GMC 1976 ZEO 6083 Maintenance Manual page 63

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AIR CONDITIONING SYSTEM
compressor should run in A/C modes above 45°F .
(7 .2°C .) ambient or 42 psi at the switch .
The switch interacts with other switches so that
in an A/C system where the compressor will not op-
erate above 45°F . (7 .2°C .) ambient the following
components should be checked for continuity :
1 . Compressor discharge pressure switch .
2. Master switch (on control head).
3. Thermostatic switch .
If both switches show proper continuity, check
the harness for shorts or improper ground condi-
tions. The switch also contains the high pressure line
service ports.
ELECTRICAL COMPONENTS
COMPRESSOR CLUTCH ASSEMBLY
The clutch assembly (figure 10) consists of the
coil, pulley and armature . The coil is basically an
electro-magnetic device charged by the battery.
When energized, it sends a magnetic force through
the soft iron in the pulley, which is constantly turn-
ing as a result of being belt driven by the engine, to
the armature . The armature is keyed to the compres-
sor shaft. When magnetically energized the armature
is pulled into the pulley causing the compressor to be
activated.
BLOWER
The blower (figure 19) is simply a device for mov-
Figure 19-Blower Location, Typical
ing air. The blower used in this vehicle is a centrifu-
gal type fan which forces air across the evaporator
and/or heat cores to the vehicle interior .
THERMOSTATIC SWITCH
The thermostatic switch (figures 15 and 16) is
basically a bimetal switch which is controlled by a
sensing tube across the outlet of the evaporator core .
As the evaporator cools the sensing tube the bimetal
switch turns off the clutch and disengages the com-
pressor until the tube becomes warm enough to turn
the compressor back on .
BLOWER SWITCH, RELAY & RESISTOR
The blower switch, blower relay and the blower
resistor must be discussed together because of their
interrelations with each other. The blower switch
located on the instrument panel regulates low,
medium and high blower speeds through a blower
resistor system . This resistor (figure 15) regulates the
amount of current fed to the blower thereby regulat-
ing the blower speed. The blower relay (figure 20)
provides the proper connections for the low and
medium speeds through the resistor assembly and
direct battery current to the blower for high speed.
VACUUM COMPONENTS
The vacuum system (figure 21) consists of three
basic components :
1. Vacuum tank .
2. Modes or vacuum switches .
3. Control Panel .
Figure 20-Blower Relay