Chevrolet 1977 light duty truck, Service Manual

Page: 48 / 861

GENERAL DESCRIPTION
FO UR-SEASO N SYSTEM (C-K M ODELS)
Both the h e a tin g a n d cooling fu n ctio n s are
p erfo rm ed by this system. A ir e n te rin g the vehicle must
pass through the cooling unit (evaporator) and through
(or aro u n d ) the heatin g unit, in that order, and the
system is thus re fe rred to as a " r e h e a t " system.
The e v a p o ra to r provides m axim um cooling o f the
air passing through the core w hen the air conditioning
system is calling for cooling. A thermostatic switch,
located on the blow er-evaporator case, acts to control
compressor o p eratio n by sensing the evaporator inlet
line tem perature.
System operatio n is as follows" Air, either outside
air o r recirculated air, enters the system an d is forced
thro u g h the system by the blower. As the air passes
th rough the evaporator core, it receives m axim um
cooling if the air conditioning controls are calling for
cooling. A fter leaving the evaporator, the air enters the
H eater and Air C o nditioner Selector Duct Assembly
where, by m eans o f div erter doors, it is caused to pass
th ro u g h or to bypass the heater core in the proportions
necessary to provide the desired outlet tem perature.
C ond itio n ed air then enters the vehicle through either
the floor distributor duct or the dash outlets. W hen,
d u rin g cooling operations, the air is cooled by the
ev a p o ra to r to below com fort level, it is then w arm ed by
the h e a te r to the desired tem perature. D uring " h e a tin g
only" operations, the e v ap o rato r will not be in operation
and am b ie n t air will be w a rm e d to the desired level in
the same m anner.
T he dash outlets are rectangular in design. The
outlets can be rotated horizontally or vertically to direct
air as desired. U n d e r the left distributor duct is located a
floor cooler which can be rotated to provide cooling air
or shut off completely.
C 6 0 SYSTEM (G M O D ELS)
This system perform s both the heating and cooling
functions: however, the h e atin g and cooling systems
o p erate in dependently o f one a n o th er — the system
provides e ith e r heating or cooling and thus is referred to
as a "p a ra lle l system".
In the a ir conditioning modes, air (either a mixture
o f inside an d outside at N O R M or inside air only at
M AX) enters the system and passes through the
e v ap o rato r core (receiving m ax im u m cooling) an d is
then directed into the passenger c o m p a rtm e n t through
the dash outlets an d foot coolers.
A therm ister clipped to the evaporator core is used
to control c o m p re sso r o p e r a t i o n by sensing air
te m p e ra tu re as it leaves the ev a p o ra to r core. As the core
te m p e ra tu re increases the resistance o f the th erm ister to
400 ohms, the amplifier (on the control) furnishes
cu rre n t to energize the com pressor clutch coil. As the
ev a p o ra to r cools down, the te m p e ra tu re o f the thermister
decreases which biases the amplifier off and compressor
clutch coil is de-energized.
The dash outlets are rectangular in design and can
be rotated horizontally or vertically to direct air as
desired. Foot coolers are pro vided on both driver and
passenger side.
In the heater-defrost modes, the air conditioning
outside air door is closed. The h e a te r air door is open
and outside air is allowed to pass through the heater core
(receiving m axim um heating) and is then directed into
the passenger co m p a rtm e n t through the heater a n d / o r
defroster outlets.
OVERH EA D SYSTEM S (C-K-G M ODELS)
These systems (C69 on C -K -G Models) operate in
conjunction with the Four-Season System (C-K Models)
or C60 System (G M o d e ls )-th e y do not operate
independently.
N O T E : O verhead system kits are available for
non-factory installation on C-K Models, providing
the vehicle is equipped with the front system.
These units are self contained, o perating on inside
(recirculated) air only. Air is drawn into the unit, passed
through the e v ap o rato r core and then directed into the
passenger co m p a rtm e n t through the air distributor duct.
System control is through the front system. T he only
control on the overhead system is a three speed blower
switch.
M O TO R H O M E CHA SSIS SYSTEM
This system p erfo rm s the cooling functions only.
W hen heating (above am bient tem peratures) is desired,
the vehicle heater must be used. W h e n air conditioning
is desired, the heater should be completely shut off.
This self-contained unit is bracket m ounted to the
dash by the body m anufacturer. It operates on inside
(recirculated) air only. Air is draw n into the unit, passed
th ro u g h the e v a p o r a to r core (receiving m a x im u m
cooling) and then directed into the vehicle through
adjustable outlets.
A thermostatic switch, located on the face plate is
used to control compressor operatio n by sensing air
tem perature as it leaves the evap o rato r core.
BASIC A IR C O N D IT IO N E R
W hen we look at an air conditioning unit, we will
always find a set o f coils or a finned ra d ia to r core
through which the air to be cooled passes. This is known
as the " e v a p o ra to r" . The re frig eran t boils in the
evaporator. In boiling, or course, the re frig e ran t absorbs
heat and changes into a vapor. By piping this vapor
outside the car we can bodily carry out the heat that
caused its creation.
Once we get v a p o r out o f the evaporator, all we
have to do is remove the heat it contains. Since heat is