Viscous Fan Drive - Jeep Wrangler 1999 Electronic Service Manual

7 - 6 COOLING SYSTEM
DESCRIPTION AND OPERATION (Continued)
Fig. 6 Hose Clamp Tool—Typical
Fig. 7 Clamp Number/Letter Location
Ordinary worm gear type hose clamps (when
equipped) can be removed with a straight screw-
driver or a hex socket. To prevent damage to
hoses or clamps, the hose clamps should be
tightened to 4 N·m (34 in. lbs.) torque. Do not
over tighten hose clamps.
When performing a hose inspection, inspect the
radiator lower hose for proper position and condition
of the internal spring.

VISCOUS FAN DRIVE

The thermal viscous fan drive (Fig. 8) 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.
Fig. 8 Water Pump Mounted 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 in (Fig. 9). 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.
Fig. 9 Viscous Fan Drive—Typical
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
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