Ignition System: Description and Operation

CONSTRUCTION
The Electronic Ignition (EI) system consists of a Crankshaft Position (CKP) sensor, coil pack, connecting wiring, and Powertrain Control Module (PCM).

PURPOSE
The ignition system is designed to ignite the compressed air / fuel mixture in an internal combustion engine by a high voltage spark from an ignition coil. The ignition system also provides timing information to the PCM for proper vehicle operation and misfire detection.

OPERATION

Calculated Spark Timing
The CKP sensor is used to indicate crankshaft position and speed by sensing a missing tooth on a pulse wheel mounted to the crankshaft. The PCM uses the CKP signal to calculate a spark target and then fires the coil pack to that target.

Firing of The Ignition Coils
The coil packs receive their signal from the PCM to fire at the calculated spark target. Each coil within the pack fires two spark plugs at the same time. The plugs are paired so that as one fires during the compression stroke, the other fires during the exhaust stroke. The next time the coil is fired the situation is reversed.

Ignition Timing is Controlled by The PCM.
The PCM acts as an electronic "switch" to ground in the coil primary circuit. When the switch is closed, battery power applied to the coil primary circuit builds a magnetic field around the primary coil. When the switch opens, the power is interrupted and the primary field collapses inducing the high voltage in the secondary coil windings and the spark plug is fired.

Ignition Diagnostic Monitor (IDM) Signal.
A kickback voltage spike occurs when the primary field collapses. The PCM uses this voltage spike to generate an IDM signal. IDM communicates information by pulsewidth modulation in the PCM.

Tachometer Signal
The PCM processes the CKP signal and uses it to drive the tachometer as the "Clean Tach Out" (CTO) signal.