GMC 1976 ZEO 6083 Maintenance Manual page 413

ature of the surrounding air, the oil contracts to
occupy less volume thus allowing room for expan-
sion when the coil heats up during normal operation.
The oil acts as an insulator to prevent high voltage
arc-over within the coil .
In the design of an ignition system, sufficient
primary circuit resistance must be present to protect
the distribution contacts from excessive arcing and
burning. In some ignition systems, part of this resist-
ance may take the form of a separate resistor or a
calibrated resistance wire connected between the ig-
nition switch and the coil primary terminal . Since the
value of this resistor along with the resistances of the
other components in the entire primary circuit af-
fects the coil performance at higher engine speeds .
During cranking, the external resistance on most
applications is by-passed to provide full battery volt-
age to the coil for improved performance and easier
starting . The by-pass wire may be connected to an
"R" terminal on the cranking motor solenoid which
contacts the contact disk during cranking, or to a
separate terminal on the ignition switch, as shown in
the previous section . The higher currents during
cranking are not sufficient to cause distributor con-
tact deterioration because of the short periods of time
in the life of contacts spent during cranking. Also,
the lowered battery voltage during cranking causes
a lower primary current, so the resistor by-pass fea-
ture is an offsetting factor . By-passing the resistor
with the engine operating will cause very rapid fail-
ure of the distributor contacts .
SPARK PLUGS
The spark plug consists of a metal shell in which
is fastened a porcelain insulator and an electrode
extending through the center of the insulator. The
metal shell has a short electrode attached to one side
and bent in toward the center electrode. There are
threads on the metal shell that allow it to be screwed
into a tapped hole in the cylinder head . The two
electrodes are of special heavy wire, and there is a
specified gap between them . The electric sparkjumps
this gap to ignite the air-fuel mixture in the combus-
tion chamber, passing from the center, or insulated,
electrode. The seals between the metal base, por-
celain, and center electrode as well as the porcelain
itself, must be able to withstand the high pressure
and temperature created in the combustion chamber
during the power stroke .
Some spark plugs have been supplied with a
built-in resistor which forms part of the center elec-
trode. The purpose of this resistor is to reduce radio
and television interference from the ignition system
as well as to reduce spark-plug-electrode erosion
caused by excessively long sparking . We have been
talking of the high-voltage surge from the ignition-
ENGINE ELECTRICAL 6Y- 33
coil secondary as though it were a single powerful
surge that almost instantly caused the spark to jump
across the spark plug gap. Acutally, the action is
more complex than that . There may be a whole series
of preliminary surges before a full-fledged spark
forms. At the end of the sparking cycle the spark
may be quenched and may reform several times. All
this takes place in only a few ten-thousandths of a
second . The effect is that the ignition wiring acts like
a radio transmitting antenna; the surges of high volt-
age send out impulses that causes radio and televi-
sion interference . However, the resistors in the spark
plugs tend to concentrate the surges in each sparking
cycle, reduce their number, and thus reduce the in-
terference and also the erosive effect on the plug
electrodes .
Heat Range System
The "heat range" of a spark plug is determined
primarily by the length of the lower insulator. The
longer this is, the hotter the plug will operate; the
shorter it is, the cooler the plug will operate.
Spark plugs, to give good performance in a par-
ticular engine, must operate within a certain temper-
ature range (neither too hot or too cool) . If the spark
plug remains too cool : oil, soot, and carbon com-
pounds will deposit on the insulator causing fouling
and missing. If the plug runs too hot, electrodes will
wear rapidly, and under extreme conditions, prema-
ture ignition (pre-ignition) of the fuel mixture may
result .
Frequently, the wrong type of spark plugs, one
with an improper heat range for the engine, may
have been installed when replacing spark plugs origi
nally fitted by the engine manufacturer and such
misapplication may lead to poor performance. The
heat range system makes it possible to select the type
of spark plug that will operate within the correct
temperature range for each specific engine .
Where abnormal operating conditions cause
chronic carbon or oil fouling of the plugs, the use of
a type one number higher (a "hotter" type) than
recommended will generally remedy the trouble; and
by the same formula, where chronic pre-ignition or
rapid electrode wear is experienced, a type with one
number lower (a "cooler" type) will generally be
found satisfactory .
The last digit of the type number indicates the
heat range position of the plug in the heat range
system . Read the numbers as you would a thermom
eter-the higher the last digit, the "hotter" the spark
plug will operate in the engine ; the lower the last
digit, the "cooler" the spark plug will operate.