Regulated by the opening and closing of a thermostatic spring, a flow of a viscous silicone fluid engages the fan clutch to operate at different speeds depending on the temperature. This helps power consumption and saves fuel. Whilst a hot engine means a fast clutch, fan a cool engine's fan runs much slower.
If a fan clutch slips, sticks or loosens too much it may not be pulling sufficient air over the radiator to do the job. In this case it's time for a replacement. For best results always use quality

fan clutches

from Eurocarparts.com.