Viscous Fan Drive - Jeep cherokee 2000 Service Manual

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DESCRIPTION AND OPERATION (Continued)
Fig. 12 Hose Clamp Tool
1 – HOSE CLAMP TOOL 6094
2 – HOSE CLAMP

VISCOUS FAN DRIVE

DESCRIPTION
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 overheat-
ing.
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 cou-
pling 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 tempera-
tures.
A thermostatic bimetallic spring coil is located on
the front face of the viscous fan drive unit. A typical
viscous unit is shown in (Fig. 13). 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.
Vehicles equipped with 2.5L engines have what is
know as an hybrid cooling fan system. This means
that not only do they have a viscous fan but they also
have an electric fan as well. The hybrid viscous fan
drive has a low speed characteristic. This causes the
mechanical fan speeds to be very low 200–400 rpm
range when not engaged allowing the engine to have
additional performance and horsepower gaines.
Fig. 13 Typical Viscous Fan Drive
1 – VISCOUS FAN DRIVE
2 – MOUNTING HUB
3 – THERMOSTATIC SPRING
OPERATION
Only when sufficient heat is present, will the vis-
cous 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.
COOLING SYSTEM 7 - 7