Chevrolet Sonic Repair Manual: Electronic Ignition System Description

Electronic Ignition System Operation

The electronic ignition system produces and controls the high energy secondary spark. This spark ignites the compressed air/fuel mixture at precisely the correct time, providing optimal performance, fuel economy, and control of exhaust emissions. The engine control module (ECM) collects information from the crankshaft position sensor and the intake/exhaust camshaft position sensors to determine the sequence, dwell, and timing of the spark for each cylinder. The ECM transmits a frequency signal to the ignition coil module on the individual ignition control circuits to fire the spark plugs.

Crankshaft Position Sensor

The crankshaft position sensor is an externally magnetically biased digital output integrated circuit sensing device. The sensor provides a pulse for each magnetic pole of the encoder wheel on the crankshaft. The sensor produces an ON/OFF DC voltage of varying frequency, with 58 output pulses per crankshaft revolution. The frequency of the sensor output depends on the velocity of the crankshaft. The ECM uses sensor signal pulse to determine crankshaft speed and position to calculate the best timing for ignition and fuel injection. The ECM also uses the crankshaft position sensor information to control camshaft phasing and to detect cylinder misfire.

The ECM also has a dedicated replicated crankshaft position sensor signal output circuit that may be used as an input signal to other modules for monitoring engine RPM.

The crankshaft position sensor is connected to the engine control module by the circuits listed below:

• A 5 V reference circuit
• A low reference circuit
• A signal circuit

Crankshaft Encoder Wheel

The crankshaft encoder wheel is part of the crankshaft. The encoder wheel consists of 58 tooth and a reference gap. Each tooth on the encoder wheel is spaced 6° apart with a 12° space for the reference gap. The pulse from the reference gap is known as the sync pulse. The sync pulse is used to synchronize the ignition coil module firing sequence with the crankshaft position while the other tooth provides cylinder location during a revolution.

Camshaft Position Sensors

The intake and exhaust camshaft position sensors are each triggered by a notched reluctor wheel built onto the camshaft sprockets. The four signal pulses occur every camshaft revolution. Each notch is a different size which is used to identify the compression stroke of each cylinder and to enable sequential fuel injection. The camshaft position sensors are connected to the ECM by the circuits listed below:

• A 5 V reference circuit
• A low reference circuit
• A signal circuit

Ignition Coil Module

The ignition coil module integrates the 4 coils and the ignition control module within a single sealed component.

The ignition coil module has the following circuits:

• An ignition voltage circuit
• A ground
• A low reference circuit
• 4 ignition coil control circuits

The ECM controls the individual coils by transmitting timing pulses on the ignition coil control circuit to each ignition coil to enable a spark event.

The spark plugs are connected to each coil by a short boot. The boot contains a spring that conducts the spark energy from the coil to the spark plug. The spark plug electrode is coated with platinum for long wear and higher efficiency.

Engine Control Module (ECM)

The ECM controls all ignition system functions and constantly adjusts the spark timing. The ECM monitors information from various sensor inputs that include the following:

• The crankshaft position sensor
• The accelerator pedal position (APP)
• The manifold absolute pressure (MAP) sensor
• The intake air temperature (IAT) sensor
• The vehicle speed sensor (VSS)
• The engine knock sensor
• The engine coolant temperature (ECT) sensor
• The mass airflow (MAF) sensor
• The camshaft position sensors