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 TM 9-8000
not shielded. This shielding is a metal covering of woven
15-13. Distributor.
construction that surrounds the wires. Plain metallic
shields cover the distributor blocks and booster coil. The
a. The magneto distribution operates much the
shielding is grounded to the engine so that it can pick up
same as the battery ignition distributor (para 15-3). The
the undesirable radiations from the magneto and carry
distributor rotor, which directs the electrical impulses in
them directly to a ground. The radiations are thus
proper order to the cylinders, usually is driven through
prevented from reaching the vehicle's radio aerial and
suitable gearing, at one-half crankshaft speed. This ratio
interfering with reception.
ensures that each cylinder will be fired during the cycles
of the engine. The cam is much larger than those used
15-14. Booster Coil. Magneto speed during cranking
in battery ignition distributors and contains the contact
is not high enough to develop a hot spark. An external
spring and cam electrode that, together, serve the same
source of high-tension current for starting is provided,
purpose as the rotor segment in a battery ignition
either by a booster magneto or by a high-tension coil,
distributor. That is, the cam electrode connects the high-
with the primary current being supplied by a battery. The
tension center electrode (connected to secondary of coil)
coil method is most common. Current from the booster
to each of the outer distributor electrodes in turn. These
coil, which operates just like the coil in the battery ignition
outer electrodes are connected by high-tension leads to
system, is conducted to the booster electrode of the
the spark plugs.
magneto. This connects the booster coil with the
b. One end of the secondary winding is connected
magneto primary coil when the contacts open, thereby
causing a current surge through the primary (from the
to the primary. The other end terminates at the high-
booster coil), which produces a rapid change of magnetic
tension insert, which is a piece of metal extending
field strength in the primary. This action Induces in the
through the Bakelite case covering the coil. The high-
secondary a high-voltage surge sufficiently strong to fire
tension voltage developed in the secondary coil passes
the plugs.
through the insert to a contactor such as the high-tension
contact spring in the magneto cover. From there, it goes
to the center electrode and then, internally, through the
15-15. Magneto Switches(Fig. 15-24).
distributor rotor to the outer electrodes. The rotor is
timed so that the cam electrode will line up with the
a. Because the magneto is self-energizing, it
center electrodes to which spark plug leads are
cannot be turned off by disconnecting it from some
connected, at the proper time for igniting the fuel-air
external source of energy as in the battery ignition
charges in the cylinders.
system. Instead, the magneto coil must be grounded.
Because one end of the magneto coil is grounded
c. The spark plug assembly provides a gap where
already, grounding the other (or breaker point) end
a surge of high voltage may cause a spark to ignite the
effectively will prevent the magneto from producing high-
fuel-air mixture. One spark plug electrode is connected
voltage surges. This is accomplished by means of a
to the high-tension cables from the distributor blocks; the
grounding switch on the vehicle instrument panel. When
other is fastened to a ground. Most air-cooled engines
the switch is turned off, contacts in the switch are closed
have two spark plugs for each cylinder and two separate
and the magneto coil is grounded through them. The
magnetos for the ignition system. The second spark
magneto, booster, and main-engine starter switches all
plug ensures better combustion and is added insurance
are located together in the same housing. The main
against ignition failure.
magneto switch actually has four positions because, in
the application shown, four magnetos are used (fig. 15-
d. The high-tension wires that conduct the current
24). The four positions are OFF (all magnetos ground-
from the distributor blocks to the spark plugs are
ed), A (two of the magnetos that fire the plugs in the
commonly called the ignition harness. As a magneto
accessory ends of cylinders are ungrounded and
ignition system transmits a form of high-frequency
operative), F (the two magnetos that fire the plugs in the
current, radiations emanating from it during operation will
flywheel ends of the cylinders are ungrounded and
interfere with radio reception if the Ignition system is
operative), and BOTH (all four magnetos are operative).
15-21
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