Ignition System: Description and Operation

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 the spark advance of the ignition system. Electronic ignition has the following advantages over a mechanical distributor system:
^ No moving parts.
^ Less maintenance.
^ Remote mounting capability.
^ No mechanical load on the engine.
^ More coil cool down time between firing events.
^ Elimination of mechanical timing adjustments.
^ Increased available ignition coil saturation time. The electronic ignition system does not use the conventional distributor and coil. The ignition system consists of three ignition coils, an ignition control module, a camshaft position sensor, two Hall effect crankshaft position sensors, an 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.

Conventional ignition coils have one end of the secondary winding connected to the engine ground. In this ignition system, neither end of the secondary winding is grounded. Instead, each end of a coil[prirne ]s secondary winding is attached to a spark plug. Each cylinder is paired with the cylinder that is opposite it (14, 2-5, 36). These two plugs are on "companion" cylinders, i.e., on top dead center at the same time.

When the coil discharges, both plugs tire 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 the compression stroke. The same process is repeated when the cylinders reverse roles. This method of ignition is called a 'waste spark' ignition system.

Since the polarity of the ignition coil primary and secondary windings is fixed, one spark plug always fires with normal polarity and its "companion" plug tires with reverse polarity. This differs from a conventional ignition system that fires all the plugs with the same polarity. Because the ignition coil requires approximately 30% more voltage to fire a spark plug with reverse polarity, the ignition coil design is improved, with saturation time and primary current flow increased. This redesign of the system allows higher secondary voltage to be available from the ignition coils - greater than 40 kilovolts (40,000 volts) at any engine RPM. The voltage required by each spark plug is determined by the polarity and the cylinder pressure. The cylinder on compression requires more voltage to fire the spark plug than the one on exhaust.

It is possible for one spark plug to fire even though a plug wire from the same coil may be disconnected from its companion plug. The disconnected plug wire acts as one plate of a capacitor, with the engine being the other plate. These two "capacitor plates" are charged as a 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 that is still connected. Secondary voltage requirements are very high with an open spark plug or spark plug wire. The ignition coil has enough reserve energy to fire the plug that is still connected at idle, but the coil may not fire the spark plug under high engine load. A more noticeable misfire may be evident under load; both spark plugs may then be misfiring.

The 24X crankshaft position sensor, secured in an aluminum mounting bracket and bolted to the front side of the engine timing chain cover, is partially behind the crankshaft balancer. The 7x crankshaft position sensor uses a two wire connector at the sensor and a three way connector at the ignition control module.

The 24X crankshaft position sensor contains a Hall effect switch. The magnet and Halleffect switch are separated by an air gap. A Hall effect switch reacts like a solid state switch, grounding a low current signal voltage when a magnetic field is present. When the magnetic field is shielded from the switch by a piece of steel placed in the air gap between the magnet and the switch, the signal voltage is not grounded. If the piece of steel (called an interrupter) is repeatedly moved in and out of the air gap, the signal voltage will appear to go ON OFF ON OFF. Compared to a conventional mechanical distributor, this ON OFF signal is similar to the signal that a set of breaker points in the distributor would generate as the distributor shaft turned and the points opened and closed.

In the case of the electronic ignition system, the piece of steel is a concentric interrupter ring mounted to the rear of the crankshaft balancer. The interrupter ring has blades and windows that, with crankshaft rotation, either block the magnetic field or allow it to reach the Hall effect switch. The Halleffect switch is called a 24X crankshaft position sensor because the interrupter ring has 24 evenly spaced blades and windows. The 24X crankshaft position sensor produces 24 ON OFF pulses per crankshaft revolution.

The 7X crankshaft position sensor is the other Hall effect switch closer to the crankshaft. The interrupter ring is a special wheel cast on the crankshaft that hes seven machined slots, six of which are equally spaced 60 degrees apart. The seventh slot is spaced 10 degrees from one of the other slots. as the interrupter ring rotates with the crankshaft, the slots change the magnetic field. this will cause the 7x the Hall effect switch to ground the 3X signal voltage that is supplied by the ignition control module. The ignition control module interprets the 7X ON OFF signals as an indication of crankshaft position. The ignition control module must have the 7X signal to fire the correct ignition coil. The 24X interrupter ring and Halleffect switch react similarly. The 24X signal is used for better resolution at a calibrated RPM.