Fan and Shroud

The cooling system fan pulls a large volume of air through the radiator core that cools the hot water circulating through the radiator. A fan belt or an electric motor drives the fan A fan driven by a fan belt, is known as an engine-powered fan and is bolted to the water pump hub and pulley. Sometimes a spacer fits between the fan and pulley to move the fan closer to the radiator. Besides removing heat from the coolant in the radiator, the flow of air created by the fan causes some direct cooling of the engine itself.

Fan blades are spaced at intervals around the fan hub to aid in controlling vibration and noise. They are often curled at the tip to increase their ability to move air. Except for differences in location around the hub, most blades have the same pitch and angularity.

Bent fan blades are very common and result in noise, vibration, and excess wear on the water pump shaft. You should inspect the fan blades, pulleys, pump shaft end play, and drive belt at every preventive maintenance inspection.

A variable pitch (flex) fan has thin, flexible blades that alter airflow with engine speed (fig. 6-9). These fan blades are made to change pitch as the speed of the fan increases so that the fan will not create excessive noise or draw excessive engine power at highway speeds. At low speeds, the fan blades remain curved and pull air through the radiator. At higher speeds, the blades flex until they are almost straight. This reduces fan action and saves engine power.

The fluid coupling fan clutch is designed to slip at high speeds, performing the same function as a flexible fan. The clutch is filled with silicone-based oil. Fan speed is controlled by the torque-carrying capacity of the oil. The more oil in the coupling, the greater the fan speed; the less oil in the coupling, the slower the fan speed.

The thermostatic fan clutch has a temperature sensitive, bimetallic spring that controls fan action. The spring controls oil flow in the fan clutch When cold, the spring causes the clutch to slip, speeding engine warm-up. After reaching operating temperature, the spring locks the clutch, providing forced air circulation.

An electric engine fan uses an electric motor and a thermostatic switch to provide cooling action (fig.6-10). An electric fan is used on front-wheel drive vehicles having transverse mounted engines. The water pump is normally located away from the radiator.

The fan motor is a small, direct current (dc) motor. It mounts on a bracket secured to the radiator. A metal or plastic fan blade mounts on the end of the motor shaft.

A fan switch or temperature-sensing switch controls fan motor operation. When the engine is cold, the switch is open, keeping the fan from spinning, and speeds engine warm-up. When coolant temperature reaches approximately 210°F, the switch closes to operate the fan and provide cooling.

An electric engine fan saves energy and increases cooling system efficiency. It only functions when needed. By speeding engine warm-up, it reduces emissions and fuel consumption. In cold weather, the electric fan may shut off at highway speeds. There may be enough cool air rushing through the grille of the vehicle to provide adequate cooling. On some models a timed relay may be incorporated that allows the fan to run for a short time after engine shutdown. This, in conjunction with thermosiphon action, helps to prevent boilover after engine shutdown.

Figure 6-9.—Variable pitch fan.