GMC 1976 ZEO 6083 Maintenance Manual page 120

3 . Liquid Line - A liquid line restriction
will
be
evidenced by low discharge and suction pressure,
and insufficient cooling.
SIGHT GLASS DIAGNOSIS
At temperatures higher than 70°F . (21 .1°C .), the
sight glass may indicate whether the refrigerant
charge is sufficient . A shortage of liquid refrigerant
is indicated after about five minutes of compressor
operation by the appearance of slow-moving bubbles
(vapor) or a broken column of refrigerant under the
glass. Continuous bubbles may appear in a properly
charged system on a cool day. This is a normal situa-
tion . If the sight glass is generally clear and perform-
ance is satisfactory, occasional bubbles do not
indicate refrigerant shortage.
If the sight glass consistently shows foaming or
a broken liquid column, it should be observed after
partially blocking the air to the condenser. If under
this condition the sight glass clears and the perform-
ance is otherwise satisfactory, the charge shall be
considered adequate .
In all instances where the indications of refriger-
ant shortage continues, additional refrigerant should
be added in 1/4 lb . increments until the sight glass
is clear. An additional charge of 1/2 lb . should be
added as a reserve after the glass clears . In no case
should the system be overcharged.
ELECTRICAL SYSTEM
For electrical connections and routings, refer to
the wiring diagram at the end of the manual . Also see
Figure 109.
VACUUM SYSTEM DIAGNOSIS
Start the engine and allow it to idle - move the
selector lever to each position and refer to the
vacuum diagrams and operational charts for proper
AIR CONDITIONING SYSTEM 1- 103
airflow, air door functioning and vacuum circuits . If
airflow is not out of the proper outlets at each selec-
tor lever position, then proceed as follows:
1 . Check for good hose connection -
at the vacuum
actuators, control head 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 Vacuum Diagram) . Idle
the engine and read the vacuum (a normal vacuum
is equivalent to manifold vacuum) at all selector
lever positions.
a.
Vacuum Less Than Normal At All Positions -
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 normal, then the problem lies down-
stream .
b.
Vacuum Less Than Normal at Some Positions -
If vacuum was low at one or several of the
selector lever positions, a leak is indicated in these
circuits .
c.
Vacuum Normal at All Positions -
If vacuum was normal and even at all posi-
tions, then the malfunction is probably caused by
improperly connected or plugged lines or a defective
vacuum valve or valves .
3 .
Specific Vacuum Circuit Check
Place the selector lever in the malfunctioning po-
sition and check for vacuum at the pertinent vacuum
actuators. If vacuum exists at the actuator but the
door 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 deter-
mine whether the cause is the vacuum harness or the
vacuum valve. Check the vacuum harness first.