Ignition System LT1

The following are the primary components of the distributor ignition (DI) system:

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 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 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

Refer to Distributor Ignition (DI) System Check in Engine Controls.

Ignition System L36

The 3800 VIN K electronic ignition system uses a coil pack. For each two cylinders in the engine, the coil pack uses one ignition coil. The ignition control module (ICM) is mounted under the ignition coils. The ICM performs ignition coil switching functions and interacts with the powertrain control module (PCM) in order to optimize ignition system operation.

The 3800 ignition system spark timing is controlled electronically. The ICM controls the spark timing during engine start-up. The ICM provides a back-up timing system that will allow the engine to run in the event of an open or ground in the ignition control circuit. Once the engine starts and is running above 400 rpm, the PCM takes over the spark timing and sends signals to the ICM for ignition coil switching.

The electronic ignition system uses a waste spark method of spark distribution. Each cylinder is paired with its opposing cylinder in the firing order. One cylinder on compression fires simultaneously with the opposing cylinder on exhaust. The exhaust stroke spark is referred to as the waste spark.

The spark plugs in the two opposing cylinders are connected to the two secondary terminals of the same ignition coil. The spark voltage appears at the center electrode of one of the spark plugs and jumps the side electrode. The spark voltage then passes through the cylinder head to the other spark plug. At the second spark plug, the spark jumps from the side electrode to the center electrode. The spark then completes the series circuit back to the ignition coil. The high level of energy available from the ignition coil fires both plugs simultaneously. Since the waste spark requires very little voltage to fire, the coil output voltage fires the cylinder that is on the compression stroke.