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Index• Multi function switches other than Red Dot’s are being used
in some applications. The concept is the same as described
above.
4. Fan Clutches, Radiator Shutters & Override Controls
An engine fan clutch and radiator shutter assembly may be used to
control and maintain engine (coolant) temperature. Engine fan and radia-
tor shutter operation are closely related to radiator mounted AC con-
denser efficiency (heat energy movement). This is the main reason for fan
clutch and radiator shutter override controls.
There are three types of fan clutches:
•
Viscous Drive
•
Air Actuated
•
Electric
All but the viscous type may have a fan override control as part of the AC
system. The
viscous drive fan clutch
has a high viscosity silicone fluid
that moves inside the clutch to increase or decrease fan speed. The fluid is
controlled by a temperature sensor, valve and centrifugal force.
Air actuated fan clutches
are either on or off. Control is by a
thermostatic valve which measures engine coolant temperature. When
coolant is hot, the valve opens and air pressure enters the fan body
causing the clutch to engage. Some air actuated clutches use air pressure
for the off condition.
The electric fan clutch
has a preset engine coolant temperature
sensor. When the set temperature is reached, the clutch is engaged for
increased air movement through the condenser and radiator. When cool-
ant temperature drops the clutch disengages the fan.
Radiator shutters
control air (outside air) flow through the radiator.
These shutters are opened and closed by thermal or air activated spring
loaded valves. Both types of valves sense and maintain engine coolant
within a narrow temperature range (4 to 6 degrees). The narrow 4 to 6
degree Fahrenheit temperature range selected may fall anywhere be-
tween 160 to 220 degrees Fahrenheit. Thus, override controls are re-
quired to bypass radiator shutter controls for efficient or safe condenser
function and AC system operation.
Chapter Review
• Basic system controls begin at a control panel inside the cab
or sleeper compartment. We can decide on heating or cooling
and where and how much air flow we want. We can select a
temperature range for the inside of the cab or sleeper.
• When a heater, air conditioner or HVAC system is turned on,
the automatic control devices take over system control. They
are designed to keep the system in balance and maintain the
operator selected temperature range inside the vehicle.
Chapter Review