Ignition System: Description and Operation

The Electronic Ignition System consists of a distributor, an Ignition Control Module (ICM), engine knock sensor and an B+ relay.
The distributor is mounted at the rear of the cylinder head and is driven by a gear on the camshaft. It contains no electrical components other than the rotor and distributor cap, and its sole function is to transmit high voltage to the spark plugs in the correct firing sequence.
The Ignition Control Module (ICM), mounted in the engine compartment on the center of the bulkhead, consists of an ignition coil and ICM operated by the Electronic Control Unit (ECU). Based on information received from various sensors, the ECU calculates the precise ignition timing for all engine operating conditions. As timing is calculated, the ECU provides the ICM with a signal to trigger the ignition coil. This signal is received by the module in the form of a 5 volt square wave, and is transmitted through terminal B of connector 2 on the power module. As the leading edge of the wave contacts the ignition circuitry in the module, the module charges the coil primary windings. When coil saturation occurs, the module circuitry opens the primary windings to collapse the magnetic field in the windings. This induces a high voltage in the coil secondary windings, which is transferred through to the coil wire, rotor, distributor cap, and plug wire to the spark plug.
The Knock Sensor is an electro-mechanical device capable of measuring vibration and converting it into an electrical signal. During engine knock, a variation in pressure on the sensor body produces current, which is transmitted to the ECU through a cable consisting of two shielded wires. Proper spark advance is fully automatic, and is provided by a load table built into the ECU. When the knock sensor signals the ECU that pre-ignition is about to occur, the ECU will automatically retard ignition timing as required. When the threat of pre-ignition is eliminated as determined by engine load, the ECU will then slowly return ignition timing to its preset value.