Chevrolet Light Duty Truck 1973, Руководство по обслуживанию

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E N G IN E ELECTRICAL 6 Y -33
Fig. lO i—C ro s s -S e c tio n of S p a r k Plu g
longer this is, the hotter the plug will operate; the shorter
it is, the cooler the plug will operate (Fig. 1 li).
Spark plugs, to give good performance in a particular
engine, must operate within a certain temperature range
(neither too hot nor too cool). If the spark plug remains
too cool: oil, soot, and carbon compounds will deposit on
the insulator causing fouling and missing. If the plug
runs too hot, electrodes will wear rapidly, and under
extreme conditions, premature 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 originally 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
------- COLDER HOTTER ------- ►
Fig. l l i — S p a r k P lu g H e a t R a n g e S y s te m
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 thermometer-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.
Spark Plug Reach and Threads
Spark plugs are manufactured in a number of thread
sizes and "reaches." Reach is the distance from the
gasket seat to the end of the shell. Spark Plugs have a
type number on the insulator which designates plug
thread size as well as the relative position in the heat
range system as previously explained.
Secondary Ignition Cables
The secondary wiring consists of the high tension cables
connected between the distributor cap, the spark plugs,
and the high tension terminal of the ignition coil. These
cables carry the high voltage surges to the spark plugs
and are heavilty insulated to contain the high voltages.
The cables are neoprene jacketed and have a multiple
cloth thread core impregnated with a graphite solution to
give the correct conductivity and proper resistance for
suppression of radio and television interference.
Ignition Switch
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, refer to the Steering
section of this manual.
The ignition switch is key operated through the actu­
ator rod assembly to close the ignition primary circuit
and to energize the starting motor solenoid for crank­
ing. The ignition switch has five positions: OFF,
LIGHT DUTY TRUCK SERVICE MANUAL