GMC 1982 Light Duty Truck, Service Manual

Page: 908 / 1435

6 D -3 2 ENGINE ELECTRICAL
GROUND
TERMINAL ^ T E R M IN A L
(CONNECTED TO
IGNITION SWITCH)
LATCH (4)
CONNECT TACHOMETER
FROM THIS TERMINAL
TO GROUND.
TACH TERMINAL
(SOME TACHOMETERS
MUST CONNECT FROM
THIS TERMINAL TO
ENERGIZER POSITIVE (+)
CONSULT TACHOMETER
MANUFACTURER.)
Fig. 6D-13D--HEI Distributor Assembly
IG N IT IO N T IM IN G
Timing specifications for each engine are listed in
Section 6E and on the Vehicle Emissions Control
Information label on the radiator support. When using a
timing light, connect an adapter between the No. 1 spark
plug and the No. 1 spark plug wire, or use an inductive type
pick-up. Do not pierce the plug lead. Once the insulation of
the spark plug cable has been broken, voltage will jump to
the nearest ground, and the spark plug will not fire properly.
The timing procedure remains the same as the conventional
ignition system. Always follow Vehicle Emissions Control
Information label procedures when adjusting timing.
Some engines will incorporate a magnetic timing probe
hole for use with special electronic timing equipment.
Consult manufacturer’s instructions for use of this
equipment.
S E C O N D A R Y W IR IN G
The spark plug wiring used with the HEI system is a
carbon impregnated cord conductor encased in an 8mm
diameter silicone rubber jacket. The silicone wiring will
withstand very high temperatures and also provides an
excellent insulator for the higher voltage of the HEI system.
The silicone spark plug boots form a tight seal on the plug
and the boot should be twisted 1/2 turn before removing.
Care should also be exercised when connecting a timing
light or other pick-up equipment. Do not force contacts
between the boot and wiring or through the silicone jacket.
Connections should be made in parallel using an adapter.
DO NOT pull on the wire to remove. Pull on the boot, or use
a tool designed for this purpose.
SPARK PLUGS (FIG . 6 D -1 7 D )
Resistor type, tapered seat spark plugs are used on all
gasoline engines. No gasket is used on these tapered seat
plugs. See Fig. 6D-18D for an explanation of letter coding
on spark plugs.
See Engine Exhaust Emissions Section (6E) for spark
plug application and gap sizes. Always replace plugs with
the correct plug listed on the Vehicle Emissions Control
Information label.
Normal or average service is assumed to be a mixture
of idling, slow speed, and high speed operation with some of
each making up the daily total driving. Occasional or
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-and-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.
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