Distributor Operation

The ignition system consists of a distributor assembly, control circuits for ignition timing in the Powertrain Control Module (PCM), a separate ignition coil/ignition control module assembly, vacuum harness, primary and secondary wiring, and the spark plugs. All ignition timing is controlled by the PCM, there is no "bypass" mode built into the ignition control module as in other ignition systems.

The distributor assembly is mounted directly to the front cover behind the water pump assembly. It directs the spark from the ignition coil assembly to the proper spark plug secondary wire through a rotor. Spark plug secondary wire connectors in the distributor cap are arranged for convenience in routing spark plug wire harness assemblies. The corresponding cylinder number is molded into the distributor cap next to each spark plug secondary wire connector.

The distributor assembly also contains a signal disk and two optical Camshaft Position (CMP) sensor assemblies that provide signals to the PCM for timing control. One CMP sensor assembly senses 360 equally-spaced slots in the signal disk to provide 720 signals per camshaft revolution for high resolution timing control. The other CMP sensor assembly senses eight disk slots of variable length, providing a low-resolution signal that tells the PCM which spark plug is being fired. Power for the optical CMP sensor assemblies, a system ground, and the two CMP sensor assembly signals are transferred between the PCM and the distributor assembly through a shielded harness connected to the four-terminal connector on the distributor,

The ignition system supplies two timing inputs to the PCM, a high resolution signal (360 pulses per one camshaft revolution) and a low resolution signal (eight pulses per one camshaft revolution). The PCM can determine if one of the timing inputs is not being received by comparing the two inputs. If the PCM detects one timing pulse without detecting the other timing pulse, Diagnostic Trouble Code (DTC) 16 or 36 will set.

The reference signals toggle between 0 and 5 volts as the camshaft assembly turns. Therefore, an open, a short to voltage, a short to ground, or a defective sensor inside the distributor assembly can prevent the voltage from pulsing at the PCM.

The ignition coil/ignition control module assembly provides spark to the distributor assembly, timed by signals from the PCM. Power (B+) for the ignition coil primary circuit and the ignition control Module is supplied by the ignition switch. The PCM combines the camshaft position information supplied by the distributor with other system parameters and calculates the required spark advance and coil dwell. The PCM signals the ignition control module, which turns on the primary current to the ignition coil by grounding the primary circuit, and then turns it off by removing the ground. When the primary current flow stops, high voltage induced in the ignition coil secondary winding becomes the spark voltage for the spark plug. The spark voltage is delivered to the distributor assembly through the coil output (secondary) wire, and then directed to the proper spark plug connector by the distributor rotor.