Figure 15-19. Vacuum Timing Controls.

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Capacitive-Discharge Ignition

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 15-20. Centrifugal Timing Controls.

TM 9-8000

Figure 15-19. Vacuum Timing Controls.

admits manifold vacuum to the vacuum passage,

and close the contacts earlier in the cycle.

This

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.

15-20).

The

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.

TA233599

15-16