Ignition coil

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Ignition Coil from Puch Moped
Ignition Coil Internals

An Ignition coil is a specialty transformer used to step the output voltage of the smaller ignition coil on the magneto generator up to a voltage high enough to cause a spark at the spark plug.

General transformers

A transformer is simply two individual wires wrapped around a common metal core. Each of these wires is called a winding. When current is run through the primary winding it induces a magnetic field that causes current to flow in the secondary winding. This transfer is dependent on the ratio between the number of turns in each winding. For example if the primary and secondary winding have the same number of turns (each wire has been wrapped around the metal core the same number of times) then the output voltage will be the same as the input. However, if the primary winding has 10 times as many turns as the secondary then the output voltage would be 10 times larger.

In relation to mopeds

When the points open the magneto generator provides 6/12VAC power to the primary winding. The primary winding on the ignition coil has fewer turns than the secondary winding (on the order of 1:100) this induces the very high voltage (on the order of 10,000V) required to jump the air gap at the spark plug which is connected to the secondary winding.

Notes about transformers

  • Transformers only work with AC power. If DC power is provided to the primary winding of a transformer the winding will act as a short circuit and no power will be transfered to the secondary winding.
  • The primary and secondary windings are not physically connected. Taking an Ohm meter reading between the primary and secondary winding should show an open circuit.

Symptoms of a Bad Ignition Coil

Most coils stay functional for many years. Problems with the ignition coil are often coupled with condenser and loose wire issues. Ignition coil problems should be addressed as a last resort type of issue.

  • No Spark
  • Spark exists when cold, after warming up, the engine dies. It sometimes dies when it's already warm and finally let to idle.

The wrapped wires inside the actual coating tend to heat up due to resistance. As they expand, they shift around each other and cause wear on the coating that is on them. Over time this coating becomes brittle and thin, which leads to ignition coil failure.


There are several areas which can fail on an ignition coil. Most coil damage is attributed to heat and corrosion. The following are checkpoints for troubleshooting:

  • The Grounded Mounting Screws
    • These are subject to galvanic cell corrosion. make sure that the galvanized screws are still shiny and new, if not, they should be replaced. Make sure that the plate which the screws attached to are also clean.
  • Magneto Signal Wire
    • Clean up any corrosion, tarnish, and test the continuity of this wire from the magneto to the actual terminal on the ignition coil.
  • High Voltage Spark Plug Wire
    • This joins the ignition coil by fitting the core of the wire to a spike in the ignition coil. Assure that there is good contact and no corrosion.
  • Spark Plug Boot
    • Make sure this is well mounted on the wire and free of dirt. Make sure that the clip (if relevant) that attaches to the tail of the spark plug is present.

If these checkpoints pass, chances are, the coil needs replacing.