Ignition System: Description and Operation

Distributor Ignition (DI) System:

DESCRIPTION
The distributor ignition system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide improved engine performance, fuel economy, and control of exhaust emissions, the Powertrain Control Module (PCM) controls distributor spark advance (timing) with an Ignition Control (IC) system.

To properly control ignition and combustion timing the PCM needs to know:
^ Camshaft position.
^ Engine speed (RPM).
^ Engine load (manifold pressure or vacuum).
^ Atmospheric (barometric) pressure.
^ Engine coolant temperature.
^ The amount of detonation.

OPERATION
The distributor ignition system is a net build distributor (no timing adjustment) that provides angle based timing information to the PCM for individual cylinder spark timing. The ignition module is capable of providing both 4X and 18OX timing pulses each crankshaft revolution. Using these timing pulses, the PCM processes ignition spark timing and sends an IC signal to the ignition coil module to activate the secondary ignition system.

The PCM will also compare the 4X and the 18OX signals to each other to determine the location of the number one cylinder and top dead center. In addition, if only one signal is being received by the PCM a Diagnostic Trouble Code (DTC) will be set. DTC P1371 will be set if the 4X signal is missing and DTC P0372 will be set if the 18OX signal is missing. The vehicle will not run if the 4X (Reference) signal is not available at the PCM for processing.

COMPONENTS/CIRCUITS
The Ignition Control (IC) system consists of the following:

^ Camshaft Position Sensor
^ Ignition Coil
^ Ignition Coil Module
^ Powertrain Control Module.

These circuits perform the following functions:

Low resolution signal reference
- This provides the PCM with reference signals, firing order, and camshaft position information. If the low resolution signal circuit becomes open or grounded, the engine will not run because the PCM will not operate the ignition coil module, fuel pump, or the fuel injectors.
- A DTC P1371 will set if the PCM receives high resolution signal references without the low resolution signals. If the engine cranks but will not start, refer to Engine Cranks but Does Not Run.

High resolution signal reference
- This provides the PCM with detailed reference signals and crankshaft position information.
- If this circuit becomes opens or grounded a DTC P0372 will set and the engine will still run.
- When there is a malfunction with the high resolution circuit, excessive crank times will be experienced.

System ground
- This circuit is grounded at the PCM and provides ground for the distributor to generate low and high resolution signal reference signals.
- If this circuit becomes open, the engine will not run since there will be no reference information.

Ignition Control (IC)
- This circuit controls the ignition coil ON and OFF time. It signals the ignition coil module to begin primary coil dwell current when the IC is high. The ignition coil module shuts OFF ignition coil current when the signal goes low.
- If this circuit becomes open, shorted to voltage, or grounded, the engine will not start and a DTC P1351 or P1361 will set.