Ignition System: Description and Operation

ELECTRONIC IGNITION (EI) SYSTEM DESCRIPTION
The electronic ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system uses an individual coil for each cylinder. The ignition coils and ignition control module (ICM) are contained within two assemblies, one for each cylinder bank. The assemblies are mounted in the center of each camshaft cover, with short boots connecting the coils to the spark plugs. The driver modules within each ICM are commanded ON/OFF by the powertrain control module (PCM). The PCM primarily uses engine speed and position information from the crankshaft (CKP) and camshaft position (CMP) sensors to control the sequence, dwell, and timing of the spark.

The EI system consists of the following components:

Camshaft Position (CMP) Sensor
The camshaft position (CMP) sensor is also a magneto resistive sensor, with the same type of circuits as the crankshaft position (CKP) sensor. The CMP sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor information is used by the PCM to determine the position of the valve train relative to the crankshaft position.

Camshaft Reluctor Wheel
The camshaft reluctor wheel is part of the camshaft sprocket. The reluctor wheel profile is a smooth track, half of which is of a lower profile than the other half. This allows the CMP sensor to supply a signal as soon as the key is turned ON, since the CMP sensor reads the track profile, instead of a notch.

Crankshaft Position (CKP) Sensor
The crankshaft position (CKP) sensor is a three wire sensor based on the magneto resistive principle. A magneto resistive sensor uses two magnetic pickups between a permanent magnet. As an element such as a reluctor wheel passes the magnets the resulting change in the magnetic field is used by the sensor electronics to produce a digital output pulse. This system uses two sensors within the same housing for the V6 engine, and two separate sensors for the V8 engine. The PCM supplies each sensor a 12-volt reference, low reference, and a signal circuit. The signal circuit returns a digital ON/OFF pulse 24 times per crankshaft revolution.

Crankshaft Reluctor Wheel
The crankshaft reluctor wheel is part of the crankshaft. The notches on the reluctor wheel provide a unique pattern for each pair of cylinders that are at top dead center (TDC) at the same time. This is known as pulse width encoding. This pulse width encoded pattern allows the PCM to quickly recognize which pair of cylinders are at TDC.

Ignition Coil/ICM Assembly
Each ignition control module (ICM) has the following circuits:
- An ignition 1 voltage circuit
- A chassis ground
- An ignition control circuit for each cylinder
- A low reference circuit

The PCM controls spark by pulsing the ignition control circuits to the ICM to trigger the coils and fire the spark plugs. The PCM and ICM are internally protected against shorts to power and ground on the ignition control circuits.

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 plugs are tipped with platinum for long wear and higher efficiency.

Powertrain Control Module (PCM)
The PCM controls all ignition system functions, and constantly corrects the basic spark timing. The PCM monitors information from various sensor inputs that include the following:
- The throttle position (TP) sensor
- The engine coolant temperature (ECT) sensor
- The mass airflow (MAF) sensor
- The intake air temperature (IAT) sensor
- The vehicle speed sensor (VSS)
- The transmission gear position or range information sensors
- The engine knock sensor (KS)

Modes of Operation
Both CKP sensors provide identical pulses, although one signal is shifted several degrees of crankshaft rotation to the other. This amount depends on whether the sensors are separate or integrated with each other. The two CKP signals allow the PCM to perform an angle-based decode operation. This is considered a self-clocked system, where one sensor acts as a clock and the other is a data signal. The advantage of angle-based decoding is the increased accuracy and consistency of signals, even during engine acceleration and deceleration. If one sensor is not operating correctly, the PCM uses a time-based decode operation. This mode will read the pulse width of the remaining signal, and thereby provide a means of back-up with the minimum of performance loss. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.