Cherokee 4WD L6-4.0L VIN S (1998)
Fan Clutch: Description and Operation
The thermal viscous fan drive is a silicone-fluid filled coupling used to connect the fan blades to either the engine or the water pump shaft. The
coupling allows the fan to be driven in a normal manner. This is done at low engine speeds while limiting the top speed of the fan to a predetermined
maximum level at higher engine speeds.
2.5L vehicles with A/C are equipped with a viscous fan drive which is designed to "free wheel" during most of the ambient conditions encountered by
the vehicle and will only engage during high heat loads as seen in trailer towing or high ambient temperatures.
Fig.8 Typical Viscous Fan Drive
A thermostatic bimetallic spring coil is located on the front face of the viscous fan drive unit. A typical viscous unit is shown. This spring coil reacts to
the temperature of the radiator discharge air. It engages the viscous fan drive for higher fan speed if the air temperature from the radiator rises above a
certain point. Until additional engine cooling is necessary, the fan will remain at a reduced rpm regardless of engine speed.
Only when sufficient heat is present, will the viscous fan drive engage. This is when the air flowing through the radiator core causes a reaction to the
bimetallic coil. It then increases fan speed to provide the necessary additional engine cooling.
Once the engine has cooled, the radiator discharge temperature will drop. The bimetallic coil again reacts and the fan speed is reduced to the previous
disengaged speed.
CAUTION: Engines equipped with poly-V drive belts have reverse rotating fans and viscous fan drives. They are marked with the word REVERSE
to designate their usage. Installation of the wrong fan or viscous fan drive can result in engine overheating.
CAUTION: If the viscous fan drive is replaced because of mechanical damage, the cooling fan blades should also be inspected. Inspect for fatigue
cracks, loose blades, or loose rivets that could have resulted from excessive vibration. Replace fan blade assembly if any of these condo ions are
found. Also inspect water pump bearing and shaft assembly for any related damage due to a viscous fan drive malfunction.
NOISE
It is normal for fan noise to be louder (roaring) when:
-
The underhood temperature is above the engagement point for the viscous drive coupling. This may occur when ambient (outside air temperature)
is very high.
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Engine loads and temperatures are high such as when towing a trailer.
-
Cool silicone fluid within the fan drive unit is being redistributed back to its normal disengaged (warm) position. This can occur during the first 15
seconds to one minute after engine start-up on a cold engine.