GMC 4000 Series Service Manual page 104

INSUFFICIENT CO O LING D IA G N O SIS CHART NOTES
NOTE "A "
Suction throttling valve piston sticking; if stuck
closed, no cooling due to lack of flow of refriger­
ant through the evaporator core; if stuck open no
controlled cooling and cab may get too cold - evap­
orator may freeze. Replace valve.
NOTE "B"
System with excess discharge pressure should be
slowly depressurized at the receiver-dehydrator
inlet connection, observing the behavior of the high
pressure gauge indicator.
1. If discharge pressure drops rapidly, it in­
dicates air (with the possibility of moisture) in the
system . When pressure drop levels but still indi­
cates in excess of specifications shown in the
OPERATIONAL TEST DATA CHART, slowly bleed
system until bubbles appear in the sight glass and
stop. Add refrigerant until bubbles clear, then add
one (1) pound of refrigerant. Recheck operational
p ressu res. If discharge pressure still remains
above specifications and the suction pressure is
slightly above normal, then a restriction exists in
the high pressure side of the system .
2. If discharge pressure drops slowly, it in­
dicates excessive refrigerant. If pressure drops to
specifications and sight glass remains clear, stop
depressurizing and recheck operational p ressu res.
If p ressu res are satisfactory, depressurize until
bubbles appear in the sight glass, stop depressur­
izing, then add one (1) pound of refrigerant. Re­
check operational p ressu res.
3. If discharge pressure remains high after
depressurizing the system , continue depressuriz-
ing until bubbles appear in the sight gla ss. If sue-
tion p ressu res also remain high, then the suction
throttling valve may require adjustment, as well
Refer to "Refrigerant Line Connections Torque Specifications" chart on following page.
G M C S E R V I C E M A N U A L
(Used in Conjunction With Chart on Opposite Page)
as a possibility of a restriction in the high p res­
sure side of the refrigeration system . The system
will have high pressure control more frequently
under this condition.
Install gauge set and bleed off refrigerant from
com pressor suction and discharge side for 20 s e c ­
onds. After 20 seconds close valves and recheck
operating p ressu res. Repeat until discharge p res­
sure is normal. Check sight glass; if bubbles ap­
pear it indicates that air was in system . Charge
with refrigerant as follows: 2000 engine rpm,
"OUTSIDE" air, "HI" blower and maximum cooling.
Add refrigerant until sight glass clears, then add
1 pound additional.
NOTE "C"
Check for presence of bubbles or foam. If bubbles
or foam is noted, charge with refrigerant as fol­
lows: 2000 engine rpm, "OUTSIDE" air, "HI"
blower and Maximum Cooling. Add refrigerant
until sight glass clears, then add an additional 1
pound.
NOTE: It is not unusual for bubbling to occur on
minimum cooling and "LO" blower in mild weather
even with a fully charged system .
NOTE "D "
Remove insulation and inspect for clearance be­
tween tube and bulb. If gap exists, move bulb to
establish contact, reclamp and reinsulate.
NOTE " E"
Remove expansion valve and inspect internal screen
for foreign objects. If present, there is a p ossi-
bility seat is being held open. Install new expan-
sion valve; if condition is corrected, discard the
valve removed.
Sec. 1
Page 97
AIR CONDITIONING