its effectiveness after a certain number of years, so the coupling should be replaced. The replacement is very simple; just unscrew the fan from the pulley and replace the coupling by unscrewing the relevant fixing screws.

Updated on 19 February 2026 by ScegliAuto

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