The electronic ignition system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide optimum engine performance, fuel economy, and control of exhaust emissions, the PCM controls the spark advance of the ignition system. Electronic ignition has the following advantages over a mechanical distributor system:

The electronic ignition system does not use the conventional distributor and coil. The ignition system consists of three ignition coils, an ignition control module, a camshaft position sensor, 2 Hall-effect crankshaft position sensors, an engine crankshaft balancer with interrupter rings attached to the rear, related connecting wires, and the Ignition Control (IC) and fuel metering portion of the PCM.

Conventional ignition coils have one end of the secondary winding connected to the engine ground. In this ignition system, neither end of the secondary winding is grounded. Instead, each end of a coil's secondary winding is attached to a spark plug. Each cylinder is paired with the cylinder that is opposite it (1-4, 2-5, 3-6). These two plugs are on companion cylinders, i.e., on top dead center at the same time.

When the coil discharges, both plugs fire at the same time to complete the series circuit. The cylinder on compression is said to be the event cylinder and the one on exhaust is the waste cylinder. The cylinder on the exhaust stroke requires very little of the available energy to fire the spark plug. The remaining energy will be used as required by the cylinder on the compression stroke. The same process is repeated when the cylinders reverse roles. This method of ignition is called a waste spark ignition system.

Since the polarity of the ignition coil primary and secondary windings is fixed, one spark plug always fires with normal polarity and its companion plug fires with reverse polarity. This differs from a conventional ignition system that fires all the plugs with the same polarity. Because the ignition coil requires approximately 30% more voltage to fire a spark plug with reverse polarity, the ignition coil design is improved, with saturation time and primary current flow increased. This redesign of the system allows higher secondary voltage to be available from the ignition coils - greater than 40 kilovolts (40,000 volts) at any engine RPM. The voltage required by each spark plug is determined by the polarity and the cylinder pressure. The cylinder on compression requires more voltage to fire the spark plug than the one on exhaust.

It is possible for one spark plug to fire even though a plug wire from the same coil may be disconnected from its companion plug. The disconnected plug wire acts as one plate of a capacitor, with the engine being the other plate. These two capacitor plates are charged as a spark jumps across the gap of the connected spark plug. The plates are then discharged as the secondary energy is dissipated in an oscillating current across the gap of the spark plug that is still connected. Secondary voltage requirements are very high with an open spark plug or spark plug wire. The ignition coil has enough reserve energy to fire the plug that is still connected at idle, but the coil may not fire the spark plug under high engine load. A more noticeable misfire may be evident under load; both spark plugs may then be misfiring.

The 24X crankshaft position sensor (1), secured in an aluminum mounting bracket (3) and bolted to the front side of the engine timing chain cover (2), is partially behind the crankshaft balancer (refer to the 24X Crankshaft Position Sensor graphic).

The 7x crankshaft position sensor uses a two wire connector at the sensor and a three-way connector at the ignition control module.

The 24X crankshaft position sensor contains a Hall-effect switch. The magnet and Hall-effect switch are separated by an air gap. A Hall-effect switch reacts like a solid state switch, grounding a low current signal voltage when a magnetic field is present. When the magnetic field is shielded from the switch by a piece of steel placed in the air gap between the magnet and the switch, the signal voltage is not grounded. If the piece of steel (called an interrupter) is repeatedly moved in and out of the air gap, the signal voltage will appear to go ON-OFF-ON-OFF-ON-OFF. Compared to a conventional mechanical distributor, this ON-OFF signal is similar to the signal that a set of breaker points in the distributor would generate as the distributor shaft turned and the points opened and closed.

In the case of the electronic ignition system, the piece of steel is a concentric interrupter ring mounted to the rear of the crankshaft balancer. The interrupter ring has blades and windows that, with crankshaft rotation, either block the magnetic field or allow it to reach the Hall-effect switch. The Hall-effect switch is called a 24X crankshaft position sensor, because the interrupter ring has 24 evenly spaced blades and windows. The 24X crankshaft position sensor produces 24 ON-OFF pulses per crankshaft revolution.

The 7x crankshaft position sensor is the other Hall-effect switch closer to the crankshaft. The interrupter ring is a special wheel cast on the crankshaft that hes seven machined slots, six of which are equally spaced 60 degrees apart. The seventh slot is spaced 10 degrees from one of the other slots. as the interrupter ring rotates with the crankshaft, the slots change the magnetic field. this will cause the 7x the Hall-effect switch to ground the 3X signal voltage that is supplied by the ignition control module. The ignition control module interprets the 7x ON-OFF signals as an indication of crankshaft position. The ignition control module must have the 7x signal to fire the correct ignition coil.

The 24X interrupter ring and Hall-effect switch react similarly. The 24X signal is used for better resolution at a calibrated RPM.

The camshaft position sensor is located on the timing cover behind the water pump near the camshaft sprocket. As the camshaft sprocket turns, a magnet in it activates the Hall-effect switch in the camshaft position sensor. When the Hall-effect switch is activated, it grounds the signal line to the PCM, pulling the camshaft position sensor signal circuit's applied voltage low. This is interpreted as a CAM signal. The CAM signal is created as piston #1 is on the intake stroke. If the correct CAM signal is not received by the PCM, DTC P0341 will be set.

Three twin-tower ignition coils are individually mounted to the ignition control module. Each coil provides spark for two plugs simultaneously (waste spark distribution). Each coil is serviced separately. Two terminals connect each coil pack to the module. Each coil is provided a fused ignition feed. The other terminal at each coil is individually connected to the module, which will energize one coil at a time by completing and interrupting the primary circuit ground path to each coil at the proper time.

The ignition control module performs the following functions:

The 3X reference signal sent to the PCM by the ignition control module is an ON, OFF pulse occurring 3 times per crankshaft revolution.

To properly control ignition timing, the PCM relies on the following information:

The Ignition Control (IC) system consists of the following components:

The electronic Ignition Control Module (ICM) connector terminals are identified as shown in the Electronic Ignition System graphic. These circuits perform the following functions:

There are important considerations to point out when servicing the ignition system. The following Noteworthy Information will list some of these, to help the technician in servicing the ignition system.

The PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions. To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating Ignition Control (IC) spark timing:

The ignition system uses the same four ignition module-to-PCM circuits as did previous Delco engine management systems using distributor-type ignition. Ignition Control (IC) spark timing is the PCM's method of controlling spark advance and ignition dwell when the ignition system is operating in the IC Mode. There are two modes of ignition system operation:

In Bypass Mode, the ignition system operates independently of the PCM, with Bypass Mode spark advance always at 10 (BTDC. The PCM has no control of the ignition system when in this mode. In fact, the PCM could be disconnected from the vehicle and the ignition system would still fire the spark plugs, as long as the other ignition system components were functioning. (This would provide spark but no fuel injector pulses. The engine will not start in this situation.) The PCM switches to IC Mode (PCM controlled spark advance) as soon as the engine begins cranking. After the switch is made to IC Mode, it will stay in effect until one of the following conditions occur:

If a PCM/IC fault is detected while the engine is running, the ignition system will switch to Bypass Mode operation. The engine may quit running, but will restart and stay in Bypass Mode with a noticeable loss of performance.

In the IC Mode, the ignition spark timing and ignition dwell time is fully controlled by the PCM. IC spark advance and ignition dwell is calculated by the PCM using the following inputs:

The following describes the PCM to ignition control module circuits:

The IC output circuitry in the PCM issues IC output pulses anytime crankshaft reference signal input pulses are being received. When the ignition system is operating in the Bypass Mode (no voltage on the bypass control circuit), the ignition control module grounds the IC pulses coming from the PCM. The ignition control module will remove the ground from the IC circuit only after switching to the IC Mode. (The PCM commands switching to IC Mode by applying 5 volts on the bypass circuit to the ignition control module.) The PCM monitors its own IC output, and expects to see no pulses on the IC circuit when it has not yet applied 5 volts on the bypass control circuit. When the second 3X reference pulse at the start of crank is seen by the PCM, it applies 5 volts to the bypass control circuit and the IC pulses should no longer be grounded by the ignition control module. The PCM constantly monitors its IC output, and should detect the IC pulses only when commanding the IC Mode.

If the IC circuit is open, the PCM will detect IC output pulses while attempting to start the engine (in the Bypass Mode) due to the ignition control module not being able to ground the IC pulses. Three things will occur:

If IC circuit is grounded, the PCM would not detect a problem until the change to IC Mode is commanded by the PCM. When the PCM applies 5 volts to the bypass control circuit, the ignition control module will switch to IC Mode. With the IC circuit grounded, there would be no IC pulses for the ignition control module to trigger the ignition coils, and the engine may falter. The PCM will quickly revert back to Bypass Mode (turn OFF the 5 volts on the bypass circuit), DTC P1361 will set, and the ignition system will operate in Bypass Mode until the fault is corrected and the engine is stopped and restarted.

If bypass circuit is open or grounded, the ignition control module cannot not switch to IC Mode. In this case, the IC pulses will stay grounded in the ignition control module, and DTC P1361 will be set. The engine will start and run in Bypass Mode.

An open or ground in the IC or bypass circuit will set DTC P1350 or P1361. If a fault occurs in the IC output circuit when the engine is running, the engine may falter or quit running but will restart and run in the Bypass Mode once the ignition has been cycled. A fault in either circuit will force the ignition system to operate on Bypass Mode timing (10° BTDC), which will result in reduced performance and fuel economy.

The PCM uses information from the engine coolant temperature sensor in addition to RPM to calculate spark advance values as follows:

Therefore, detonation could be caused by high resistance in the engine coolant temperature sensor circuit. Poor performance could be caused by low resistance in the engine coolant temperature sensor circuit.

If the engine cranks but will not run or immediately stalls, Engine Cranks But Will Not Run diagnostic table must be used to determine if the failure is in the ignition system or the fuel system. If DTC P0300, P0321, P0341, P0336 P1200, P1350 P1361 or P1374 is set, the appropriate diagnostic trouble code chart must be used for diagnosis.

If a misfire is being experienced with no DTC set, refer to Symptoms section for diagnosis.