The induction winding is connected to the ignition module and forms part of an internal module control circuit. With the ignition switched on and the engine stationary, battery voltage is applied to terminal and to the positive coil terminal.
However, there is no current flow through the primary winding as the switching transistor is in the "off" state.
Battery voltage therefore can also be read at the coil negative terminal and at terminal.
With engine rotation, the reluctor teeth on the distributor shaft approach the stator teeth and a voltage of positive polarity is induced in the winding.
This voltage signals the control circuit to turn the primary current on and a voltage is applied to the base/emitter of the switching transistor.
Current can now flow through the ignition coil primary winding, and through the collector/emitter of the switching transistor to ground.
The magnetic field is thus established in the ignition coil.
Further rotation of the reluctor causes the teeth to align with the stator teeth and then move away.
The changing magnetic field now induces a voltage of opposite polarity in the winding. When this voltage reaches a pre-determined value, the control circuit switches off the voltage applied to the base of the switching transistor.
The transistor is thus switched "off" breaking the primary circuit.
This causes self induction in the primary winding and a mutually induced high tension voltage in the secondary winding. This provides the spark at the spark plug.
