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ENGINE
ENGINE COOLANT THERMOSTAT - 5.9L DIESEL (Continued)
Fig. 28 Thermostat Removal/Installation
1 - THERMOSTAT HOUSING
2 - CYLINDER HEAD
3 - THERMOSTAT
(7) Start the engine and check for coolant leaks.
Run engine to check for proper thermostat operation.
FAN DRIVE VISCOUS CLUTCH
DESCRIPTION
The thermal viscous fan drive (Fig. 29) is a sili-
cone-fluid- filled coupling used to connect the fan
blades to 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.
OPERATION
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. 30). 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 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.
Fig. 29 Viscous Fan
1 - WATER PUMP BYPASS HOSE
2 - FAN BLADE ASSEMBLY
3 - VISCOUS FAN DRIVE
4 - WATER PUMP AND PULLEY
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.
Fig. 30 Viscous Fan Drive—Typical
1 - VISCOUS FAN DRIVE
2 - THERMOSTATIC SPRING
3 - MOUNTING NUT TO WATER PUMP HUB
DIAGNOSIS AND TESTING - VISCOUS FAN
DRIVE
NOISE
NOTE: It is normal for fan noise to be louder (roar-
ing) when:
• The underhood temperature is above the engage-
ment point for the viscous drive coupling. This may
DR
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