Chevrolet Light Duty Truck 1973 Service Manual page 113

VACUUM SYSTEM DIAGNOSIS
(C-K FOUR-SEASON SYSTEM)
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 airflow, air door
functioning and vacuum circuits. If airflow is not out of
the proper outlets at each selector lever position, then
proceed as follows:
1. Check for good hose connections-at
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 Daigram). 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
downstream.
b.
Vacuum Less Than Normal at Some Positions-
If vacuum was low at one or several of the
HEATER AND AIR C O N D IT IO N IN G
selector lever positions, a leak is indicated in
these circuits.
c.
Vacuum Normal at All Positions-
If vacuum was normal and even at all positions,
then the malfunction is probably caused by
improperly connected or plugged lines or a
defective vacuum valve or valves.
3.
Specific Vacuum Circuit Check
the vacuum
Place the selector lever in the malfunctioning
position 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 determine whether the cause is the vacuum
harness or 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.
1A-87
LIGHT DUTY TRUCK SERVICE MANUAL