Motorola GM Series Installation Manual page 28

2-4 REDUCING NOISE INTERFERENCE
HV Lead
Distributor
Ignition
Breaker
Coil
Ignition
Points
Switch
Spark plug leads
Spark
Cam
Battery
Plugs
Condenser
Rotor Gap
Ignition
Breaker
Coil
Ignition
Lead Inductance
Points
Switch
Stray
Capacitance
Cam
Spark Plug Gap
Battery
Condenser
FL0830261-O
Figure 2-2 Typical Vehicular Ignition System, Simplified Schematic Diagram
The battery is connected to the primary winding of the coil through the ignition switch. The primary
circuit is returned to the battery through the breaker points, which are bypassed by the condenser.
The points are normally closed. As the cam shaft is rotated by the engine, its lobes or corners open
and close the points in proper synchronization with the piston in each cylinder.
With the ignition switch on and the breaker points closed, coil primary current builds up at a rate
determined by the coil inductance.
When the breaker points open, primary current decreases and, by self-induction, an electromotive
force is induced in the primary which is many times greater than the battery voltage. The high
voltage inducted in the coil secondary causes a spark across the distributor rotor-to-spark plug wire
gap and then across the spark plug gap for a short interval of time when the breaker points open.
The condenser reduces arcing of the points.
The secondary circuit of the ignition coil, including the distributor rotor gap and the spark gap, is the
main source of ignition interference. The lead inductance and stray capacitance provide a tuned
circuit. Because the discharge of the circuit is through a low resistance (ionized spark gap), the
circuit tends to oscillate. The frequency and amplitude of oscillation vary as current changes in the
spark gap.