BMW cooling fans are one of two types - engine mounted mechanical and full electric (not driven by the engine). As the fan spins it draws air from the front of the car, pulls it through the radiator (where the air cools the water vanes in the radiator), and then directs the airflow over and around the engine. The faster you drive the car, the faster air is being drawn through the radiator. But at idle or in traffic, and especially warm days, the lack of airflow could lead to overheating if your fan is malfunctioning. The fan should always be spinning fast. A fan that barely spins at all is a sign of a failed fan clutch and should be replaced right away. The clutch is mounted in the fan and can be replaced on its own. The actual fan rarely goes bad unless one of the blades is damaged. Electric fans require a complete replacement if it's not working (the motor cannot be replaced separately).
