Chevrolet Light Duty Truck 1973 Service Manual page 105

pressures. Outlet tube restrictions will be indicated by
low head pressures and little or no cooling. An
excessively cold receiver-dehydrator outlet may be
indicative of a restriction.
Expansion Valve
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.
The first three conditions require valve replacement. The
last two may be corrected by replacing the valve inlet
screen and by properly installing the power element
bulb.
Attachment of the expansion valve bulb to the
evaporator outlet line is very critical. The bulb must be
attached tightly to the line and must make good contact
with the line along the entire length of the bulb. A loose
bulb will result in high low side pressures and poor
cooling.
Indications of expansion valve trouble are provided by
Performance Tests; consult Diagnostic Charts.
VALVE STUCK OPEN
Noisy Compressor.
No Cooling - Freeze Up.
VALVE STUCK CLOSED, BROKEN POWER
ELEMENT OR PLUGGED SCREEN
Very Low Suction Pressure.
No Cooling.
POORLY LOCATED POWER ELEMENT BULB
Normal Pressure.
Poor Cooling.
Diagnosis 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 (evaporator pressure) will be above
specifications.
2. The loss of system performance is not as evident
when the compressor head pressure is below 200
psi. Therefore, it may be necessary to increase the
system head pressure by partially blocking the
HEATER A N D AIR C O N D IT IO N IN G
condenser. Disconnect the blower lead wire and
repeat the "perform ance check" to determine if the
evaporator pressure can be obtained.
3. The system will also indicate a low refrigerant
charge by bubbles occurring in the sight glass.
Evaporator
When the evaporator is defective, the trouble will show
up as an inadequate supply of cool air. A partially
plugged core due to dirt, a cracked case, or a leaking seal
will generally be the cause.
Refrigerant Line Restrictions
Restrictions in the refrigerant lines will be indicated as
follows:
1. Suction Line - A restricted suction line will cause
low suction pressure at the compressor, low
discharge pressure and little or no cooling.
2. Discharge Line - A restriction in the discharge line
generally will cause the pressure relief valve to
open.
3. Liquid Line - A liquid line restriction will be
evidenced by low discharge and suction pressure,
and insufficient cooling.
Sight Glass Diagnosis (G and Motor Home
Chassis Units)
At temperatures higher than 70 degrees F, the sight glass
may indicate whether the refrigerant charge is sufficient.
A shortage of liquid refrigerant is indicated after about
five minutes of compressor operation by the appearance
of slow-moving bubles (vapor) or a broken column of
refrigerant under the glass. Continuous bubbles may
appear in a properly charged system on a cool day. This
is a normal situation. If the sight glass is generally clear
and performance is satisfactory, occasional bubbles do
not indicate refrigerant shortage.
If the sight glass consistently shows foaming or a broken
liquid column, it should be observed after partially
blocking the air to the condenser. If under this condition
the sight glass clears and the performance is otherwise
satisfactory, the charge shall be considered adequate.
In all instances where the indications of refrigerant
shortage continues, additional refrigerant should be
added in 1/4 lb. increments until the sight glass is clear.
An additional charge of 1/2 lb. should be added as a
reserve after the glass clears. In no case should the
system be overcharged.
1A-79
LIGHT DUTY TRUCK SERVICE MANUAL