<P> Formerly, ignition coils were made with varnish and paper insulated high - voltage windings, inserted into a drawn - steel can and filled with oil or asphalt for insulation and moisture protection . Coils on modern automobiles are cast in filled epoxy resins which penetrate any voids within the winding . </P> <P> A single - spark system has one coil per spark plug . To prevent premature sparking at the start of the primary pulse, a diode or secondary spark gap is installed in the coil to block the reverse pulse that would otherwise form . </P> <P> In a coil meant for a dual - spark system, the secondary winding has two terminals isolated from the primary, and each terminal connects to a spark plug . With this system, no extra diode is needed since there would be no fuel - air mixture present at the inactive spark plug . </P> <P> In a low - inductance coil, fewer primary turns are used, so primary current is higher . This is not compatible with the capacity of mechanical breaker points, so solid - state switching is used . </P>

What is a coil pack and what does it do