Thermostatic Switch; Checking For Proper Operation - Chevrolet 10 1971 Series Chassis Service Manual

HEATER AND AIR CONDITIONING 1A-18
G M Chevrolet Air Conditioning
(Refrigerant Charge - 3 lbs.-4 oz.)
Temperature of
Air Entering 70°
Condenser
Engine rpm
Compressor 110-
Head Pressure* 120
Suction Pres­
6
sure psi*
40-
Discharge Air
45
Temperature*
*When compressor clutch disengages.
EVAPORATOR CONTROL VALVE (POA) (Fig. 26)
The only check for proper POA valve operation is to
check the suction pressure at the valve during a per­
formance test. The POA valve is an absolute valve and
will provide different gauge readings based on the alti­
tude where the readings are being taken. Correct gauge
reading at sea level is 29.5 psig.
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 psig. — Sea Level
30.0 psig. — 1000 ft.
30.5 psig. — 2000 ft.
31.0 psig. — 3000 ft.
31.5 psig. — 4000 ft.
32.0 psig. — 5000 ft.
32.5 psig. — 6000 ft.
33.0 psig. — 7000 ft.
33.5 psig. -- 8000 ft.
34.0 psig. — 9000 ft.
34.5 psig. — 10000 ft.

THERMOSTATIC SWITCH

Roof-Mounted and G M Chevrolet Systems
These systems make use of a thermostatic switch with
an air sensing capillary.
switch by sensing the temperature of the air leaving the
fins (fig. 27).

Checking for Proper Operation

1. Install the gauge set and set up the vehicle as des-
scribed 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 continued to operate regardless of
the knob adjustment, it indicates that the switch
points are fused which will lead to evaporator
freeze-up. Replace the switch.
• If the compressor does not operate, regardless of
the position of the knob, a loss of the power element
120°
80° 90° 100° 110°
2000
135- 160- 190- 220- 260-
145 200 230 270
170
7 9 10 10 13
41- 41- 42- 44- 44-
46 46 47 49 49
Gauge readings will
This capillary controls the
OUTLET
Fig. 26— Evaporator Pressure Control Valve (PO A )
charge is indicated (provided that it has been estab­
lished that power is supplied to the switch). This,
of course, results in no cooling. Replace the switch.
THERMOSTATIC SWITCH THERMOSTATIC
CAPILLARY INSTALLED
IN BRACKET
G M CHEVROLET SYSTEM
Fig. 2 7 — Thermostatic Switch Installation
BLOWER
SWITCH SWITCH
10-30 CHEVROLET TRUCK SERVICE MANUAL