Fan Clutches
Fan Clutch Operation & Troubleshooting
The Fan Clutch is a component of the cooling system that varies the fan speed to meet the cooling needs of the engine. Nearly all rear wheel drive vehicles use Fan Clutches to control fan speed because of their ability to provide high volumes of air and good fuel economy. Despite their popularity, they are one of the least understood cooling components. There are two types of Fan Clutches, the Non-Thermal (also called centrifugal clutches) and the Thermal Fan Clutch.
Non-Thermal Fan Clutch
- Low cost alternative for some standard thermal clutches.
- Always engaged, less fuel savings than a thermal clutch.
- Spins at about 30-60% of the water pump speed.
- Shorter life expectancy.
- Cannot replace a heavy-duty clutch.
- Identified by the smooth, steel faceplate, (without a thermal spring assembly), on the front.
Thermal Fan Clutch
- Varies the fan speed with temperature of the air behind the radiator.
- Engaged (high speed) operation provides maximum cooling.
- Disengaged (low speed) operation provides fuel savings and noise reduction.
- Greater life expectancy than a non-thermal clutch.
- Briefly engaged at cold start-up.
- Engages at about 170° radiator air temperature, (about 30° lower than coolant temperature).
The air temperature coming through the radiator is sensed by the bi-metal thermal spring on the front of the thermal fan clutch. It expands and contracts with the change in air temperature operating a valve inside of the clutch. When cold, the silicone drive fluid is pumped from the working area to the reservoir. When hot, the valve opens allowing fluid from the reservoir to be transferred to the working area thereby increasing the fan speed. The clutch disengages as the air temperature decreases, closing the valve and allowing the silicone fluid to be pumped back into the reservoir.
A thermal fan clutch is engaged on a cold startup because the fluid drains into the working area when the engine is shut off. The fan clutch will slow down shortly after startup as a result of a pumping action produced by a difference in speed between the shaft and the body of the clutch.
Most models are designed to duplicate original equipment performance. Some Chevrolet/GMC truck models are specifically designed to engage at lower temperatures than the original equipment parts that they replace.
Thermal Fan Clutch Design Types
Hayden makes three different types of thermal fan clutches; Standard, Heavy and Severe Duty. Each type is engineered to simulate the performance, fit and appearance of the original equipment clutch that it replaces. All fan clutches are for specific applications and should only be applied for which they are cataloged. Use of the wrong fan clutch may result in poor cooling, excess noise, reduced mileage, or fan clutch failure.
Standard Duty Thermal
- Turns fan 60-70% of shaft speed when engaged.
- Disengage to 20-30% of the shaft speed
- Used with lighter pitch fans. (1-1/2” of pitch).
- Flat plate impeller design with 11.4 Sq. In. of working surface.
- Identified by a smooth steel faceplate & thermal spring assembly on the front side.
Heavy-Duty Thermal
- Turns the fan 80-90% of the shaft speed when engaged for increased cooling.
- Turns the fan 20-30% of the shaft speed when disengaged.
- Used with deeper pitch fans. (2 -1/2” of pitch).
- Land and groove design with 26.0 Sq. In. of working area allows higher operating RPM’s.
- Identified by finned aluminum faceplate and thermal spring on the front.
Severe Duty Thermal
- Turns the fan 80-90% of the shaft speed when engaged.
- Turns the fan 20-30% of the shaft speed when disengaged.
- Used with deeper pitch fans. (2 -1/2° of pitch).
- Land and groove design with 70 Sq. In. of working area.
- Larger working surface provides cooler running and longer life expectancy.
- Thicker body and deep finned faceplate dissipate more heat.
- Can be used in place of many heavy-duty clutches.
Hayden now has available Truck Fan Drives for Ford and General Motors medium-duty trucks and buses.