Ignition System: Description and Operation

Ignition Control Module And PCM Circuit:

GENERAL DESCRIPTION
The Electronic Ignition (EI) 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 Powertrain Control Module (PCM) controls spark advance of the ignition system. EI has several advantages over a mechanical 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.

OPERATION
The Electronic Ignition system does not use the conventional distributor and coil. This ignition system consists of three ignition coils, an Ignition Control Module (ICM), a Camshaft Position Sensor, a duel "Hall-effect" crankshaft position sensor, an engine crankshaft balancer with crankshaft position sensor interrupter ring attached to the rear, related connecting wires, and the ignition control 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, both ends of each coil secondary winding is attached to a spark plug. Each cylinder is paired with the cylinder that is opposite it (1 - 4, 2 - 5, 3 - 6).

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 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 a forward current flow and its "companion" plug fires with a reverse current flow. This is different from a conventional ignition system that fires all the plugs with the same direction of current flow. Since it requires approximately 30% more voltage to fire a spark plug backwards, the ignition coil design is improved, with saturation time and primary current flow increased. This 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 still-connected spark plug.

Secondary voltage requirements are very high with an "open" spark plug or wire. The ignition coil has enough reserve energy to fire the still-connected plug at idle, but possibly not under high engine load. A more noticeable misfire may be evident under load; both spark plugs may then be misfiring.

CIRCUITS AFFECTING IGNITION CONTROL
To properly control timing, the PCM relies on the following information:

- Engine load (manifold pressure or vacuum).
- Atmospheric (barometric) pressure.
- Engine temperature.
- Intake air temperature.
- Crankshaft position.
- Engine speed (rpm).

IC System
The IC system consists of the ignition coil and module assembly (ignition coils, electronic ignition control module), the 3X crankshaft position sensor and the 24X crankshaft position sensor, PCM and connecting wires. The Ignition Control Module (ICM) connector terminals are lettered as shown in the image. These circuits perform the following functions:

3X reference high - CKT 647
The crankshaft position sensor sends a signal to the electronic ignition control module which generates a reference pulse which is sent to the PCM. The PCM uses this signal to calculate crankshaft position and engine speed (also used to trigger the injector).

3X reference low - CKT 453
This wire is grounded through the module and makes sure the ground circuit has no voltage drop between the ignition module and the PCM, which if open, could affect performance.

Ignition control bypass - CKT 424
During initial cranking, the PCM will look for synchronizing pulses from the camshaft and 3X crankshaft position sensor indicating the position of #1 piston and intake valve. 5 volts is applied to the bypass circuit the instant these signals are received by the PCM. This generally occurs within 1 or 2 revolutions of the crankshaft. An open or grounded bypass circuit will set a DTC 42 and the engine will run at base timing. A small amount of advance is built into the ignition control module to enhance performance.

IC - CKT 423
The PCM uses this circuit to trigger the ignition control module. The PCM uses the crankshaft reference signal to base its calculation of the amount of spark advance needed under present engine conditions.

24X reference signal
Additional to the electronic ignition system is the 24X crankshaft position sensor. Its function is to increase idle quality and provide good low speed driveability.