Pontiac FIREBIRD 1972 Service Manual page 89

1A-30 1972 PONTIAC SERVICE MANUAL
EVAPORATOR CORE PURGING
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F SERIES
The evaporator case (left half) on F Series models contains
an air door which is controlled by a vacuum diaphragm
mounted on top of the case. The door and diaphragm are
designed to work with the blower motor to purge conden-
sation (moisture) from the evaporator core.
The purge door and diaphragm operate as follows:
The purge door rests against the opening in the left
evaporator case toward the engine. When the engine is
started and the air conditioning control panel is set to the
OFF position, vacuum is applied to the diaphragm and the
purge door swings to seal the dash opening from the
evaporator core.
On air conditioned F Series models, the blower motor
operates whenever the ignition switch is ON. When the
key is on and the A/C control panel is in the OFF posi-
tion, the blower motor will operate in the low position.
With the purge door against the dash opening, the blower
motor passes inlet air through the evaporator case and out
the opening in the left half of the case. This air flow will
pick up any moisture from the evaporator core and empty
into the engine compartment.
When the control panel is switched to any position other
than OFF vacuum is no longer present at the diaphragm
and the purge door returns to seal the opening in the left
evaporator case half and inlet air passes through the
evaporator core and into the dash opening.
Because moisture is not allowed to collect at the evapora-
tor core, dry air will be passed into the air distribution
system when the control panel is switched from OFF to
any other position. This eliminates windshield fogging
when system is turned on.
SUCTION THROTTLING (P.O.A.) VALVE
FUNCTION
The main function of the P.O.A. valve (Fig. 1A-21) is to
maintain the evaporator pressure at a sufficiently high
level to avoid freezing of moisture on the evaporator core
and at the same time provide maximum cooling efficiency.
BASIC OPERATION (Figs. 1A-22 and 1A-23)
A bronze bellows is used to control a small needle valve,
which in turn controls a large piston. The bellows is con-
structed so that it has a tendency to expand when the
pressure surrounding it goes below 28.5 psi, or contract
when the pressure goes above 28.5 psi. Each time the
bellows expands and closes, the needle valve-pressure sur-
rounding the bellows increases. When the pressure in-
creases sufficiently, the bellows contracts and opens the
needle valve, then pressure surrounding the bellows drops.
When the pressure drops sufficiently, the bellows expands,
etc.
Because of the compressor drawing on the outlet end of
the P.O.A. valve, a lower pressure exists at the outlet than
at the inlet. When the bellows expands and the pressure
around the bellows starts to increase, simultaneously the
lower pressure on the top side of the piston approaches the
pressure on the underside. The closer the two pressures
come to being equal, the more the spring pushes the piston
closed. The more the two pressures become unequal, the
more the bottom (higher) pressure pushes the piston open.
SUMMARY
When the bellows expands, the pressure increases on top
of the piston to nearly equal the pressure below the piston
with the result that the spring pushes the piston closed.
When the bellows contracts and the pressure drops on top
of the piston, the higher pressure below the piston pushes
it open.
COMPRESSOR ASSEMBLY
The compressor (Figs. 1A-24, 1A-25 and 1A-26) is the
same for all series except for belt pulley size. The A Series
pulley with the 6 cylinder engine is slightly larger. Both
units are of basic double-action piston design. Three hori-
zontal double acting pistons make up a six-cylinder com-
pressor, and are mounted axially around the compressor
shaft to operate in a front and rear cylinder assembly.
These pistons operate in a 1 1/2" bore, have a 1 3/16"
stroke and are actuated by a swash plate pressed on the
compressor crankshaft.
Reed-type suction and discharge valves are mounted in
valve plates between the cylinder assembly and the head
at each end of the compressor. The heads are connected
with each other by gas-tight passage ways which direct
refrigerant gas to a common output.
Fig. 1A-24 Compressor Assembly