The purpose of the ignition system is twofold: first to create a voltage high enough
(20,000+) to arc cross the gap of a spark plug, thus creating a spark strong enough to
ignite the air/fuel mixture for combustion; second to control the timing so that the spark
occurs at the right time and at the right cylinder.
The basic principle of the electrical spark ignition system has not changed for over 75
years. What has changed is the method by which the spark is created. The automobile has
evolved from a mechanical system (distributor) to a solid state electronic system. Both
systems control a low voltage primary circuit through an ignition
coil which will induce a high voltage in the secondary circuit which is then
directed to the right spark plug at the right time.
The primary circuit consists of an ignition switch, ballast resistor, some type of
off/on switch , ( in the older cars a set of breaker
points) , which is usually inside a distributor, an ignition coil and the
connecting wires. The purpose of the primary circuit is to allow low voltage from the
battery, to pass through the ignition coil where the voltage is stepped up from 12 volts
to as much as 40,000 volts.
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The ignition coil contains both the
primary and secondary winding circuits. The coil primary winding contains 100 to 150 turns
of heavy copper wire. The turns of this wire must be insulated from each other or they
would short out and not create the primary magnetic field that is required. The primary
circuit wire goes into the coil through the positive terminal and exits through the
negative terminal. The coil secondary winding circuit contains 15,000 to 30,000 turns of
fine copper wire, which also must be insulated from each other. To further increase the
coils magnetic field both windings are install around a soft iron core. To withstand the
heat of the current flow, the coil is filled with oil for cooling. The ignition coil is
the heart of the ignition system. As current flows through the coil a strong magnetic
field is built up. When the current is shut off, the collapse of this magnetic field
induces a high voltage which is released through the large center terminal through the
distributor to the spark plugs.
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The secondary circuit consists of the secondary windings of the ignition coil, which
produces the high voltage needed to arc across the spark plug gap. This voltage is sent
out the center coil tower, through a high tension wire to the distributor cap where a
rotor distributes the spark through the distributor cap, to the right spark plug at the
right time.
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Breaker points have not been used since the mid 70’s, but many older cars still
have them. Because of the simplicity of breaker points, it is a good starting point in
understanding the switching mechanism that controls the current flow through the coil. The
points are made up of a fixed contact point and a movable contact point. The movable point
is spring loaded and rides on a 4,6, or 8 lobe cam (depending on the number of cylinders).
The points are located inside a distributor. As the engine rotates, the camshaft turns the
distributor, which then opens and closes the breaker points as many as 15,000 to 25,000
times a minute. When the points are closed, current is allowed to flow through the
ignition coil, thereby building a magnetic field around the windings. When the points are
opened, they interrupt that current flow, thereby collapsing the magnetic field and
releasing a high voltage surge. This high voltage enters the top of the distributor, where
an ignition rotor distributes that voltage through a cap to the right spark plug at the
right time. The distributor also contains a condenser that prevents arcing by absorbing
excess current when the points open. The difficulty with the breaker point system is that
the part that rubs against the cam wears. This wear causes a constant need for adjustment
and eventual replacement. In the mid 70’s this problem was corrected through the use
of solid state electronics and transistors as switching devices.
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