Chevrolet 70 Series 1970, Service Manual

Page: 145 / 841

CAB AND BODY MOUNTINGS 1-114
Figure 15—Evaporator Pressure Control Valve (P.O.A.I
SUCTION THROTTLING VALVE
(CONVENTIONAL CAB MODELS)
The m ain function of the Suction Throttling
V alve (fig. 16) is to m aintain the evaporator p r e s ­
su r e at a p r e ssu r e su fficien tly high to a v o id fr e e z -
ing of m oisture on the evaporator core and at the
sa m e tim e provide maximum cooling effic ien c y .
The opposing fo r c e s , spring p r e ssu r e and at­
m osp h eric p r e ssu r e on the one sid e of the valve
Figure J 6—Suction Throttling Valve
diaphragm and evaporator p r e s s u r e on the opposite
sid e of the diaphragm constitute a balanced valve
and control its operation.
When the evaporator p r e ssu r e r i s e s above the
manual spring settin g , this in c r e a se in evaporator
p r e s s u r e is exerted against the v a lv e p iston and
through the b leed h oles in the p iston to the under­
sid e of the diaphragm . The p r e s s u r e s under the
diaphragm o v erco m es the sp rin g p r e ssu r e causing
the valve p iston to m ove in an opening direction .
A s the p iston opens, it rea ch e s a balanced p osition
and the evaporator p r e ssu r e returns to it s origin al
settin g (23-24 p sig ).
When the evaporator p r e ssu r e drops below the
pred eterm in ed settin g , the sp rin g p r e s s u r e o v e r ­
co m e s the opposing evaporator p r e s s u r e a n d fo rces
the p iston in a c lo sin g direction until the p r e d e te r ­
m ined p r e ssu r e setting i s m aintained (23-24 p sig ).
The m inim um operating p r e s s u r e of the suction
throttling valve is p r e - s e t by an adjustm ent sc r e w .
M aximum operating p r e ssu r e may be obtained by
m eans of a cable operated from the instrum ent
panel to the le v e r arm on the v a lv e. When reduced
load conditions require elevated control p r e s s u r e s ,
the le v e r arm actu ates the inner sp rin g a ssem b ly
causing the p isto n to th rottle in a clo sin g direction.
T his r e su lts in a higher evaporation p r e s s u r e and
the d e sir e d in c r e a se in d isch arge a ir tem p eratu re.
COMPRESSOR
The co m p r e sso r i s of b a sic double-action
p iston design. T hree horizontal double acting
p isto n s make up a six -c y lin d e r co m p r e sso r , and
a r e mounted axially around the co m p r e sso r shaft
to operate in a front and re a r cylin d er a ssem b ly .
T h ese p isto n s operate in a VA" b ore, have a 1 -3 /1 6 "
strok e and a r e actuated by a sw ash p late p r e s s e d
on the c o m p resso r crankshaft (s e e fig . 17).
R eed-type suction and d isch arge v a lv e s are
mounted in valve p la tes betw een the cylin d er a s ­
sem b ly and the head at each end of the co m p r e sso r .
The heads a r e connected with each other by g a s-
tight p a ssa g e w ays which d irect refrig era n t gas to
a com m on output.
The c o m p resso r m ainshaft is driven by the
pulley when the clutch c o il is en erg ized . It extends
through the c o m p r e sso r front head, to the com ­
p r e s s o r rea r head and d riv es the o il pump in the
rea r head pump c a v itie s.
The co m p resso r is fitted with a high p r e ssu r e
r e lie f v a lv e. If the d isch arge p r e s s u r e ev er ex­
cee d s approxim ately 440 p s i, the r e lie f valve opens
autom atically to r e lie v e the p r e s s u r e and c lo s e s
again when the p r e ssu r e r e c e d e s.
The p ulley a sse m b ly contains an e le c tr ica lly
con trolled m agnetic clutch, perm ittin g the com ­
LEVER HELPER
SPRINGS
ADJUSTING
PIN NUT ADJUSTING
FROM
EVAPORATOR
CONTROL
LEVER
ASSEMBLY OUTER
SPRING
TO COMPRESSOR
T-3310
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL