GMC 1976 ZEO 6083 Maintenance Manual page 62

The operation of the valve is quite simple . It is a
matter of controlling opposing forces produced by a
spring and the refrigerant pressures. For example:
The pressure in the power element is trying to push
the seat away from the orifice, while the adjusting
spring is trying to force the seat toward the orifice.
These opposing pressures are established in the de-
sign of the valve so that during idle periods the ad-
justing spring tension and the refrigerant pressure in
the cooling coil are always greater than the opposing
pressure in the power element. Therefore, the valve
remains closed . When the compressor is started, it
will reduce the pressure and temperature of the re-
frigerant in the cooling coil to a point where the
vapor pressure in the power element becomes the
stronger . The seat then moves off the orifice and
liquid starts to flow through the valve orifice into the
cooling coil .
The purpose of the power element is to help de-
termine the quantity of liquid that is being metered
into the cooling coil . As the temperature of the low
pressure line changes at the bulb, the pressure of the
vapor in the power element changes, resulting in a
change of the position of the seat . For example, if the
cooling coil gets more liquid than is required, the
temperature of the low pressure line is reduced and
the resultant lowering of the bulb temperature
reduces the pressure of the vapor in the power ele-
ment, allowing the seat to move closer to the orifice.
This immediately reduces the amount of liquid leav-
ing the valve. Under normal operation, the power
element provides accurate control of the quantity of
refrigerant to the cooling coil .
To employ our tire pump analogy once more for
clarity, it is the same situation that would exist if you
were inflating a tire with a very slow leak . Providing
you pumped the air into the tire as fast as it leaked
out, you would be able to maintain pressure even
though the air would merely be circulating through
the tire and leaking out through the puncture .
EVAPORATOR
The function of the evaporator (figures 15 and
16) is to cool and dehumidify the air flow before it
enters the passenger compartment . The evaporator
assembly consists of an aluminum core enclosed in a
sheet metal housing located in the front of the vehicle
chassis. Two water drain holes are located in the
bottom of the housing. Two refrigerant lines are con-
nected to the sides of the evaporator core : the small
inlet line on the right, and the larger outlet line on
the left .
The temperature sensing bulb of the expansion
valve is clamped to the outlet pipe of the evaporator
core . The high pressure liquid refrigerant, after it is
metered through the expansion valve, passes into the
AIR CONDITIONING SYSTEM
1- 45
evaporator core where it is allowed to expand under
reduced pressure . As a result of the reduced pressure
the refrigerant begins to expand and return to the
original gaseous state. To accomplish this transfor-
mation it begins to boil .
The boiling action of the refrigerant demands
heat . To satify the demand for heat, the air passing
over the core gives up heat to the evaporator and is
subsequently cooled .
Figure 16 shows the evaporator housing on vehi-
cles equipped with the additional air conditioning
outlets shown in Figure 25 . The vacuum actuator rod
operates an auxiliary air flow door .
LOW REFRIGERANT CHARGE
PROTECTION SYSTEM
The compressor discharge pressure switch (fig-
ure 18) performs the function of shutting off the
compressor when it senses low refrigerant pressure .
The switch is located in the evaporator inlet line
(high pressure) . The switch electrically is wired in
series between the compressor clutch, the thermo-
static switch, and the master switch on the control.
When the switch senses low pressure it breaks con-
tact and opens the circuit to the compressor clutch,
thus shutting off the A/C system and preventing
compressor failure or seizure.
The compressor discharge pressure switch also
performs the function of the ambient switch as the
pressure at the switch varies directly with ambient
temperatures . The compressor should not run below
25 °F. (-3 .9°C .) ambient or 37 psi at the switch . The
Figure 18-Compressor Discharge Pressure Switch,
Typical