Chevrolet Light Duty Truck 1973 Service Manual page 64

1A-38 HEATER A N D AIR C O N D IT IO N IN G
That the drying agent in the receiver or accumulator is
Activated Silica Alumina.
That the inert gas in the expansion valve-capillary line is
carbon dioxide.
SYSTEM CONTROLS
Four-Season System (C-K Models) - Fig. 41
The system selector level (air control lever) determines the
mode of operation: OFF, A/C, VENT, HEATER, BILEVEL
or DEF. When the system selector lever is placed in the A/C,
BI-LEVEL or DEF positions, electrical circuit connection is
made to the compressor clutch through the control panel
switch and the discharge pressure switch. If the switch is
closed (ambient temperature above 40°F.), the compressor
will run. In the OFF, VENT or HEATER positions, the com­
pressor is not energized.
The system selector lever also determines the direction of
outlet air flow. Moving the lever from mode to mode
varies the position of a rotary vacuum valve on the
control. The position of the vacuum valve will supply
vacuum to, or vent, vacuum diaphragms which position
the upper and lower mode and defroster air doors in the
selector duct assembly. The position of these air doors
determines if output air flow is from heater outlet
(OFF), the heater outlet with slight air flow from
defroster nozzles (HEATER), heater and A /C outlets
(VENT), A /C outlet only (A/C), heater, A /C and
defroster outlets (BI-LEVEL), or the defroster nozzles
with slight air flow from the heater outlet (DEF).
Temperature Control
The temperature lever determines the temperature of
outlet airflow by positioning the temperature door in the
selector duct assembly, through the motion of a bowden
cable linking the control panel lever to the temperature
door. In addition, the temperature lever is connected to a
second vacuum valve on the control panel.
The vacuum valve supplies vacuum to the air inlet
diaphragm through the system selector lever rotary
vacuum valve. When the system is in A /C mode and the
temperature lever is at full COLD, the air inlet door is
positioned to reduce the supply of outside air to the
system from 100% to approximately 20%. The remainder
of the air input (80%) to the A /C system is then taken
from the interior of the passenger compartment. This
H I
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Off A /C
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FAN
Fig. 41-Four-Season System Controls (C-K Models)
VINT HEM ER Bl LEV EL Off
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recirculation of interior air (recirc. operation) provides a
source of fast cool down of interior temperatures.
Fan Switch
The blower (FAN) switch provides a means of selecting
the amount of airflow from the system by regulating the
speed of the blower motor. There are, however,
limitations to the control of blower speed. To provide
constant ventilation, the blower motor electrical circuitry
prevents the blower motor from being shut off when the
ignition switch is on. Therefore, the blower speeds
available are HI, LO and two medium speeds.
The control panel also has a "recirc. override" switch
which overrides the blower speed switch and automati­
cally provides HI blower speed when the system selector
lever is in A /C and the temperature lever is set to full
COLD.
Overhead System (C-K Models)
This system operates in conjunction with the Four-
Season System. Since refrigerant flow is controlled by the
front system, the only control provided for on the
overhead system is a three-speed fan switch (LOW),
MED, HI). The fan switch is mounted in the instrument
panel, to the right of the steering column (fig. 42).
In the OFF position, the blower is inoperative; however,
refrigerant is circulating in the system if the Four-
Season System is ON. In any of the three blower
positions (LOW, MED, HI), the blower will be operative
regardless if the Four-Season System is ON.
NOTE: To obtain maximum cooling, the
Four-Season System should be on A/C,
temperature lever on COLD, blower switch
on HI and the overhead unit blower switch
should be on HI.
Floor Mounted System (G Models)
Controls for the front floor mounted system consist of a
Fig. 42-Overhead Unit Control (C-K Models)
LIGHT DUTY TRUCK SERVICE MANUAL