HEATING, VENTILATION AND AIR CONDITIONING (HVAC) 1A–33
Expansion Valve
This expansion valve (1) is an external pressure type and
it is installed at the evaporator intake port.
The expansion valve converts the high pressure liquid
refrigerant sent from the receiver/drier to a low pressure
liquid refrigerant by forcing it through a tiny port before
sending it to the evaporator (2).
This type of expansion valve consists of a temperature
sensor, diaphragm, ball valve, ball seat, spring
adjustment screw, etc.
The temperature sensor contacts the evaporator outlet
pipe, and converts changes in temperature to pressure. It
then transmits these to the top chamber of the
diaphragm.
The refrigerant pressure is transmitted to the diaphragm’s
bottom chamber through the external equalizing pressure
tube.
The ball valve is connected to the diaphragm. The
opening angle of the expansion valve is determined by
the force acting on the diaphragm and the spring
pressure.
The expansion valve regulates the flow rate of the
refrigerant. Accordingly, when a malfunction occurs to
this expansion valve, both discharge and suction
pressure get low, resulting in insufficient cooling capacity
of the evaporator.
The calibration has been changed to match the
characteristics of HFC-134a.
874RY00003
Evaporator
The evaporator cools and dehumidifies the air before the
air enters the passenger compartment. High-pressure
liquid refrigerant flows through the expansion valve (2)
into the low-pressure area of the evaporator. The heat in
the air passing through the evaporator core (1) is lost to
the cooler surface of the core, thereby cooling the air.
As heat is lost between the air and the evaporator core
surface, moisture in the vehicle condenses on the outside
surface of the evaporator core and is drained off as water.
When the evaporator malfunctions, the trouble will show
up as an inadequate supply of cool air. The cause is
typically a partially plugged core due to dirt, or a
malfunctioning blower motor.
The evaporator core with a laminate louver fin is a
single-sided tank type where only one tank is provided
under the core.
874RY00007
Electronic Thermostat
The thermostat consists of the thermo sensor (1) and
thermostat unit (4) which functions electrically to reduce
the noises being generated while the system is in
operation.
The electronic thermo sensor (1) is mounted at the
evaporator core (2) outlet and senses the temperature of
the cool air from the evaporator (3). Temperature signals
are input to the thermostat unit. This information is
compared by the thermo unit and results in the output to
operate the A/C thermostat relay and turn the magnetic
clutch ON or OFF to prevent evaporator freeze-up.