LO 11-7 Identify and describe the types, features, and applications of cooling fans
1. Fans and Fan Drives
1.4. On/Off Fan Hubs
LO 11-07: Identify and describe the types, features, and applications of cooling fans. |
On/Off Fan Hubs
A more traditional fan engagement mechanism is an on/off fan hub, which uses air pressure to either engage or release a spring-loaded friction clutch Figure 67 and Figure 68. The air supplied to the fan hub is electrically signalled through an electric over-air control valve. Either the ECU provides an electrical signal to open or close the air supply solenoid or a temperature-sensitive switch in the cooling system activates the solenoid. If the refrigerant pressures in the condenser become too high, an A/C high-pressure switch supplies an input signal to the fan, which then pulls air across the condenser Figure 69. Typically, a timer is connected to the circuit to keep the fan engaged for 90–120 seconds to prevent excessive cycling between on and off. Longer and fewer fan cycles prevent premature wear of the friction materials in the fan clutch and remove more heat when the A/C refrigerant pressure is excessive.
FIGURE 11-67: An expanded view of an on/off fan hub. |
FIGURE 11-68: A schematic of an electrical circuit controlling an on/off fan hub. |
FIGURE 11-69: The engine fan pulls air across multiple heat exchangers to increase the effectiveness of heat removal from the heat exchangers. |
The coolant temperature at which the fan engages is 10–15° F (−12.2 to −9.4° C) above the normal engine operating temperature, which allows vehicle movement to first move air across the radiator and ATAAC and only engages the fan if airflow provided by vehicle speed is insufficient. Because engine fans consume a lot of engine power—easily 25 hp (19 kW) on HD engines—the fan is often activated with the engine brake to increase the engine’s resistance to driveline rotation. During engine warm-up operation, the fan may also engage to apply a load to the engine when the transmission is in neutral. A two-speed fan hub is also available, which provides a combined variable-speed operating mechanism and spring-engaged, on/off operation.
| Technicians Tip | ||
| Sensors and other electrical devices that are exposed to engine coolant have the potential to short out. Electrical current passing through the coolant from these sensors quickly corrodes brass and aluminum oil coolers, injector tubes, and other metals in the cooling system. To minimize this problem, engine blocks use a number of ground straps not only to lower resistances for engine-mounted electrical devices but also to minimize electrical conduction through engine coolant, which quickly eliminates nitrite additives in coolant, leading to further engine damage. |
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