2
FIGURE 3
FAN DISENGAGED
FIGURE 4
FAN ENGAGED
Depending upon optional equipment installed on the vehicle
such as radiator shutters and radiator mounted air
conditioning condenser, different piping arrangements are
necessary. These will be explained in the paragraph on
control systems.
DISENGAGED
When the vehicle is started with a cold engine the thermo-
pneumatic control valve is open. As brake system air
pressure is built up, air passes through the control valve to
the fan clutch. Air enters the inlet port in the bracket of the
fan clutch and travels through the drilled passage in the
shaft to fill the piston cavity. When air in the piston cavity
reaches a pressure of 70-75 psi, the piston slides on the
shaft moving the pressure plate assembly and fan to the
disengaged position. In this position the clutch lining is out
of contact with the pulley and the fan is no longer driven by
the engine (see Fig. 3).
ENGAGED
As the engine coolant rises in temperature, the engine
thermostat opens to circulate radiator coolant through the
engine. With further increase in coolant temperature, the
fan drive control valve closes and exhausts air from the
piston cavity when the engine coolant temperature reaches
the setting of the control valve. The fan clutch spring forces
the piston and pressure plate back on the shaft. When the
pressure plate lining contacts the pulley, the fan is engaged
and is driven by the engine (see Fig. 4).
CONTROL SYSTEMS
The fan clutch, in addition to its primary function of operating
the fan as needed to maintain the cooling liquid within a
certain temperature range, must be coordinated with other
devices when installed on the vehicle.
These are radiator shutters and radiator mounted condenser
for air conditioning. Four basic configurations are possible
as follows:
Figure 5.
Fan Clutch only
Figure 6.
Fan Clutch with shutters
Figure 7.
Fan Clutch with radiator mounted air
conditioning condenser
Figure 8.
Fan Clutch, shutters and condenser
The objective in the case of Figure 5 is to not engage the fan
until the engine thermostat is fully open. The thermo-
pneumatic control valve should therefore be calibrated to
release the air from the fan clutch when the coolant
temperature is at least 10°F. higher than the full open point
of the engine thermostat.
The objective in the case of Figure 6 is to not engage the
fan until the shutters are fully open. The fan control valve
should, therefore, engage the fan at a coolant temperature
10°F. higher than the shutter opening point. If the shutters
are the modulating type, the fan engagement should be at
least 10°F. higher than the full-open temperature of the
shutter.
In the case of Figure 7 the fan must be controlled to perform
the supplementary function of pulling cooling air through
the air conditioning condenser when required, although the
engine coolant may be below the fan cut-in temperature.
This is most likely to occur at high ambient temperatures
and low road speed or idling. If the condenser is
inadequately cooled, the air conditioning system will start
to build up a higher than normal pressure. Therefore, a
pressure switch is connected into the air conditioning
compressor discharge line, as shown in Figure 7. When
this pressure switch senses a higher than normal pressure
in the air conditioning system (approximately 250 P.S.I.), it
closes an electrical circuit which in turn energizes a solenoid
operated air supply valve which exhausts the air supply for
