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 TM 9-8000
insulation usually has a braided structure of rayon
connected in parallel but made to operate progressively.
molded Into it to increase strength. TVRS cable has a
That is, they are so set with respect to the cam lobes that
very high resistance value that effectively eliminates
one arm opens its contacts slightly before the other. The
radio interference, but is very susceptible to conductor
circuit actually is not broken until the second set of
breakage if not handled carefully.
contacts open. Both sets of contacts are open for a
shorter period of time than they are in simultaneous
(3) An alternative to TVRS cable is shielded
operation. This allows the ignition coil a slightly longer
solid conductor cable. This type of cable is a solid
period of time in which to buildup. The breakers are
conductor encased by a layer of insulation. The cable is
arranged for progressive parallel operation.
then encased in a metallic shielding and an outer layer of
c. Alternate Operation (Fig.
This is
insulation. This type of secondary cable, though very
expensive, is very strong and widely used in military
another arrangement that increases the time of contact
vehicles.
and allows better magnetization of the coil. The two
pairs of contacts still operate electrically in parallel, but
use a cam with only half as many lobes as there are
15 4.Multiple Contact Distributors.
cylinders to be fired. The breakers are so arranged
around the cam that one pair of contacts will close
a. General. Distributors may have more than one
almost immediately after the other pair has opened.
set of contacts. Some distributors use two sets in
Thus, the two sets of breaker contacts almost overlap
parallel for longer contact closing and higher magnetic
each other's movements so there is no waste of time.
strength of the coil. Others may use two sets for
Shortly after one pair of contacts opens the circuit, the
alternate firing of cylinders. Still others have two sets for
other pair closes it, so the coil can start to build up at
dual ignition or for operation of two semi-independent
once in preparation for the next spark. One set of
ignition systems.
contacts operates for half the cylinders and the other set
for the other half.
b. Parallel Operation (Fig. 15-12). To counteract
the bounce, or chatter, of the breaker arm and to prevent
d. Dual-Circuit Operation (Fig. 15-14).
overload of the contact points, two breakers can be
connected in parallel and adjusted to open at the same
(1) Another breaker arrangement, very similar
time. If one pair of contacts has a tendency to bounce
in appearance to the one used for alternate operation, is
open at high speed, it is likely that the other pair will not
used with two separate coils for firing a large number of
bounce at exactly the same instant. Therefore, the
circuit will be closed more positively than it would be with
only one pair of contacts. The breaker arms also can be
Figure 15-12. Parallel-Connected Multiple Contacts.
15-9
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