GMC 4000 Series Service Manual page 84

PISTON
O -RIN G SEAL
OIL PUMP
REAR HEAD
DISCHARGE SUCTION
VALVE PLATE REED
The opposing forces, spring pressure and at­
mospheric pressure on the one side of the valve
diaphragm and evaporator pressure on the opposite
side of the diaphragm constitute a balanced valve
and control its operation.
When the evaporator pressure rises above the
manual spring setting, this increase in evaporator
pressure is exerted against the valve piston and
through the bleed holes in the piston to the under­
side of the diaphragm. The pressures under the
diaphragm overcome the spring pressure causing
the valve piston to move in an opening direction.
As the piston opens, it reaches a balanced position
and the evaporator pressure returns to its original
setting (23-24 p .s.i.g.).
When the evaporator pressure drops below the
predetermined setting, the spring pressure over­
com es the opposing evaporator pressure and forces
the piston in a closing direction until the predeter­
mined pressure setting is maintained (23-24 p.s.i.g.)
The minimum operating pressure of the suction
throttling valve is p re-set by an adjustment screw.
Maximum operating pressure may be obtained by
G M C S E R V I C E M A N U A L __________________ Page 77
M AINSHAFT
DRIVE BALL
THRUST
Q IL M AINSHAFT \
PICK-UP REAR SW A SH
TUBE BEARING PLATE
Figure 7— Refrigerant Compressor
SU C T IO N ^ O N T ^
SHAFT
MAINSHAFT \ \
FRONT FRONT CLUTCH
BEARING HEAD COIL
means of a cable operated from the instrument
panel to the lever arm on the valve. When reduced
load conditions require elevated control pressures,
the lever arm actuates the inner spring assembly
causing the piston to throttle in a closing direction.
This results in a higher evaporation pressure and
the desired increase in discharge air temperature.
COMPRESSOR
The com pressor is of basic double-action
piston design. Three horizontal double acting
pistons make up a six-cylinder com pressor, and
are mounted axially around the com pressor 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 com pressor crankshaft (see fig. 7).
Reed-type suction and discharge valves are
mounted in valve plates between the cylinder a s­
sembly and the head at each end of the com pressor.
The heads are connected with each other by gas-
tight passage ways which direct refrigerant gas to
a common output.
' Qpp 1
AIlT CONDITIONING
PULLEY
SEAL BEARING
ARMATURE
\ PLATE
\ A N D HUB
PULLEY ASSEMBLY
T-31 1 9