Low Refrigerant Protection - Pontiac FIREBIRD 1972 Service Manual

1A-36 1972 PONTIAC SERVICE MANUAL
LOW REFRIGERANT CHARGE PROTECTION
A low refrigerant charge protection system (Fig. 1A-27)
is incorporated in all compressor clutch coil electrical
circuits. This circuit prevents damage to the compressor
assembly as the result of refrigerant loss.
The protection system consists of:
1. The superheat shut-off switch located in the compres-
sor rear head.
2. The thermal limiter assembly located under the com-
pressor front brace on eight cylinder engine models
or strapped to the A/C engine wire harness on A
Series models with six cylinder engine.
3. A wire assembly which connects the protection sys-
tem in series with the compressor clutch coil circuit.
SUPERHEAT SHUT-OFF SWITCH
The superheat shut-off switch in the compressor rear head
(Fig. 1A-28) senses low refrigerant charge conditions in
the compressor to activate the protection system.
A cross sectional view of the superheat shut-off switch
construction is shown in Fig. 1A-29. The switch consists
of a diaphragm, sensing tube and base assembly which
threads into the switch housing and terminal assembly.
The switch terminal pin is hermetically sealed with a
glass-to-metal seal in the switch housing. The diaphragm
and sensing tube mounting base contains four holes for
TERMINAL
ELECTRICAL
Fig. 1A-29 Cross Section
-
Superheat Shut-Off Switch
passage of refrigerant vapor in and around the diaphragm
permitting the operating suction pressures to affect exter-
nal diaphragm assembly pressure. The diaphragm and
sensing tube assembly is charged with R-114 refrigerant
and the sensing tube protrudes into the suction cavity of
the rear compressor head to sense suction gas tempera-
tures. The internal pressure of the diaphragm and sensing
tube assembly is affected thermally by the suction gas
temperature and the diaphragm is affected externally by
the suction pressure.
The electrical contact welded to the diaphragm will only
contact the terminal pin during a low pressure-high tem-
perature condition. High pressure-high temperature or
low pressure-low temperature conditions will not cause
the contacts to close. Fig. 1A-30 shows the temperature-
pressure curve of the superheat shut-off switch. The con-
tacts may be either "open" or "closed" in the tolerance
zone depending on the characteristics of the switch and
accuracy of pressure and temperature readings taken.
The superheat switch is mounted and sealed in the rear
head by means of an O-ring between the switch housing
and the cavity wall of the rear head. A special formed
retaining ring holds the switch in place and electrically
grounds the switch housing to the compressor. The switch
retaining ring must be installed with the high point of the
curved sides adjacent to the switch housing. The flat side
of the retainer ring provides a positive seat in the retaining
ring groove and the tips of the ring give a more positive
electrical continuity.
THERMAL LIMITER ASSEMBLY
The thermal limiter assembly (Fig. 1A-31) consists of a
temperature sensitive fuse link between terminals B and C
SUC
TOLERANCE
TlON TEMP
ZONE
.
( 1 1 1 1 -
.
10 0 10 20 30
SUCTION PRESSURE
(PSIG)
Fig. 1A-30 Operating Characteristics
-
Superheat
Shut-Off Switch