GMC 1982 Light Duty Truck, Service Manual

Page: 74 / 1435

AIR CONDITIONING IB-31
E X P A N S IO N TUBE (ORIFICE)
R em oval a n d Installatio n
1. D ischarge the refrigerant from the system.
2. Disconnect liquid line at evaporator inlet and remove
expansion tube (orifice) from the inlet pipe.
3. To in stall, reverse the above procedure. W hen
in stallin g th e expansion tu b e (orifice) inside the
ev a p o ra tio r inlet pipe, it M U S T be in stalled w ith
" shorter screen end " inserted first.
SPECIFIC C O M P O N E N T D IA G N O S IS
The following is a description of the type of sym ptom
each refrig eran t com ponent will evidence if a defect occurs:
COM PRESSOR
A com pressor defect will appear in one of four ways:
Noise, seizure, leakage, or low discharge pressure.
NOTICE: R esonant com pressor noises are not cause for
alarm ; however, irregular noise or rattles may indicate
broken parts or excessive clearances due to wear. To
check seizure, de-energize the m agnetic clutch and
check to see if drive plate can be rotated. If rotation is
im possible, com pressor is seized (R e fe r to "F alse
C om pressor S e iz u re "). To check for a leak, refer to
leak testing. Low discharge pressure may be due to a
faulty internal seal of the com pressor, or a restriction in
the com pressor.
NOTICE: F urtherm ore, low discharge pressure m ay be
due to an insufficient refrig eran t charge or a restriction
elsewhere in the system. These possibilities should be
checked prior to servicing the com pressor. If the
com pressor is inoperative, but is not seized, check to see
if cu rren t is being supplied to the m agnetic clutch coil
term inals.
CO N D EN SER
A condenser may be defective in two ways: it may leak,
or it m ay be restricted. A condenser restriction will result in
excessive com pressor d isch arg e pressure. If a p a rtia l
re stric tio n is present, som etim es ice or frost will form
im m ediately after the restriction as the refrigerant expands
afte r passing through the restriction. If air flow through the
condenser or radiator is blocked, high discharge pressures
will result. During norm al condenser operation, the outlet
pipe will be slightly cooler than the inlet pipe.
RECEIVER-DEHYDRATO R - M O T O R H O M E
CHASSIS
A defective re c e iv e r-d e h y d ra to r m ay be due to a
restriction inside the body of the unit. A restriction at the
inlet to the receiv e r-d e h y d ra to r will cause high head
pressures. O utlet 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.
E X P A N S IO N V A LV E
A m alfunction of the expansion valve will be caused by
one of the following conditions: valve stuck open, valve stuck
closed, broken power elem ent, a restricted screen or an
im properly located or installed power elem ent bulb. The
first three conditions require valve replacem ent. The last two
may be corrected by replacing the valve inlet screen and by
properly installing the power elem ent bulb.
A tta c h m e n t of the expansion valve bulb to the
evaporator outlet line is very critical. The bulb m ust be
attached tightly to the line and m ust 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
perform ance tests; consult diagnostic charts.
1. V A LV E S T U C K O PE N
a. Noisy Com pressor
b. N o Cooling - Freeze Up.
2. Valve stuck closed, broken power elem ent or plugged
screen
a. Very Low Suction Pressure.
b. N o Cooling.
3. P O O R L Y L O C A T E D P O W E R E L E M E N T BULB
a. N orm al Pressure.
b. Poor Cooling.
Diagnosis for Defective V a lv e
T he following procedure m ust be followed to determ ine
if a m alfunction is due to a defective expansion valve.
1. C heck to d e te rm in e if the system will m eet the
p erfo rm an ce 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 perform ance is not as evident when
the com pressor head pressure is below 1 379 kPa (200
psi). Therefore, it may be necessary to increase the
system head p ressu re by p a rtially blocking the
condenser. Disconnect the blower lead wire and repeat
the "p e rfo rm a n c e c h eck " to d eterm in e if the
evaporator pressure can be obtained.
3. The system will also indicate a low refrigerant charge
by bubbles occurring in the sight glass (M otor Home
Chassis System s).
E V APO R A TO R
W hen 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
R estrictions in the refrigerant lines will be indicated as
follows:
1. Suction Line - A restricted suction line will cause low
suction pressure a t the com pressor, low d isch arg e
pressure and little or no cooling.
2. D ischarge 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 (M oto r H om e Chassis Units)
A t tem peratures higher th an 70°F (21°C ), the sight
glass m ay in d icate w h eter the re frig e ra n t ch arg e is
sufficient. A sh o rta g e of liquid re frig e ra n t is ind icated