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 TM 9-8000
period. Full voltage would cause the points to burn.
(2) At high engine speeds the points open and
close quickly. The short duration of point closure tends
to cool the ballast resistor. This causes a decrease in
resistance, which, in turn, increases the voltage to the
points. This is desirable because of the short coil
saturation period at high speeds.
h. Secondary (Spark Plug) Cables (Fig. 15-11).
The secondary ignition cables carry the high-voltage
electric current to the spark plugs.
(1) An ignition cable in its simplest form consists
of a solid or stranded conductor of steel or copper that is
Figure 15-10. Ballast Resistor
surrounded by a heavy insulation of neoprene, hypalon,
or silicon rubber. The design of the insulating material is
proportionately in resistance to any temperature
important because of the high-voltage electric current. A
increase. The ballast resistor is bypassed during engine
poorly designed insulating material will leak electric
starting. This is done to compensate for the large
current through it, causing spark plug misfire. This is
voltage drop experienced during engine cranking. This
true particularly in damp weather or when the insulation
large drop lowers available voltage below the threshold
is subjected to high engine compartment temperatures.
voltage required for ignition system operation, thus
inhibiting spark plug firing. The ballast resistor is then
(2) The secondary cable in its simplest form
placed back into the circuit as the engine is started.
usually is inadequate for modern applications because
the high voltage creates radio signals that interfere with
(1) The points open and close slowly at low
TV and radio reception. The most common alternative is
engine speeds. The longer duration of point closure
TVRS secondary cable. Rather than a conventional
tends to make the ballast resistor heat up, causing a
insulated wire, the TVRS cable is made of a fiberglass or
proportional increase in resistance. The increase in
linen stranding that is filled with carbon.
The
resistance, in turn, reduces the voltage to the points.
This is desirable because full voltage is not needed at
low speeds, where there is a longer coil-saturation
Figure 15-11. Secondary Cable Construction.
15-8
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