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Fan Clutch Diagnosis - Cooling System

What is the Fan Clutch?

The fan clutch is a coupling device that is located between the water pump shaft and the fan.  The fan clutch is designed to improve the vehicle’s cooling system efficiency while reducing the load on the engine and loss of energy caused by the fan itself.  The Fan Clutch allows the fan to operate at lower speeds and effectively detach at higher speeds when the vehicle is moving and air movement due to velocity aids to cool the engine. 

There are two types of Fan Clutches, thermal and non thermal fan clutches, also called centrifugal clutches.  Both types operate on the fluid-drive principle.

Non-thermal fan clutches

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

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.

Overheating Engine TemperatureSymptoms of a Worn / Defective
Fan Clutch that should be Replaced

How do you know when your Fan Clutch is wearing out or has failed to do it's job?  There are a few key symptoms. 

  • Excessive Free-Wheeling when spun manually (when engine is stopped) - With the engine stopped, manually spin the fan.  If the Fan spins excessively, over 3 revolutions, as though there is no resistance it should be replaced.
  • If your air conditioner does not perform well at idle or low vehicle speeds then the clutch may have failed and air is not passing over the A/C condenser efficiently enough to cool the refrigerant.
  • If the fan speed does not increase when engine is running hot or if the fan speed does not increase until engine is excessively hot.
  • Looseness of the Fan - Excessive lateral movement of the fan blades.  If the fan blade moves more than 1/4" front to back measured at the end of the blade.  Some lateral movement is a normal condition due to the type of bearing used in fan clutches. Approximately 1/4" (6.5 mm) maximum lateral movement measured at the fan tip is allowable.  
  • Vibration - Sometimes vibration can be detected due to a failed clutch.  The vibration can increase with with engine speed.   Many times this can lead to water pump failure.
  • When the engine is stopped, turning the fan blade manually turns rough, grinding or does not turn at all.
  • Leaking silicone Fluid - Excessive fluid leakage will cause the clutch to fail to engage.
  • Noise - If you hear excessive fan noise or a roar at all engine speeds.  Noise can be detected when the clutch should be engaged, during initial cold startup or when the engine is hot.  Under high speeds or higher RPMs over 2500, a locked up fan can create a roaring noise.

When the Fan Clutch Wears and Fails

An important factor when you look at the Fan Clutch is to note that the fan clutch and water pump share a common shaft.  Thus the operation of one will affect the operation of the other.  For instance if the fan clutch bearing wears, then the fan clutch will wobble.  The vibration of a wobbling fan will in turn cause the water pump bearing to wear pre-maturely and eventually fail itself.  Likewise a worn water pump bearing can cause a fan clutch bearing to fail for the same reason, vibration being transmitted from one bearing to the other.  When replacing the damaged water pump, it is wise to replace the worn fan clutch at the same time otherwise the new water pump will soon fail.  That is why the fan clutch should be replaced when a water pump is replaced.


Engine Overheating Basics

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Fan Clutch Diagnosis
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Jeep Project CJ-7

An ongoing Budgeted Rebuild/Build up of a Rock Crawling Machine.

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Ramp Travel Index
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Dick Cepek's 2008 F-250 Super Duty Project Vehicle – Project CRUSHER
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Off-Road Lights
by LightForce Product Review / Installation.
from Off-Road Lights


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Ultra-Cool Hand Throttle for Free!

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