ENGINE DIVISION SERVICE MANUAL
The vacuum advance system incorporates a vacuum
diaphragm unit which rotates the distributor sensor plate in
response to changes in carburetor throttle bore vacuum.
The two systems, mechanical and vacuum (Figure 5)
operate independently, yet work together to provide proper
Fig. 5 Spark Advance Systems
Mechanical (Centrifugal) Advance Mechanism
Vacuum Advance Diaphragm Unit
To perform effectively, the ignition system must be
properly maintained. It is important that ignition system
maintenance operations be performed at the specified time or
For ignition system maintenance intervals, procedures
and specifications, refer to the appropriate "Emission Control
Systems and Service Operations" section of the Truck Service
Manual as listed below.
For vehicles built in 1978, see
For vehicles built in 1979, see
For vehicles built in 1980, see
For vehicles built in 1981, see
Unfasten distributor cap retaining clips. Remove
distributor cap (with high tension cables) and position it
out of the way.
Disconnect vacuum hose from distributor vacuum
advance unit, if equipped.
Disconnect distributor wiring as follows:
Disconnect distributor leads from positive and
negative terminals of. ignition coil.
Disconnect deceleration throttle modulator lead
(white wire) at connector (where used).
Disconnect tachometer drive cable, if equipped.
Scribe a mark on distributor housing in line with tip of
rotor (Figure 6) and note position of rotor and
distributor housing in relation to surrounding engine
parts as reference points for reinstalling distributor.
Fig. 6 Rotor Position Reference Mark
Remove distributor hold-down bolt and clamp.
Carefully withdraw distributor from engine. Do not lose
gasket or "O" See Figure 7 for gasket or "O" ring
location on distributor.
CGES-145-U Page 5
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