GMC 1976 ZEO 6083 Maintenance Manual page 116

the magnetic clutch and check to see if drive
plate can be rotated. If rotation is impossible,
compressor is seized (See "False Compressor
Seizure"). To check for a leak, refer to leak test-
ing earlier in this section. Low discharge pres-
sure may be due to a faulty internal seal of the
compressor, or a restriction in the compressor.
Low discharge pressure may also be due to an
insufficient refrigerant charge or a restriction else-
where in the system . These possibilities should be
checked prior to servicing the compressor . If the
compressor is inoperative, but is not seized, check to
see if current is being supplied to the magnetic clutch
coil terminals.
CONDENSER
A condenser may be defective in two ways: it may
leak, or it may be restricted . A condenser restriction
will result in excessive compressor discharge pres
sure . If a partial restriction is present, sometimes ice
or frost will form immediately after the restriction as
the refrigerant expands after passing through the re-
striction. If air flow through the condenser or radia-
tor is blocked, high discharge pressures will result .
During normal condenser operation, the outlet pipe
will be slightly cooler than the inlet pipe.
RECEIVER-DEHYDRATOR
A defective receiver-dehydrator may be due to a
restriction inside the body of the unit. A restriction
at the inlet to the receiver-dehydrator will cause high
head pressures. Outlet tube restrictions will be in-
dicated by low head pressures and little or no cool-
ing. 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 ele
ment, a restricted screen or an improperly located or
installed power element bulb . The first three condi-
tions 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 pres-
sures and poor cooling.
Indications of expansion valve trouble are prov-
ided by Performance Test; consult Diagnostic
Charts .
AIR CONDITIONING SYSTEM
1- 99
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 de-
termine if a malfunction is due to a defective expan-
sion valve.
1 . Check to determine if the system will meet the
performance test as outlined previously . If the expan-
sion valve is defective, the low pressure readings
(evaporator pressure) will be above specifications .
2. The loss of system performance is not as evi-
dent when the compressor head pressure is below
200 psi. Therefore, it may be necessary to increase
the system head pressure by partially blocking the
condenser. Disconnect the blower lead wire and re-
peat the "performance 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 par-
tially 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 in-
dicated 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 dis-
charge line generally will cause the pressure relief
valve to open.