The fan clutch controls the fan between the radiator and the engine. It should be turning at engine speed when the coolant temp hits around 100C. It will be noticeable loud and should be pulling a large amount of air through the radiator.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record