Chevrolet 1977 light duty truck, Service Manual

Page: 52 / 861

the only thing that expanded the refrigerant from a
liquid to a v a p o r in the first place, removal o f that same
h e a t will let the v a p o r condense into a liquid again. T hen
we can return the liquid re frig e ran t to the evaporator to
be used over again.
Actually, the v apor com ing out o f the evaporator is
very cold. We know the liquid refrig eran t boils at
tem p eratu res considerably below freezing and th at the
vapors arising from it are only a shade w a rm e r even
though they do contain quantities o f heat. Consequently,
we c a n ’t expect to remove heat from sub-freezing vapors
by " co o lin g " them in air tem peratures that usually range
between 15 C an d 38 C (60 °F and 100 °F)...heat refuses
to flow from a cold object toward a w a rm e r object.
But with a pum p, we can squeeze the heat-laden
v a p o r into a smaller space. And, w hen we compress the
vapor, we also concentrate the heat it contains. In this
way, we can make the v a p o r hotter without add in g any
heat. T hen we can cool it in com paratively w arm air.
T h a t is the only responsibility o f a compressor in an
air conditioning system. It is not intended to be a pum p
ju st for circulating the referig eran t. Rather, its jo b is to
exert pressure for two reasons. Pressure makes the vapor
hot enough to cool off in w arm air. At the same time, the
com pressor raises the r e f r i g e r a n t ’s pressure above the
condensing point at the te m p e ra tu re o f the surrounding
air so it will condense.
As the re frig e ran t leaves the compressor, it is still a
v a p o r although it is not quite hot and ready to give up
the heat th at it absorbed in the evaporator. One o f the
easiest ways to help re frig e ran t vapor discharge its heat
is to send it through a readiator-like co m p o n en t known
as a condenser.
The condenser really is a very simple device having
no moving parts. It does exactly the sam e jo b as the
fa m ilia r ra d ia to r in a typical home steam -heating
system. There, the steam is nothing more th an w ater
vapor. In passing through the radiator, the steam gives
up its heat and condenses back into water.
T he purpose o f the condenser, as the n am e implies,
is to condense the high pressure, high tem perature
re frig e ran t vap o r discharged by the compressor into a
high pressure liquid refrig eran t. This occurs w hen the
high pressure, high tem perature re frig eran t is subjected
to the considerably cooler
metal surfaces o f the
condenser. This is due to the fu n d a m e n ta l laws, which
state th at " h e a t travels from the w arm er to the cooler
surface," an d that "w hen heat is removed from vapor,
liquid is produced."
W h e n the re frig e ran t condenses into a liquid, it
again is ready for boiling in the evaporator. So, we run a
p ipe from the condenser back to the evaporator.
MAIN UNITS OF THE SYSTEM
T h ese th ree units th en ; the e v a p o ra to r, the
compressor, an d the condenser...are the m ain working
parts in any typical air conditioning system. We have the
ev a p o ra to r w here the re frig e ran t boils and changes into
a vapor, a b so rb in g heat as it does so. We have the pu m p
or com pressor to p u t pressure on the r e frig e ran t so it can
get rid o f its heat. A nd we have a condenser outside the
car body to help discharge the heat into the surrounding
air.
Now let’s look at the compressor in detail, and some
o f the com ponents that work with these m ain units to
complete the air conditioning system.
AXIAL SIX-CYLIN DER C O M PR ESSO R
The prim e purpose o f the compressor (fig. IB-4) is
to take the low pressure refrig eran t vapor produced by
the evaporator and compress it into a high pressure, high
tem perature vapor which will be sent on to the
condenser.
It utilizes the principle that "w hen a vap o r is
compressed, both its pressure and tem perature are
ra ise d " which we have already discussed. The axial six
cylinder compressor is m ounted above the engine in a
special ru b b e r m ounted bracket and is belt driven from
the engine through an electromagnetic clutch pulley on
the compressor.
The compressor has three double-acting pistons,
m aking it a six cylinder compressor. The compressor has
a l .5 inch bore and 1.1875 inch stroke, giving it a total
displacement o f 12.6 cu. in. Identification o f the
compressor is by model and serial n u m b e r stam ped on a
plate on top o f the compressor.
Clutch-Pulley
The movable part o f the clutch drive plate is in
fro n t o f the pulley and bearing assembly. The arm ature
plate, the movable m em ber, is attached to the drive hub
through driver springs and is riveted to both members.
The hub o f the drive plate is pressed over a square drive
key located in the compressor shaft. A spacer and
retainer ring are assembled to the shaft and the
assembly is held in place with a self-locking nut. The
pulley rim, power element ring and pulley hub are
form ed into a final assembly by molding a frictional
m aterial between the rim and the hub with the power
element ring im bedded in the forward face o f the
assembly.
A two-row ball bearing is pressed into the pulley
hub and held in place by a re ta in e r ring. The entire
pulley and bearing assembly is then pressed over the
front head o f the compressor and secured by a retainer
ring.
Clutch coil
The coil is molded into the coil housing with a filled
epoxy resin and must be replaced as a complete
assembly. Three protrusions on the rear o f the housing
fit into alignm ent holes in the compressor front head. A
retainer ring secures the coil and housing in place. The
coil has 3.85 ohms resistance at 26.7X1 (80 °F) am bient
tem perature and will require no more th an 3.2 amperes
at 12 vols D.C. Since the clutch coil is not grounded
internally, a ground lead is required as well as a " h o t"
lead.
Compressor Connector
C ompressor connectors, are attached to the com pres­
sor rear head by means o f a single bolt and lock washer.