3 - 3
Section B
9803/3280
Section B
3 - 3
Issue 1
Basic Operation
Air Conditioning
System Operation (cont'd)
The refrigerant is drawn through the evaporator matrix
E
by
the suction of the compressor. The temperature of
refrigerant is now considerably below that of the air being
drawn across the evaporator matrix by the blowers. Heat is
transferred from the ambient and recirculated air to the
refrigerant, causing the low pressure liquid to vaporise and
become a low pressure gas. Moisture in the air condenses
on the evaporator matrix and is drained away via
condensate.
Cool de-humidified air is emitted through air vents into the
cab.
The low temperature, low pressure, high heat content
refrigerant gas, is now drawn by suction back to the
compressor, where the cycle is completed.
Service connection points
X
and
Y
adjacent to the
compressor are provided for connecting the pressure gauge
manifold, when charging/testing the refrigerant. Connection
X
high pressure (red) and connection
Y
low pressure (blue).
System Control
Control of the system is achieved by the cyclic action of the
compressor's electromagnetic clutch. When current is fed to
the field coil of the compressor' s clutch, a magnetic field
develops between the field coil and the armature which pulls
the field coil, complete with clutch assembly, onto the
compressor' s rotor. Since the clutch assembly is turned
constantly by the crankshaft pulley drive belt, the
compressor armature turns, starting the refrigeration cycle.
Current is fed to the field coil through three series switches
whose contacts are controlled by the following:
a
The manual A/C ON/OFF switch
F
in the control
console.
b
The freeze protection thermostat switch monitoring
the evaporator temperature.
c
The high and low level binary pressure switch.
The ON/OFF switch
F
will start the refrigeration cycle
provided that the ambient temperature in the cab is greater
than 0°C and the refrigerant pressure remains within the
specified limits.
The thermostat has its sensor inserted in the evaporator
matrix. It controls the refrigeration cycle by switching the
compressor clutch on and off to prevent freezing of the
condensate on the evaporator matrix.
The high and low level binary pressure switch assembly is
located adjacent to the expansion valve on the HVAC unit. If
the refrigerant pressure exceeds the upper pressure limit
specified or falls below the lower limit, the contacts will open
and the clutch will disengage, closing down the refrigeration
cycle.
Note:
Switch settings are given in the technical data, refer to
Technical Data - Air Conditioning (Option)
.
F
A/C ON/OFF Switch
G
Turn to switch on the 3-speed heater fan. Turn further
clockwise for faster speeds. It functions only with the
starter switch ON.
H
Temperature - Turn anti-clockwise to increase the heat.
J
Air Recirculation - Turn clockwise to recirculate air in
the cab. Turn anti-clockwise for fresh air from outsde.
The control can be adjusted to give a combination of
recirculated air and fresh air.
G
H
J
F
A401550
