Chevrolet Light Duty Truck 1973 Service Manual page 414

6 K - 4 E N G IN E C O O L IN G
WARM WATER
FROM ENGINE
RADIATOR CODE'
LOCATION
Fig. 5—Typical Cross-Flow Radiator
held against its seat by a spring, which, when
compressed, allows excessive pressure to be relieved out
the radiator overflow. The vacuum valve is also held
against its seat by a spring which, when compressed,
opens the valve to relieve the vacuum created when the
system cools.
Fan
The cooling fan is located on the end of the water pump
shaft and is driven by the same belt that drives the
pump. In some instances a fan shroud surrounds the fan.
This shroud increases the cooling system efficiency by
ensuring that all air pulled in by the fan passes through
the radiator. Basically there are two types of fans: a fixed
drive fan (which rotates at water pump - engine rpm -
speed) and the automatic fan clutch.
Automatic fan clutches, Figure 6, are hydraulic devices
used to vary the speed in relation to the engine
temperature. Automatic fan clutches are used with many
engines, especially those equipped with factory installed
air conditioning units. Automatic fan clutches permit the
use of a high delivery fan to insure adequate cooling at
reduced engine speeds while eliminating overcooling,
excessive noise, and power loss at high speeds.
The automatic fan clutch has two modes of operation,
the engaged mode and the disengaged mode. The
disengaged mode (engine cold or high speed driving)
occurs when the silicone fluid is contained in the
reservoir area of the fan clutch. As the temperature of
the engine rises so does the temperature of the bimetallic
coil. This bimetallic coil is connected to the arm shaft in
such a way that as the temperature rises the shaft moves
the arm exposing an opening in the pump plate. This
opening allows the silicone fluid to flow from the
reservoir into the working chamber of the automatic fan
clutch.
The silicone fluid is kept circulating through the fan
clutch by wipers located on the pump plate. A hole is
located in front of each wiper, Figure 7. The speed
differential between the clutch plate and the pump plate
COLD RADIATOR TRANSMISSION
FILL LEVEL OIL COOLER
LINES
COOLED WATER
TO ENGINE
develops high pressure areas in front of the wipers, thus
the fluid is forced back into the reservoir. But as the
temperature rises the arm uncovers more of the large
opening and allows more of the silicone fluid to re-enter
the working chamber.
The automatic fan clutch becomes fully engaged when
the silicone fluid, circulating between the working
chamber and the reservoir, reaches a sufficient level in
the working chamber to completely fill the grooves in the
clutch body and clutch plate.
The resistance of the silicone fluid to the shearing action
caused by the speed differential between the grooves
transmits torque to the clutch body. The reverse situation
occurs when the temperature drops. The arm slowly
closes off the return hole thus blocking the fluid flow
from the reservoir into the working chamber.
The continuous action of the wipers removes the silicone
fluid from the grooves in the working chamber and
reduces the shearing action. Thus, less torque is
transmitted to the clutch body and the speed of the fan
decreases.
The temperature at which the automatic fan clutch
engages and disengages is controlled by the setting of
the bimetallic coil. This setting is tailored to satisfy the
cooling requirements of each model.
FAN CLUTCH DIAGNOSTIC
PROCEDURE
1. NOISE
Fan noise is sometimes evident under the following
normal conditions: a. when clutch is engaged for
maximum cooling, and b. during first few minutes after
start-up until the clutch can re-distribute the silicone
fluid back to its normal disengaged operating condition
after overnight settling.
However, fan noise or an excessive roar will generally
occur continuously under all high engine speed
conditions (2500 r.p.m. and up) if the clutch assembly is
locked up due to an internal failure. If the fan cannot be
roated by hand or there is a rough grating feel as the
fan is turned, the clutch should be replaced.
2. LOOSENESS
Under various temperature conditions, there is a visible
lateral movement that can be observed at the tip of the
fan blade. This is a normal condition due to the type of
bearing used. Approximately 1/4" maximum lateral
movement measured at the fan tip is allowable. This is
not cause for replacement.
3. SILICONE FLUID LEAK
The operation of the unit is generally not affected by
small fluid leaks which may occur in the area around the
bearing assembly. However, if the degree of leakage
appears excessive, proceed to item 4.
LIGHT DUTY TRUCK SERVICE MANUAL