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CV fan clutches are part of a vehicle’s engine cooling system and used to cool the engine by regulating the fan speed


At low or regular engine loads, the cooling airflow produced during driving is normally sufficient to dissipate heat from the radiator and work as an engine cooler. In this case, the fan runs at a slow rate or is switched off completely. If the load increases and the vehicle speed drops – e.g. when climbing a hill – there is a risk of heat accumulation due to engine overheating. Thus, the CV fan clutch is engaged to ensure that sufficient air flows through the radiator. The viscous clutch ensures that the fan speed is automatically adjusted to the level necessary to cool down the engine. This reduces both fan noise and fan power consumption, with the pleasant side effect of saving fuel.

Two basic viscous coupling systems are used for CV fan clutches:


Bimetal activated viscous clutch for CV

  • The fan clutch operates in accordance with the airflow temperature behind the radiator
  • The temperature on the outgoing air from the radiator is used to control the fan
  • This version is used on passenger cars and small to medium-sized commercial vehicles.

Electronically controlled cooling fan clutch for CV

  • The vehicle computer receives signals from the engine management sensors and the temperatures at the radiator, charge-air-intercoller/ exhaust cooler, oil cooler etc. and uses them to control the fan clutch
  • This version is used for heavy commercial vehicles and agricultural vehicles.

How does a viscous fan clutch work?


If the airflow (X) is still cold, the valve is closed and all the oil (red) is in the reservoir (a). The rotor (b) revolves in the housing (c) but is not driven. The fan is either stationary or turns slowly because of slight residual torque.

However, if the airflow (X) temperature rises, the bimetallic strip (e) or solenoid opens the thermal valve (d), causing the oil (red) to flow into the space between the fan blades and the housing. This results in a power transmission, by which the rotor accelerates the speed of the housing and thereby of the cooling fan. A full-scale, adequate supply of cooling air is thus provided for.

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