Cooling Fans

When the vehicle is moving at road speed, air flows through the radiator naturally from the vehicle's forward motion. But in traffic, at idle, or in a drive-through — the vehicle is not moving and there is no natural airflow. Without a fan to pull air through the radiator and condenser, the engine overheats and the AC stops cooling.

Most modern vehicles use electric fans mounted to the radiator. The PCM or a dedicated fan control module activates the fan based on coolant temperature, AC system pressure, and transmission fluid temperature. Fans often have two speeds — low speed for normal cooling and high speed for heavy loads or AC operation. A fan that does not turn on when the engine is hot is one of the most common causes of overheating at idle or in slow traffic. Check the fan relay, the fan motor itself, and the control signal from the PCM.

Trucks and some older vehicles use a belt-driven fan with a fan clutch. The fan clutch is a temperature-sensitive coupling that engages the fan when hot air from the radiator heats a bimetallic spring in the clutch. Cold engine — the fan free-wheels and barely moves air. Hot engine — the clutch engages and the fan pulls hard. A failed fan clutch that never engages causes overheating at idle. A fan clutch that stays engaged all the time — the engine sounds like an airplane at all times — wastes power and fuel.