3
FIGURE 5
FIGURE 7
FIGURE 6
FIGURE 4
THERMO-PNEUMATIC
VALVE
THERMO-PNEUMATIC
VALVES
BLOCK
FAN
CLUTCH
RADIATOR
RADIATOR
FAN
CLUTCH
BLOCK
SHUTTER
BLOCK
BLOCK
THERMO-PNEUMATIC
VALVE
THERMO-PNEUMATIC
VALVE
FAN
OVERRIDE
SWITCH
AIR
CONDITIONING
SIGNAL
AIR COND. COMPRESSOR
SOLENOID VALVES
(NORMALLY OPEN)
FAN
CLUTCH
RADIATOR
CONDENSER
RECEIVER
DRYER
AIR
CONDITIONER
OVERRIDE
SWITCH
FAN
CLUTCH
RADIATOR
SOLENOID VALVE
AIR
CONDITIONING
SIGNAL
AIR
CONDITIONING
OVERRIDE
SWITCH
FAN
OVERRIDE
SWITCH
RECEIVER
DRYER
CONDENSER
AIR COND.
COMPRESSOR
SHUTTER
The objective in the case of Figure 5 is to not engage the fan
until the shutters are 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 temperature of the shutter.
In the case of Figure 6, 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 6. When
this pressure switch senses a higher than normal pressure
in the air conditioning system (approximately 250 psi), it
closes an electrical circuit which in turn energizes a solenoid
operated air supply valve which exhausts the air supply for
the fan control, causing the fan to engage.
Most vehicles also have an additional dual function pressure
switch connected in series with the air conditioner clutch.
This switch is normally open, closes at approximately 30
psi and opens again at approximately 400 psi. Its purpose
is to prevent operation of the air conditioning compressor if
the refrigerant is lost or to shut off the compressor if the
system pressure reaches a dangerously high pressure
(approximately 400 psi). This switch does not normally have
anything to do with fan operation; however, it is sometimes
combined with the previously mentioned override switch.
