The secondary ignition system circuit is the high voltage (Spark) side of the ignition system.

All secondary ignition systems consist of all the wiring and components between the coil output and the spark plug ground.

Wiring in the secondary ignition system circuit must have a thicker insulation than that of the primary circuit to prevent leaking (arcing) of the high voltage.

So, all ignition coils have two set of windings, the primary ignition system side and the secondary ignition system side.

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The secondary ignition system consists of:

Secondary Coil

  • The part of the coil that creates the high voltage electricity.

Coil Wire ( If it has one )

  • A highly insulated wire, that takes the high voltage from the coil, to the distributor cap.

NOTE: Distributor-less systems have no distributor cap or distributor rotor.

Distributor Cap ( If it has one )

  • A plastic cap which goes on top of the distributor, to hold the high tension wires in the right order.

Rotor ( If it has one )

  • Consequently, spins around on the top of the distributor shaft, and distributes the spark to the right spark plug.

Spark Plug Wires ( If it has one )

  • Another highly insulated wire that takes the high voltage from the cap to the plugs.

Spark Plugs

  • Take the electricity from the wires, and give it an air gap in the combustion chamber to jump across, to light the mixture.

Other Ignition Systems

Older engines use DI or Distributor Ignition systems:

  • When you have a distributor ignition there will be one coil, a coil wire, distributor cap and rotor, spark plug wires and spark plugs.

Newer engines use EI or Electronic Ignition:

  • As there are multiple coils in EI systems you have more dwell time and can make a more powerful spark. In addition the lack of moving parts (the rotor inside the distributor cap) creates a more reliable system.

Two basic styles of Electronic Ignition (EI) systems are used.

  • So, the Waste Spark system uses one coil for every two cylinders. As a result, a four cylinder would have 2 coils, 2 plug wires and 2 spark plugs.
  • The Coil On Plug system uses one coil for each spark plug. For this style a four cylinder would have four coils and four spark plugs.

Some of these systems use no plug wires. Other COP systems use a short plug wire for each cylinder.

Diagnosis Testing Tips

So, a common defect in the secondary wires is an open circuit. But, this can be easily found with an ohmmeter. Coil and plug wires have several thousand ohms per foot of length. So, do not confuse high resistance with an open circuit.

Most often a problem in the secondary ignition system will show up as a rough running engine, or one that lacks power, long before it gets bad enough to cause an engine to not start. Consequently, secondary ignition problems are much easier to diagnose with the lab scope. Reading the secondary ignition lab scope pattern on a running engine can point out problems in secondary ignition systems.

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