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TM 9-8000
Figure 15-19. Vacuum Timing Controls.
admits manifold vacuum to the vacuum passage,
and close the contacts earlier in the cycle.
thereby allowing atmospheric pressure to push the
advances the ignition timing.
diaphragm inward, compressing the return spring. This
d. Computerized Control (Fig. 15-21). The trend
movement, through the linkage from the diaphragm,
causes the breaker plate to be turned ahead so that the
in modern automotive design is to use a computer to
contacts are opened and closed earlier in the cycle. This
control ignition advance.  Electronic ignition is very
advances the spark. Usually, the vacuum line is tapped
adaptable to computer control because the amplifier
into the carburetor just at the upper edge of the throttle
circuit can be integrated right into the computer module.
valve when it is closed. This permits no advance on idle
With this arrangement the ignition timing is regulated
but causes a vacuum advance as the throttle valve is
within the module by electronically regulating the signal
opened so its edge moves past the vacuum passage.
from the distributor pickup to the amplifier. This is the
job of the computer.  The computer establishes the
c.  Centrifugal  Control  (Fig.
proper ignition timing based on signals it receives from
centrifugal advance mechanism is located in the
various sensors on the engine. The sensors provide
distributor housing and consists essentially of a pair of
information such as engine speed, throttle position,
weights mounted on pins on a weight base and linked by
coolant temperature, ambient temperature, and manifold
weight springs to the advance cam. When Idling, the
vacuum. The computer is able to change ignition timing
springs hold the advance cam in a no-advance position.
many hundreds' of times per second in contrast to
As speed increases, the centrifugal force on the weight
mechanical devices that are slow to react to the engine's
causes them to move out. This action forces the toggles
needs. Because of this, a computer- controlled system
on the weights to move against the advance cam so the
of spark advance will allow the engine to be more
cam is pushed ahead against the spring tension. As the
responsive while running on leaner fuel mixtures. This
advance cam turns ahead, the breaker cam lobes open
will result in a cleaner running, more economical engine.


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