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HEATER AND AIR CONDITIONING 1A-37
VACUUM SYSTEM COMPONENT FUNCTION
Component
Air Door Diaphragm
Power Servo
Vacuum Relay Valve
Mode Door Diaphragm
Defroster Diaphragm
Transducer
Thermo Vacuum Valve
Vacuum Applied
Air Door Open to Outside Air
(Closed to Car Body)
Maximum Heat
Vacuum Applied to One Port Opens a Passage
Through the Valve to Allow Transducer Vacuum
to be Supplied to the Power Servo
Vacuum to Linkage Side
Air Flow Out Heater
Ducts
Full Airflow Out Heater
Outlet
Vacuum to Covered Side
Airflow Out of Upper
Outlets
Full Airflow Out Defroster
Outlets (Full De-ice)
No Vacuum Applied
Air Door Open to
Re circulated
Air
(Open to Car Body)
Maximum Cooling
No Vacuum Applied Closes
Vacuum Supply to Power
Servo
Airflow Out of Upper and
Lower Outlets Door Open
1" from 100% Lower Out-
let Position
Airflow Divided 1/3 Out
Defroster Outlets
2 / 3
Out Heater Outlets
Supplies Modulated Vacuum to Power Servo
Zero Voltage Applied to Transducer
Results in Maximum Vacuum Supply
10 Volts Applied to Transducer
Results in No Vacuum Supply
(Coolant Hot) Passes Vacuum When System is Calling For Outside Air
MAINTENANCE AND ADJUSTMENTS
EVAPORATOR CONTROL VALVE (POA)
(Chevrolet, Chevelle, Comoro, and Corvette
Four Season) (Chevrolet Comfortron)
The only check for proper POA valve operation is to
check the suction pressure at the valve as during a
performance test. The POA valve is an absolute valve
and will provide different gauge readings based on the
altitude where the readings are being taken. Correct
gauge reading at sea level is 29.5 psig. Gauge readings
will be one-half psi higher for each additional 1000 feet
of elevation. The following table lists gauge readings at
different altitudes. If a valve gives improper gauge
readings, it must be replaced since it is not repairable
or adjustable.
29.5
30.0
30.5
31.0
31.5
32.0
32.5
33.0
33.5
34.0
34.5
psig.
psig.
psig.
psig.
psig.
psig.
psig.
psig.
psig.
psig.
psig.
— Sea 1
— 1000
— 2000
— 3000
— 4000
— 5000
— 6000
— 7000
— 8000
— 9000
Level
ft.
ft.
ft.
ft.
ft.
ft.
ft.
ft.
ft.
— 10000 ft.
THERMOSTATIC SWITCH
(Universal and Chevy II All-Weather System)
Thermostatic switches used in Universal and All-
Weather systems differ only in the capillary tube sensing
unit.
The Chevy n All-Weather System thermostatic switch
has an air sensing capillary which is coiled and attached
to the front of the evaporator core with plastic plugs.
This type of unit is controlled by the temperature of the
air leaving the evaporator.
Universal systems make use of a thermostatic switch
with a fin sensing capillary or a self-supporting air sens-
ing capillary. This capillary controls the switch by
sensing the temperature of the metal fins or the air
leaving the fins.
Checking for Proper Operation
1. Install the gauge set and set up the vehicle as
described under Performance Test.
2. Movement of the temperature control knob should
result in a definite change in suction pressure and
cycling of the compressor clutch.
•
If compressor continues to operate regardless of
the knob adjustment, it indicates that the points
CHEVROLET CHASSIS SERVICE MANUAL
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