Ignition System Description

The following are the primary components of the distributor ignition (DI) system:
^ The ignition coil
^ The ICM
^ The distributor
^ The PCM
^ The PCM timing control circuits
^ The vacuum harness primary wiring
^ The vacuum harness secondary wiring
^ The spark plugs
^ The camshaft position (CMP) sensors
^ The crankshaft position (CKP) sensor

The distributor is mounted on the front cover behind the water pump. The distributor directs the spark from the ignition coil through a rotor to the proper spark plug secondary wire. The spark plug secondary wire connectors in the cap ease in the routing of the spark plug wire harnesses. A cylinder number is molded on each distributor cap next to each spark plug secondary wire connector.

The distributor contains a signal disk and optical camshaft position (CMP) sensors that send signals to the PCM for timing control. The PCM controls all ignition timing. There is no bypass mode built into the ICM. One CMP sensor senses 360 equally spaced slots in the signal disk in order to provide 720 signals per camshaft revolution for high resolution timing control. The second CMP sensor senses eight disk slots of varying lengths, providing a low resolution signal. This signal tells the PCM which of the spark plugs is being fired.

The PCM determines if one of the timing inputs is not being received by comparing the inputs. If the PCM detects one timing pulse without detecting the other, DTC 16 or DTC 36 will set. The following forces are transferred between the PCM and the distributor through a shielded harness on the terminal distributor connectors:
^ The power for the optical CMP sensors
^ The system ground
^ The CMP sensor signals

The reference signals toggle between 0-5 volts as the camshaft turns. Any of the following situations can prevent the voltage from pulsing at the PCM:
1. An open circuit
2. A short to voltage
3. A short to ground
4. A defective sensor

The ignition coil and the ICM provide a spark to the distributor, timed by the PCM signals. The ignition switch provides the power for the following components:
^ The ignition coil primary circuit
^ The ICM

The PCM combines the CMP information supplied by the distributor with other system parameters. The PCM then calculates the required spark advance. The PCM signals the ICM. The ICM turns on the primary circuit to the ignition coil by the following actions:
1. Grounding the primary circuit
2. Removing the ground in order to turn off the primary circuit to the ignition coil

The high voltage induced in the ignition coil secondary winding when the primary circuit current flow stops becomes the spark voltage for the spark plug. The spark voltage undergoes two deliveries, which include:
1. The delivery to the distributor through the secondary coil output wire
2. The delivery to the proper spark plug connector by the distributor rotor.