D Ia G N O S Is; Refrigerant System - Chevrolet 1977 light duty truck Service Manual

REFRIGERANT SYSTEM

The following is a description o f the type o f
sym ptom each refrig eran t com ponent will evidence if a
defect occurs:
CO M PRESSO R
A compressor defect will ap p e a r in one o f four
ways: Noise, seizure, leakage, or low discharge pressure.
N O T E : R esonant com pressor noises are not cause
for alarm ; however, irregular noise or rattles m ay
indicate broken parts or excessive clearances due to
wear. To check seizure, de-energize the magnetic
clutch and check to see if drive plate can be
rotated. If rotation is impossible, compressor is
seized (See "False C om pressor Seizure"). To check
for a leak, refer to leak testing in the service
m anual. Low discharge pressure m ay be due to a
faulty internal seal o f
restriction in the compressor.
Low discharge pressure m ay also be due to an
insufficient refrigerant
elsewhere 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.
CO NDENSER
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 pressure. If a
partial restriction is present, sometimes ice or frost will
form im m ediately after the restriction as the refrigerant
expands after passing through the restriction. If air flow
through the condenser or rad ia to r is blocked, high
discharge pressures will result. D uring norm al condenser
operation, the outlet pipe will be slightly cooler than the
inlet pipe.
RECEIVER-DEHYD RATO R
A defective receiver-dehydrator may be due to a
restriction inside the body o f the unit. A restriction at
the inlet to the receiver-dehydrator will cause high head
pressures. Outlet tube restrictions will be indicated by
low head pressures and
excessively cold receiver-dehydrator
indicative o f a restriction.
E X P A N S IO N VALVE
A m alfunction o f the expansion valve will be caused
by one o f the following conditions: valve stuck open,
valve stuck closed, broken power element, a restricted
screen o r an im properly located or installed power
elem ent bult. The first three conditions require valve
DIAGNOSSS
the compressor, or a
charge or a restriction
little or no cooling. An
outlet may be
replacement. The last two may be corrected by replacing
the valve inlet screen and by properly installing the
power element bulb.
Attachment o f 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 o f expansion valve trouble are provided
by Perform ance Tests; consult Diagnostic Charts.
VALVE STU CK OPEN
NOISY COM PRESSOR.
N o Cooling - Freeze Up.
VALVE STUCK CLOSED, BROKEN POW ER
E L E M E N T OR P L U G G E D SCREEN
Very Low Suction Pressure.
N o Cooling.
PO ORLY LOCA TED POW ER E L E M E N T BULB
N orm al Pressure.
Poor Cooling.
Diagnosis for Defective Valve
The following procedure
determine if a malfunction
expansion valve.
1. Check to determ ine if the system will meet the
perform ance test as outlined previously. If the expansion
valve is defective, the low pressure readings (evaporator
pressure) will be above specifications.
2. The loss o f system perform ance is not as evident
when the compressor head pressure is below 200 psi.
Therefore, it m ay be necessary to increase the system
head pressure by partially blocking
Disconnect the blower
"perform ance check" to determ ine if the evaporator
pressure can be obtained.
3. The system will also indicate a low refrigerant
charge by bubbles occurring in the sight glass.
EVAPO RATOR
W hen the evaporator is defective, the trouble will
show up as an inadequate supply o f cool air. A partially
plugged core due to dirt, a cracked case, or a leaking seal
will generally be the cause.
REFR IG ER A N T LINE R E S TR IC T IO N S
Restrictions in the refrigerant lines will be indicated
as follows:
1. Suction Line - A restricted suction line will
cause low suction pressure
discharge pressure and iittle 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
must be followed
is due to a defective
the condenser.
lead wire and repeat the
at the compressor, low
to