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 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
- 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
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.
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
- 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
- Thermal fan clutches are briefly engaged at cold
- Thermal Fan Clutches engage at about 170° radiator
air temperature, which translates to about 30° lower than the
actual coolant temperature.
Symptoms 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
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.