Cooling Fan Operation

The water and antifreeze mixture (engine coolant) in the radiator is normally cooled by air forced through the radiator as you drive the vehicle. When the vehicle is not moving, air is not flowing through the radiator, so a cooling fan is used to draw air through the radiator, thus removing heat from the engine.

There are two common types of cooling fans used on today's vehicles: an electric cooling fan and a mechanical cooling fan.

Electric Cooling Fan

An electrical motor (cooling fan motor) turns fan blades that draw air through the radiator. A vehicle may have one or more cooling fans. The motor is activated when the engine temperature reaches a specified amount and will continue to operate until the temperature drops below a certain amount. It is not uncommon to hear the fan operating after you turn off your vehicle.

The engine temperature is monitored by an engine temperature sensor, and the cooling fan motor is controlled by an electrical switch. This switch may be the temperature sensor itself, a fan control relay, or the engine control module, depending on the type of vehicle. Many vehicles have fans that operate at different speeds depending on the engine temperature.

Symptoms of Failure
Engine temperature will rise and the engine may overheat, especially when driving at slower speeds. The A/C may not blow cold air.

Mechanical Cooling Fan

Trucks and older vehicles may use mechanical fan blades mounted on a pulley, which is driven by the engine or a fan belt. Mechanical fan blades are operated in conjunction with a coupling device called a fan clutch. This device uses a hub containing silicon gel that expands when the engine temperature rises. This expansion locks the fan hub and the fan blades then draw air through the radiator. When the air being drawn through the radiator has sufficiently cooled down the coolant in the radiator, the gel contracts and no longer “locks” the fan blades. Since power from the engine is needed to operate the fan, this device only operates the fan as needed, thus preventing unneeded
power loss.

Symptoms of Failure
If the fan hub coupling fails, it may fail in a locked position, which may result in increased fuel consumption and a noise when accelerating. This sound is sometimes referred to a "roaring" noise. If the silicon gel in the fan hub leaks out, the fan hub will not engage the fan blades, which can cause the engine to overheat, especially at lower speeds.