Ignition System: Description and Operation

Crankshaft Position Sensor Wiring Circuit:

Purpose:
The electronic ignition system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide optimum engine performance, fuel economy, and control of exhaust emissions, the PCM controls spark advance of the ignition system.

Operation:
The ignition system consists of two ignition coils, and ignition control module, a dual hall effect crankshaft position sensor, and engine crankshaft balancer with interrupter rings attached to the rear, related connecting wires, and the Ignition Control (IC) and fuel metering portion of the PCM.

In this ignition system, neither end of the secondary winding is grounded. Instead, each end of a coil's secondary winding is attached to a spark plug. Each cylinder is paired with an opposite cylinder (1-4, 2-3).

These two plugs are on "companion" cylinders, i.e., on top dead center at the same time. When the coil discharges, both plugs fire at the same time to complete the series circuit. The cylinder on compression is said to be the "event" cylinder and the one on exhaust is the "waste" cylinder.

The cylinder on the exhaust stroke requires very little of the available energy to fire the spark plug. The remaining energy will be used as required by the cylinder on compression stroke. The same process is repeated when the cylinders reverse roles. This method of ignition is called a "waste spark" ignition system.

It is possible in a no load condition for one plug to fire even though the spark plug lead from the same coil is disconnected from the other spark plug. The disconnected spark plug lead acts as one plate of a capacitor, with the engine being the other plate. These two "capacitor plates" are charged as a current surge (spark) jumps across the gap of the connected spark plug. The "plates" are then discharged as the secondary energy is dissipated in an oscillating current across the gap of the spark plug still connected. Because of the direction of current flow in the primary winding and thus, in the secondary winding, one plug fires from the center electrode to the side electrode while the other fires from side electrode to center electrode.

This system utilizes the IC signal from the PCM, as does a distributor type ignition system equipped with IC, to control spark timing. To properly control ignition timing, the PCM relies on the following information:

^ Engine load (manifold pressure or vacuum).
^ Engine coolant temperature.
^ Intake air temperature.
^ Crankshaft position.
^ Engine speed (RPM).
^ Knock.