Ignition System: Description and Operation

SYSTEM OPERATION
The Electronic Ignition (EI) system does not use the conventional distributor and coil. This ignition system consists of two separate ignition coils, an electronic Ignition Control Module (ICM) and Crankshaft Position (CKP) sensor as well as the related connecting wires and the Ignition Control (IC) portion of the Powertrain Control Module (PCM).

A distributorless ignition system such as this one uses a "waste spark" method of spark distribution. Each cylinder is paired with the cylinder that is opposite it (1 and 4 or 2 and 3). The spark occurs simultaneously in the cylinder coming up on the compression stroke and in the cylinder coming up on the exhaust stroke.

The cylinder on the exhaust stroke requires very little of the available energy to fire the spark plug. The remaining energy will he used as required by the cylinder on the compression stroke. The same process is repeated when the cylinders reverse roles.

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 will fire from the center electrode to the side electrode while the other will fire from side electrode to center electrode.

These systems utilize the Ignition Control (IC) signal from the PCM, as do distributor type ignition systems, to control spark timing.

Under 400 RPM, the electronic Ignition Control Module (ICM) controls spark timing (bypass timing mode) and over 400 RPM, the PCM controls spark timing (IC mode).

To properly control ignition 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).

IGNITION CONTROL (IC)
This system uses the same IC to PCM circuits that distributor type systems use. Following is a brief description for each of the IC circuits:

^ EI reference - CKT 430 The Crankshaft Position (CKP) sensor generates a signal to the electronic Ignition Control Module (ICM) which results in a reference pulse being sent to the PCM. The PCM uses this signal to calculate crankshaft position and engine speed and injector pulse width.

^ Reference ground - CKT 453 - This wire is grounded through the ignition control module and insures that the ground circuit has no voltage drop between the ignition control module and the PCM, which could affect performance.

^ Bypass - CKT 424 - At about 400 RPM, the PCM applies 5 volts to this circuit to switch spark timing control from the ignition control module to the PCM.

^ IC - CKT 423 - The ignition control module sends a reference signal to the PCM when the engine is cranking. While the engine is under 400 RPM, the ignition control module controls the ignition timing. When the engine speed exceeds 400 RPM, the PCM applies 5 volts to the bypass line to switch the timing to PCM Ignition Control (IC).