Pontiac FIREBIRD 1972 Service Manual page 96

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CUSTOM AIR CONDITIONING 1A-37
S B C
'v
FUSE
LINK
I
RESISTANCE
TYPE HEATER
Fig. 1A-3 1 Thermal Limiter Assembly
and a resistance type heater between terminals C and S.
Between terminals B and S are the fuse link and heater in
series. The heater portion of the limiter provides a time
delay by gradually heating up until the specific melt tem-
perature of the fuse link is achieved and the fuse link opens
(blows). The thermal limiter must be replaced once the
fuse link is blown.
Since the thermal limiter fuse link is temperature sensitive,
location of the limiter assembly is critical. Excessive ex-
terior heat may cause the fuse link to blow without a loss
of refrigerant. If the limiter is located in a cold area, the
fuse link may not receive enough heat from the heater to
cause the link to open when refrigerant charge is lost.
SYSTEM OPERATION
Compressor operation, with or without the low refrigerant
charge protector, is achieved by applying 12 volts to the
compressor clutch coil to activate the compressor clutch.
This 12 volt source is supplied through a switch in the
control panel and through' the ambient temperature
switch. With the low refrigerant charge protection system
in the compressor clutch coil circuit, the 12 volt source is
applied to terminal B of the thermal limiter rather than
directly to the clutch coil connector. From terminal B to
ground there are two possible circuits.
Under normal operating conditions, the superheat shut-
off switch contacts are open and the only complete circuit
is through the fuse link in the thermal limiter to terminal
C and on through the clutch coil to ground.
If refrigerant loss occurs, the superheat shut-off switch
contacts close as a result of the superheat switch sensing
decreased pressure and increased temperature of suction
gas in the compressor. When the switch contacts close, the
terminal on the superheat switch is grounded and a circuit
is completed from terminal B of the limiter through the
fuse link and heater in the limiter, to terminal S and
through the superheat shut-off switch to ground. When
this circuit is completed, the heater in the thermal limiter
starts to get warm and increases in temperature until the
fuse link is heated to its specific melt temperature and
opens the circuit to the compressor clutch coil and the
circuit to the superheat shut-off switch. This stops com-
pressor operation and prevents damage to the compressor
assembly.
When the cause of refrigerant loss is detected and cor-
rected and the system is recharged, the thermal limiter
must be replaced with a new part to restore compressor
operation with low refrigerant charge protection.
CLUTCH AND PULLEY ASSEMBLY
The pulley assembly contains an electrically controlled
magnetic clutch. The compressor clutch (Fig. 1A-32) and
the compressor are off when the system selector lever on
the control panel is in the OFF position. On B Series the
clutch and compressor are also off in the VENT and
HEATER positions. In the other positions of the system
selector lever the clutch will be engaged or disengaged
depending upon ambient temperature and the position of
the Vent-Normal switch (except B Series) on the control
panel. At approximately 32 O F . ambient temperature, the
ambient switch located in the air inlet assembly will close
to complete the circuit to the compressor clutch. Below
32°F. the ambient switch is open and the compressor can-
not operate. Above 32°F. (ambient switch closed), com-
pressor operation is controlled by the Vent-Normal switch
(except B Series). With a given control panel setting of the
system selector, placing the Vent-Normal switch to
VENT will turn the compressor off. As the system selector
lever is moved from one position to another, the Vent-
Normal switch will automatically return to the NORM
(normal) position and the compressor will be running. If
refrigeration is not desired, the switch must be returned to
the VENT position every time the system selector lever is
moved.
The armature plate is.the movable member of the clutch.
The plate is attached to a driven ring by driver springs,
which are riveted to the armature plate and the driven
ring. The driven ring is attached to the clutch hub by a
rubber disc, which is bonded to both the driven ring and
the clutch hub. The clutch hub is pressed onto the com-
pressor shaft and is aligned with a square drive key located
in the keyway of the compressor shaft. This hub and drive
plate assembly is retained by a spacer and retainer ring
(assembled to the shaft) and is held in place with a hexa-
gonal lock nut.
The rubber disc isolates the compressor shaft from the
drive pulley to prevent vibrations from being transmitted
either into or out of the compressor shaft.
The pulley hub and ring assembly consists of three parts:
1. Pully rim, which contains the belt groove.
2. Power element ring.
3. Pulley hub.