Non-thermal fan clutches operate based on the water pump shaft speed.  When the water pump shaft is spinning at low and idle speeds, the clutch will spin the fan at about a 1:1 ratio.  During high speed rotation the silicone fluid within the fan clutch reduces its ability to spin the fan at full speed and the energy transferred from the water pump shaft, through the clutch to the fan is reduced.  The result is less load on the engine as the fan almost free-wheels.

Benefits, drawbacks and key points about Non-Thermal Fan Clutches:

• Generally cost less than standard thermal clutches
• Non-Thermal Clutches spin at about 30-60% of the water pump speed.
• Non-Thermal Clutches have a shorter life expectancy, than Thermal Clutches
• Non-Thermal Clutches cannot replace a heavy-duty clutch
• Always creates drag on the engine since they are always engaged, which translates into less fuel savings than a thermal clutch, which will disengage when engine bay temperatures are lower.
• Non-Thermal Clutches are identifiable on the front of the clutch by the smooth, steel faceplate lacking a thermal spring assembly
   

Thermal Fan Clutches are a little more sophisticated in operation than non-thermal fan clutches.  The thermal clutch responds based on temperature changes within the engine bay.  Using a bi-metal thermostatic coil they sense the engine bay temperature and engage or disengage the fan from the water pump shaft.  When engine bay temperatures are cold, the fan clutch is disconnected.  As temperatures rise and reach a set point temperature, the bi-metal coil engages the clutch and the fan spins, cooling the engine.  Likewise as temperatures move lower, the clutch will disengage reducing drag on the engine when the fan is not needed.

As the vehicle travels down the road, the temperature of the air coming through the radiator passes over the bi-metal thermal spring located on the front of the thermal fan clutch.  The bi-metal thermal spring expands or contracts with the changes in air temperature, which move a valve inside the clutch.  When temperatures are cool, this valve is in the position to allow silicone fluid to be pumped away from the working area of the reservoir, allowing the fan clutch to free wheel.  When temperatures are hot, the valve is in the position to allow silicone fluid to be pumped into the working area of the reservoir, engaging the clutch and the fan.  As the fan speed comes up, cool air is pulled faster over the radiator, cooling the engine and the coolant temperatures.   This cycle of engaging the clutch as needed continues.

The Thermal Fan Clutch is engaged at startup because a non-spinning thermal clutch will drain the fluid into the working area of the reservoir.  When the clutch begins to spin, the pumping action of the clutch in it's cool state moves the fluid way from the working area until the valve moved it back.

Benefits, drawbacks and key points about Thermal Fan Clutches:

• Controls fans speed based on temperature
• When at high speed, the Thermal Fan Clutch can provide high speed operation provides maximum cooling with it may be needed.
• When engine speed is low,  the fan may be disengaged if not needed, providing fuel savings and noise reduction. Again, tied to temperature rather than shaft speed.
• Thermal fan clutches have a greater life expectancy than a non-thermal clutch
• Thermal fan clutches are briefly engaged at cold start-up
• Thermal Fan Clutches engage at about 170° radiator air temperature, which translates to about 30° lower than the actual coolant temperature.

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