GMC 1976 ZEO 6083 Maintenance Manual page 339

6K- 8
ENGINE COOLING
Figure 9-MotorHome Fan Clutch Assembly
The automatic fan clutch has two modes of oper-
ation, 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 temper-
ature 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 tempera-
ture rises the shaft moves the arm exposing and
opening 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.
The speed differential between the clutch plate
and the pump plate 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 suffi-
cient 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 requirement of the vehicle . ..
Trans Mode fan clutch, not shown, is similar to the
MotorHome fan clutch but, provides a greater cool-
ing capacity required for commercial use .
ing.
FAN CLUTCH TROUBLE
DIAGNOSIS
1 . NOISE
Fan noise is sometimes evident under the follow-
ing normal conditions :
a. When clutch is engaged for maximum cool-
b. During first few minutes after start-up un-
til the clutch can re-distribute the silicone fluid back
to its normal disengaged operating condition after
overnight setting.
However, fan noise or an excessive roar will gen-
erally occur continuously under all high engine
speed conditions (2500) rpm and up) if the clutch
assembly is locked up due to an internal failure . If the
fan cannot be rotated 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 normal condition due to
the type of bearing used . Approximately 1/4" max-
imum 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.
4. ENGINE OVERHEATING
a. Start with a cool engine to insure complete
fan clutch disengagement.
b. If the fan and clutch assembly free-wheels
with no drag (revolves over 5 times when spun by
hand), the clutch should be replaced . If clutch per-
forms properly with a slight drag go to Step C.
NOTE : Testing a fan clutch by holding the small
hub with one hand and rotating the aluminum