Theory of Operation
tube. The switch closes when the evaporator is above
The actual cooling and drying of cab air takes place at
the switch setting and opens when the evaporator is
the evaporator. Flow of the high pressure liquid
cooled to the switch setting. The switch has a low
refrigerant is controller by the expansion valve. The
temperature setting that prevents the evaporator from
expansion valve causes the temperature and pressure
becoming cold enough to freeze moisture that
of the refrigerant to drop, where it becomes a low
condenses on the coil.
pressure liquid (L). The expansion valve is a valve that
uses a variable orifice to control the flow of high
The B28 A/C and heater thermistor senses
pressure liquid refrigerant into the evaporator to
temperature of the evaporator coil through a refrigerant
maintain a constant pressure and temperature.
filled capillary tube.
The M6 A/C and heater blower motor (N) pulls a
The moisture, from the warm air, is condensed as it
mixture of warm cab and outside air through the
contacts the cool evaporator coil during the cooling
evaporator where it is cooled by the refrigerant. The
process and is drained away through drain tubes
heat absorbed by the evaporator causes the refrigerant
connected to the drain pan under the evaporator. With
to vaporize into a low pressure gas.
the cab air cooled and dehumidified, the refrigerant
cycle is complete.
A freeze control switch (M) senses temperature of the
evaporator coil through a refrigerant filled capillary
HEATER AND AIR CONDITIONER CIRCUIT
The following conditions must exist for air conditioning
circuit to function:
Key switch ON
Air conditioning switch turned ON
Any of the four blower motor On speeds selected on air