Chevrolet 1977 light duty truck, Service Manual

Page: 60 / 861

Fig. IB -1 3 -O ve rhe a d U n it Control (C-K Models)
lever on C O LD , blower switch on HI and the
overhead unit blower switch should be on HI.
C 6 0 SYSTEM (G M O D ELS)~FIG . IB -1 4
T h e system selector lever (air control lever)
determ ines the mode o f operation: OFF, MAX, N O R M ,
HEAT, D E F O G or D E F . W h e n the system selector lever
is placed in the A / C , positions, electrical circuit
connection is m ade to the compressor clutch through the
control panel switch and the discharge pressure switch. If
the switch is closed (am b ien t tem p eratu re above 4 .4 XI.),
the compressor will run. In the O F F , HEAT, D E F O G or
D E F positions, the compressor is not energized.
T he system selector lever also determ ines the
direction o f outlet air flow. M oving the lever fro m mode
to m ode varies the position o f a rotary vacuum valve on
the control. The position o f the vacuum valve will supply
vacuum to, or vent, vacuum d ia p h ra g m s which position
the air and defroster doors in the heater distributor duct
and the outside air door in the right side o f the cowl
plenum . The position o f these doors determ ines if ouput
air flow is from the A / C outlets using recirc air (MAX),
or recirc and outside air (N O R M ), the heater outlet
(HEAT), heater an d defroster outlets (D E F O G ), or the
d efroster nozzles with slight air flow from the heater
outlet (DEF).
T em perature Control
(A /C Modes)
T he tem p eratu re lever activates a p otentiom eter on
the control assembly (connected in series with the
am plifier on the control and t h e rm is te r on the
evaporator) to control compressor operation.
(Heater-Defrost Modes)
The tem peature lever determ ines the tem perature o f
outlet air flow by positioning the tem perature door in the
heater distributor assembly, through the motion o f a
bowden cable linking the control panel lever to the
tem perature door.
Fan Switch
The blower (F A N ) switch provides a m eans o f
selecting the am o u n t o f air flow from the system by
regulating the speed o f the heater or A / C blower motor.
There is no O F F position on the f a n switch; however the
blower is inoperative if the selector lever is placed in the
O F F position. If the selector lever is placed in any o f the
heater or A / C modes, the ap p ro p riate blower is
operative. The blower speeds available are LO, M l, M2
and HI.
O VERHEAD SYSTEM (G M ODELS)
This system opeates in conjunction with the C60
system. Since refrig eran t is controlled by the C60 system,
the only control provided on the re a r overhead system is
a three speed blower switch (fig. 1B -15).
In the O F F position, the blower is inopertive;
however, refrig eran t is circulating in the system if the
front system is ON. To operate the rear overhead system,
simply select the desired blower speed (LOW, M ED, HI).
W hen air circulation only is desired, the re a r A /C
Fig. IB -1 5 -O ve rhe a d U n it C ontrol (G Models)
Fig. 1B-14--C60 System C ontrols (G Models)
Fig. lB -1 6 -M o to r Home U n it Control