Spark Plug Wires; Spark Plugs - GMC G Service Manual

6D-36 ENGINE ELECTRICAL
ENGINES WITH TBI
On engines with TBI, spark timing changes are deter­
mined electronically by the ECM (computer), which moni­
tors information from various engine sensors, computes the
desired spark timing, and signals the distributor.
The distributor does not contain centrifugal weights,
springs, or a vacuum advance unit.
Each TBI equipped engine also has a knock sensor
mounted in the engine block and connected by a blue wire
to a spark control module (called a "half-function box") (fig­
ure 45). In response to engine knock, a signal is sent from
the sensor to the module and then to the ECM, which com­
putes how much to retard spark timing to reduce knock. A
retard command is then sent to the distributor.
Information on TBI and the computer controlled ignition
system, system components and their locations, and diag­
nostic charts are located in the Light Duty Truck Fuel and
Emissions Service Manual.
IGNITION TIMING (Figures 52 and 53)
Timing specifications for each engine are listed on the
Vehicle Emissions Control Information label on the radiator
support. Always follow Vehicle Emissions Control Informa­
tion label procedures when adjusting timing. When using a
timing light, connect an adapter between the number 1
spark plug and the number 1 spark plug wire, or use an
inductive type pick-up. On engines with the timing pointer
mounted at the oil pan, connect timing light at the number 8
(or number 5) spark plug/wire. Do not pierce the plug lead.
Once the insulation of the spark plug cable has been bro­
ken, voltage will jump to the nearest ground, and the spark
plug will not fire properly.
Some engines incorporate a magnetic timing probe hole
for use with special electronic timing equipment. Consult
manufacturer's instructions for use of this equipment.

SPARK PLUG WIRES

The spark plug wiring is a carbon impregnated cord con­
ductor encased in an 8mm diameter rubber jacket. The
silicone spark plug boots form a tight seal on the plugs.
Refer to "Spark Plug Wires" later in this section for service
precautions.

SPARK PLUGS

Resistor type, tapered seat spark plugs are used on all
engines. No gasket is used on these tapered seat plugs.
Refer to figures 46 and 47 for an explanation of letter cod­
ing on spark plugs. Refer to the Vehicle Emissions Control
Information label on the radiator support for correct gap
information.
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 inter­
mittent high-speed driving is essential to good spark plug
performance as it provides increased and sustained com­
bustion 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 engine mis­
fire, even though a film is allowed to accumulate on
exposed portion of the plug porcelains.
Figure 45 — Knock Sensor and Spark Control
Module on P-Truck
Do not mistake corona discharge for flash-over or a
shorted insulator. Corona is a steady blue light appearing
around the insulator, just above the shell crimp. It is the
visible evidence of a 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 combus­
tion gases have blown out between the shell and insulator.