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ENGINE ELECTRICAL
6D-27
intermittent high-speed driving is essential to good spark plug
performance as it provides increased and sustained
combustion heat that burns away any excess deposits of
carbon or oxide that may have accumulated from frequent
idling or continual stop-an-go or slow-speed driving. Spark
plugs are protected by an insulating nipple made of special
heat-resistant material which covers the spark plug terminal
and extends downward over a portion of the plug insulator.
These nipples prevent flash-over with resultant missing of
engine, even though a film is allowed to accumulate on
exposed portion of plug porcelains.
Fig. 6D-35 Spark Plug
-
Tapered Seat
NOTE: Do not mistake corona discharge for flash-over
or a shorted insulator. Corona is a steady blue light
appearing around insulator, just above the shell crimp.
It is the visible evidence of high-tension field, and has
no effect on ignition performance. Usually it can be
detected only in darkness. This discharge may repel dust
particles, leaving a clear ring on the insulator just above
the shell. This ring is sometimes mistakenly regarded as
evidence that combustion gases have blown out between
shell and insulator.
All spark plugs have'a type number on the insulator
which designates thread size as well as relative position of the
plug in the Heat Range. Type numbers starting with 4 are
14 mm. thread size.
The last digit of the type number indicates the Heat
Range position of the plug in the Heat Range System. Read
these numbers as you would a thermometer; the higher the
last digit, the hotter the plug will operate in engine; the lower
the last digit the cooler the plug.
IGNITION SWITCH
The switch is located in the steering column on the right
hand side just below the steering wheel. The electrical
switching portion of the assembly is separate from the key
and lock cylinder. However, both are synchronized and work
in conjunction with each other through the action of the
actuator rod assembly.
For a complete explanation of the key and lock cylinder,
and the actuator rod assembly, see STEERING, Section 3B.
The ignition and starting switch is key operated through
the actuator rod assembly to close the ignition primary circuit
and to energize the starting motor solenoid for cranking.
The ignition switch used on all cars has five positions:
Two " O F F positions ("OFF" and "OFF-LOCKED"),
"ACCESSORY", "RUN" and "START". "OFF" is the
center position of the key-lock cylinder, and "OFF-
LOCKED" is the next position to the left.
"ACCESSORY" is located one more detent to the left
of "OFF-LOCKED". Turning the key to the right of the
"OFF" position until spring pressure is felt will put the
ignition switch in the "RUN" position, and when turned fully
to the right against spring pressure, the swtich will be in the
"START" position.
The connections to the ignition switch are shown in the
Chassis Electrical Wiring Diagrams at the end of the manual.
The charts included on the diagrams show how the switches
are internally connected in each switch position. Fig. 6D-36
also shows how the switch is internally connected.
VACUUM ADVANCE SYSTEMS
All 1977 Pontiac engines are equipped with distributor
vacuum spark advance. Vacuum advance provides smooth
part throttle operation and good fuel economy. There are two
types of vacuum advance: 1) ported vacuumn and 2) full
manifold vacuum.
Ported vacuum advance systems have a timed port in the
carburetor throttle body above the throttle plates. The timed
port provides vacuum during open throttle operation only.
Full manifold vacuum advance systems use manifold
vacuum from either a manifold vacuum from either a
manifold vacuum port on the carburetor or a fitting in the
intake manifold. Vacuum is provided whenever the engine is
running.
The vacuum advance systems of various engines also
include some of the following vacuum controls:
Thermal Vacuum Switches
-
TVS
Vacuum Delay Valves
-
VDV
Vacuum Modulator Valves
-
VMV
Vacuum Reducer Valves
-
VRV
How each of the above vacuum controls is used on each
engine type is described below. Refer to Section 6E for proper
installation of vacuum hoses. Fig 6D-37 shows the various
switches, their connections, and their calibration valves.
L4-140 Engine
The L4-140 engine is equipped with a vacuum delay
valve, VDV, to maintain vacuum advance during quick
throttle openings when the engine coolant temperature is
below approximately 1 15°F. This feature improves engine
response during cold engine operation. The VDV is by-passed
thru the distributor spark TVS when the engine coolant
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