Chevrolet 1977 light duty truck, Service Manual

Page: 63 / 861

evidenced by low discharge and suction pressure, and
insufficient cooling.
S ight Glass Diagnosis (G and M o to r H om e
Chassis U nits)
At tem peratures higher th an 21 XT (70 degrees F),
the sight glass may indicate w heter the refrig eran t
charge is sufficient. A shortage o f liquid re frig eran t is
in dicated a fte r above five minutes o f compressor
o p e ra tio n by the app earan ce o f slow-moving bubbles
(vapor) or a broken column o f re frig eran t under the
glass. C ontinuous bubbles m ay a p p e a r in a properly
c h arg ed system on a cool day. This is a n o rm a l situation.
If the sight glass is generally clear and pe rfo rm a n c e is
satisfactory, occasional bubbles do not indicate re frig e r­
an t shortage.
If the sight glass consistently shows fo am in g o r a
broken liquid column, it should be observed after
p artially blocking the air to the condenser. If und er this
condition the sight glass clears an d the p erfo rm an ce is
otherwise satisfactory, the charge shall be considered
adequate.
In all instances w here the indications o f re frig e ran t
shortage continues, additional r e frig e ran t should be
a d d ed in 1/4 lb. increments until the sight glass is clear.
A n additional charge o f 1/2 lb. should be added as a
reserve a fte r the glass clears. In no case should the
system be overcharged.
VACUUM SYSTEM DIAGNOSIS
(C-K-G FOUR-SEASON S YS TE M )
Start the engine and allow it to idle - move the
selector lever to each position and refer to the vacuum
d ia g ra m s and operational charts for pro p er airflow, air
d oor functioning and vacuum circuits. If air flow is not
out o f the p ro p e r outlets at each selector lever position,
then proceed as follows:
1.
Check fo r good hose co n n ectio n s-at
the
vacuum actuators, control h e a d valve, reservoir, tees, etc.
2. Check the vacuum source circuit as follows:
Install vacuum tee and gauge (with restrictor) at the
vacuum tank outlet (see V acuum D iagram ). Idle the
en g in e and read the vacuum (a norm al vacuum is
equivalent to m anifold vacuum ) at all selector lever
positions.
a.
Vacuum Less Than N orm al At All P osition s-
Remove the tee and connect the vacuum gauge
line directly to the tank - read the vacuum. If
still low, then the problem lies in the feed circuit,
the feed circuit to the tank or in the tank itself.
If vacuum is now norm al, then the problem lies
downstream.
b.
Vacuum Less T han N orm al at Some Posi­
tio n s -
If vacuum was low at one or several o f the
selector lever positions, a leak is indicated in
these circuits.
c.
Vacuum N orm al at Alt P o sition s-
If vacuum was n o rm al and even at all positions,
then the m alfunction is pro b ab ly caused by
im properly connected or plugged lines or a
defective vacuum valve or valves.
3. Specific Vacuum Circuit Check
Place the selector lever in the m alfunctioning
position and check for vacuum at the p ertinent
vacuum actuators. If vacuum exists at the actuator
but the d oor does not move, then the actuator is
defective or the door is mechanically bound. If low
or no vacuum exists at the actuator, then the next
step is to d eterm ine w hether the cause is the vacuum
harness o r the vacuum valve. Check the vacuum
harness first.
4. Vacuum Harness Circuit Check
a. Disconnect the vacuum harness at the control
head.
b. The black line ( # 1 ) should show engine
vacuum - if not, trace back through connector to vacuum
tank.
c. To check any individual circuit place the
selector lever at the involved circuit position and check
for vacuum presence.
C H EC K IN G SYSTEM O P E R A T IO N
1. O perate system for a m axim um o f five minutes
at m axim um cooling, high blower speed and with engine
operating at 2000 R P M (exhaust should be vented if
inside).
2. W hen system is stabilized, the pressure gauges
on the charging station should read pressures corre­
sponding to values listed und er P E R F O R M A N C E
DATA.
3. W h e n correct system pressures are observed,
check system charge as described under " R e frig e ra n t
Quick Check Procedure".
4.
Feel outlet air distribution to ensure th at cold
air is being distributed.
5. Disconnect gauge lines and cap fittings.
C A U T IO N :
When removing gauge lines fr o m
fittings, be sure to remove the adapters fr o m the
fittin g s rather than the gauge lines fr o m the
adapters.
PER FO R M A N C E TEST
U n d e r norm al circumstances, it will not be necessary
to Perform ance Test a sytem as outlined below; however,
in certain instances, the following procedure m ay be
advantageous in diagnosing system malfunction.
The following fixed conditions must be adhered to
in o rder to make it possible to com pare the pe rfo rm a n c e
o f the system being tested with the standards below:
1. Doors and window closed. (Vehicle inside or in
shade.)
2. Hood up and engine exhaust suitably ventilated.
3. Vehicle in N E U T R A L with engine run n in g at
2000 rpm.
4.
Air C o nditioning controls set for -
• M axim um cooling.
• High blower speed