Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

The Ignition: Primary and Secondary Circuits and Components 241
Figure 11-24. Typical Ford EDIS ignition system with a coil pack and separate spark plug wires for the secondary
system. (Courtesy of Ford Motor Company)
firing voltage oscillation. At about 15,000 volts,
the spark plug air gap ionizes and becomes conductive.
This is the ionization voltage level, also
called the firing voltage, or required voltage.
As soon as a spark has formed, less voltage is
required to maintain the arc across the air gap.
This reduces the energy demands of the spark
causing the secondary voltage to drop to the much
lower spark voltage level. This is the inductive
portion of the spark. Spark voltage is usually
about one-fourth of the firing voltage level.
Figure 11-32 shows the entire trace of the spark.
The spark duration or burn time of the trace indicates
the amount of resistance and efficiency of the
spark voltage. Burn time on most ignition systems
is between 1.6 and 1.8 milliseconds. The traces