General System Description

Distributor Ignition (DI) System:

PURPOSE
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.

TIMING CONTROL
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
Timing Control
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.

Ignition Control Module
The PCM provides power, ground, and two 5 volt reference voltages to the ignition control module. As the camshaft turns (during crank or run), a slotted two-track timing disk is rotated inside the ignition control module. Each time a slot in either track of the disk is properly aligned, the ignition control module will pulse one of the 5 volt reference voltages to ground.

Reference Signal
A reference signal is generated each time the PCM detects that the reference voltages have been grounded by the ignition control module. When the PCM detects reference signals, ignition timing can be processed.

4X and 180X Signals Compared
The PCM will also compare the 4X and the 180X 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 180X signal is missing. The vehicle will not run if the 4X (REFERENCE) signal is not available at the PCM for processing.

SPARK ADVANCE CALCULATION
The PCM uses information from the MAP and engine coolant temperature sensor in addition to RPM to calculate spark advance as follows:

^ Cold engine = More spark advance.

^ Engine under minimum load based on RPM and low amount of air flow - More spark advance.

^ Hot engine = Less spark advance.

^ Engine under heavy load based on RPM and high amount of air flow - Less spark advance.


RESULTS OF INCORRECT OPERATION
An open IC circuit will set a DTC P1351. A grounded IC circuit will set a DTC P1361. An open or grounded IC circuit will result in an engine cranks but will not run. An open or ground in the low resolution signal circuit will set a DTC P1371 and the engine will not start. If the high resolution signal circuit becomes open or grounded a DTC P0372 would set. This will cause reduced performance and poor fuel economy.

An inoperative distributor vent system may cause premature distributor failure. To check system operation perform Distributor Vent System Check