Chevrolet Light Duty Truck 1973 Service Manual page 78

1A-52 HEATER A N D AIR C O N D IT IO N IN G
5. If the outlet temperature was less than the
prescribed temperature at the end of each cooling
cycle, turn the adjusting screw a partial turn
counterclockwise (fig. 53). If the outlet temperature
was more than prescribed temperature, turn the
adjusting screw clockwise.
NOTE: One turn of the adjusting screw will
change the outlet temperature
4 degrees.
6. Reinstall switch end plate and reinstall switch.
Reinstall face plate (Motor Home Chassis Units) or
evaporator cover (G models) before attempting a
performance test. Be sure that the air sensing
capillary has been replaced properly.
7. Check system performance. If further adjustment is
needed, repeat Steps 3 through 6 until the
prescribed pressure is reached.
NOTE: Do not attempt to run a Performance
Check with the system disassembled since
inaccurate readings would be the result.
ALWAYS reinstall switch and capillary and
any duct work before running a performance
check.
EXPANSION VALVE (Fig. 54)
An expansion valve is used on C-K Model Overhead
Systems, all G Model systems and Motor Home Chassis
Units.
A malfunction of the expansion valve will be caused by
one of the following conditions; valve stuck open, valve
stuck closed, broken power element, a restricted screen or
an improperly located or installed power element bulb.
Attachment of the expansion valve bulb to the
evaporator outlet pipe is very critical. The bulb must be
attached tightly to the pipe and must make good contact
with the pipe along the entire length of the bulb. A loose
THERMOSTATIC
SWITCH TO-
COVER SCREWS
Fig. 53-Thermostatic Switch Adjustment
approximately
SWITCH
FOAM
INSULATION
COVER
Fig. 54-Expansion Valve
bulb will result in high "high side" pressures and poor
cooling. On bulbs located outside the evaporator case,
insulation must be properly installed.
Indications of expansion valve trouble provided by the
Performance Test are as follows:
VALVE STUCK OPEN
Noisy Compressor.
No Cooling - Freeze Up.
VALVE STUCK CLOSED, PLUGGED SCREEN
OR BROKEN POWER ELEM ENT
Very Low Suction Pressure.
No Cooling.
POORLY LOCATED POWER ELEMENT BULB
Normal Pressure.
Poor Cooling.
Check for Defective Valve
The following procedure must be followed to determine
if a malfunction is due to a defective expansion valve.
1. Check to determine if the system will meet the
performance test as outlined previously. If the
expansion valve is defective, the low pressure
readings will be above specification.
2. The loss of system performance is not as evident
when the high side pressure is below 200 PSI.
Therefore, it may be necessary to increase the
system high side pressure by partially blocking the
condenser. Disconnect the blower lead wire and
repeat the "Performance Check" to determine if
the low side pressure can be obtained.
LIGHT DUTY TRUCK SERVICE MANUAL