GMC 4000 Series Service Manual page 85

Sec. 1
Page 78
AIR CONDITIONING
Figure 8 — Receiver-Dehydrator
The com pressor mainshaft is driven by the
pulley when the clutch coil is energized. It extends
through the com pressor front head, to the com­
pressor rear head and drives the oil pump in the
rear head pump cavities.
The com pressor is fitted with a high pressure
relief valve. If the discharge pressure ever ex­
ceeds approximately 440 psi, the relief valve opens
automatically to relieve the pressure and closes
again when the pressure recedes.
The pulley assembly contains an electrically
controlled magnetic clutch, permitting the com­
pressor to operate only when air conditioning is
desired.
The clutch actuating coil is molded into the
coil housing with a potted epoxy resin; therefore,
the coil and housing are replaceable only as a com­
plete assembly. The coil has 3.85 ohms resistance
at 80 F. (surrounding temperature) and should not
demand more than 3.2 amperes of 12 V.D.C.
Compressor Operation
When the "AIR COND." lever is placed in "ON,"
the electrical circuit to the compressor clutch
clo ses.
Current flowing through the coil creates a
magnetic force which flows through the pulley to
draw the armature plate (forward of the pulley a s­
sembly) rearward toward the pulley. As the arm­
ature plate moves toward the pulley, it contacts
the pulley shaft face (which rotates freely about
the com pressor shaft).
G M C S E R V I C E M A N U A L
LIGHT
MIRROR
SIG H T G L A SS
When the "AIR COND." knob is placed to "OFF,"
the electrical circuit to the com pressor clutch is
opened and the magnetic pull on the clutch no long­
er exists. The armature plate to driven ring ac­
tuating springs will then pull the armature plate
away from the pulley and the plate loses contact
with the pulley.
A gauge fitting containing a Schrader valve is
located in both the discharge and suction passage
to permit pressure gauge readings.
CONDENSER
The condenser is sim ilar to the ordinary rad­
iator but is designed to withstand much higher
p ressures. It is made up of tubes which carry the
refrigerant and cooling fins which provide rapid
transfer of heat.
The condenser is located in front of the engine
cooling system radiator so that it receives a high
volume of air from the movement of the vehicle
and from the engine fan. Air passing over the con­
denser cools the hot high pressure refrigerant gas,
causing it to condense into high pressure liquid
refrigerant.
RECEIVER-DEHYDRATOR
ASSEMBLY
The receiver-dehydrator assembly is mounted
vertically along the right rear side of the radiator
support.
The purpose of the receiver part of this s y s­
tem is to insure a solid column of liquid refrig­
erant to the expansion valve at all tim es, provided
the system is properly charged.
The liquid indicator (many tim es referred to
as a sight glass) is in the refrigeration system as
an aid to diagnosis (see fig. 8). The appearance of
bubbles or foam beneath the sight glass (liquid in­
dicator) above 70°F., ambient indicates air or a
partial discharge of refrigerant in the system . A
solid liquid column as seen in the sight glass is
difficult to tell from one that has no refrigerant in
the system at all. Two ways to establish whether
the system is properly charged or empty are to
feel the suction pipe in the suction throttling valve
or to disconnect the com pressor clutch while ob­
serving the sight glass. If the system has the proper
refrigerant charge, the suction line at the suction
throttling valve will be cool. Also, the refrigerant
column in the sight glass will be seen to collapse
soon after the clutch has been disconnected. Foam
may be noted in the sight glass below 70°F., even
when the system is free of air and properly charg­
ed. Details of these conditions are in the "Insuf­
ficient Cooling Diagnosis Chart,"
Liquid refrigerant from the condenser enters
the receiver to flow into the upper portion of the
receiver which contains desiccant confined in a