Ignition System: Description and Operation

GENERAL DESCRIPTION
The Computer Controlled Coil Ignition (C3I) system consists of a camshaft sensor, crankshaft position sensor, the Electronic Spark Timing (EST) portion of the PCM, an ignition module, three ignition coils, in a one piece unit called a coil pack, and the related connecting wires.

Cam And Crankshaft Sensors:

ENGINE SENSORS
The PCM monitors sensors on the engine, compares the signals and calculates the exact position of the cam and crankshaft in the firing order, for ignition timing control.

CAMSHAFT SENSOR
The camshaft sensor sends a signal to the PCM which uses it as a "sync pulse" to trigger the injectors in proper sequence. The cam signal is passed through the ignition module. It is not processed in any way.

CRANKSHAFT SENSOR
The crankshaft sensor sends a signal through the ignition module to the PCM. The PCM uses this reference signal to calculate engine speed and crankshaft position.

KNOCK SENSOR
A knock sensor is installed in the engine block to sense engine detonation. When a knock occurs, a signal is generated that the PCM can detect. The PCM responds by retarding the ignition timing until the knocking stops, then advances the timing in small increments back to the programmed setting. This helps to maintain optimum spark advance while avoiding damaging detonation.

Ignition Current Flow:

Ignition Coils And Module:

IGNITION COILS
Three separate coils are mounted to the module assembly. Each coil provides the spark for two plugs simultaneously. Each coil can also be replaced separately. A "Waste Spark" method of distribution is used on this system. Each cylinder is paired with its opposing cylinder in firing order so that one cylinder on the compression stroke fires simultaneously with its opposing cylinder on the exhaust stroke. Requiring less voltage to fire the plug on the exhaust stroke, most of the available voltage is sent to the compression stroke cylinder. This process is reversed as the cylinders' roles are reversed.

IGNITION MODULE
The ignition module controls spark timing under 400 RPM. Below this engine speed, the timing is fixed at a factory preset value, for easy starting. The module must receive a "sync-pulse" from the camshaft sensor followed by a Crank Signal in that order to enable engine start up. At engine speeds greater than 400 rpm, the PCM controls ignition timing through signals to the module.

PCM - EST MODE
The module uses the EST signal from the PCM to determine spark timing. Above 450 rpm, the PCM applies a 5 volt reference signal to the by-pass circuit of the ignition module. While this voltage is present, the modules timing control is by-passed and the PCM controls timing and dwell (EST signal.) The module uses this EST signal to trigger the ignition coils in proper sequence and time. This is called EST mode. In this mode, the PCM also monitors the knock sensor signal and retards the ignition timing if a knock occurs.