Clutched cooling fan system

A clutched cooling fan system uses a fan clutch to regulate engine temperature by controlling the engagement of the cooling fan based on heat levels. The most common type is the thermal fan clutch, which uses a temperature-sensitive bimetal coil spring on the front to react to the heat of the air passing through the radiator. When the air is hot, the spring expands, opening an internal valve that allows silicone fluid to flow into the clutch mechanism, reducing slippage and increasing fan speed for enhanced cooling. As the air cools, the spring contracts, closing the valve and increasing slippage, which slows the fan and reduces cooling. Most thermal fan clutches engage at approximately 170°F (about 180-190°F engine temperature) and reduce the temperature by about 20°F before disengaging.

The clutch operates as a fluid coupling filled with silicone-based oil, where the flow of fluid between the clutch plates determines the amount of engagement. Standard duty thermal fan clutches typically turn the fan at 50-60% of the water pump speed when engaged, while heavy-duty and severe-duty versions can achieve 80-90% of the water pump speed, providing greater airflow and improved cooling efficiency. A slipping fan clutch, often due to fluid deterioration over time (about 200 rpm per year), can lead to engine overheating if not addressed.