The electronic ignition (EI) system produces a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system uses one coil for each pair of cylinders. Companion cylinders are a pair of cylinders that are at top dead center (TDC) at the same time. The cylinder that is at TDC of the compression stroke is called the event cylinder. The cylinder that is at TDC of the cylinder exhaust stroke is called the waste cylinder. When the coil is triggered both companion cylinder spark plugs fire at the same time, completing a series circuit. Because the lower pressure inside the waste cylinder offers very little resistance, the event cylinder uses most of the available voltage to produce a very high energy spark. This is known as waste spark ignition. The EI system consists of the following components:

The crankshaft position (CKP) sensor has a 4-wire harness connector that plugs into the CKP sensor and connects to the ignition control module (ICM). The CKP sensor contains two hall-effect switches within the same housing, and shares a magnet between the switches. The magnet and each hall-effect switch are separated by an air gap. A hall-effect switch is a solid state switching device that produces a digital ON/OFF pulse when a rotating element passes the sensor pick-up and interrupts the magnetic field of the sensor. The rotating element is called an interrupter ring. There are two interrupter rings built into the crankshaft balancer. When the sync interrupter ring window is between the magnet and the switch, the magnetic field will cause the hall-effect switch to ground the supplied voltage from the ICM. The outer ring and the outer switch provide the ICM with 18X signals or 18 identical pulses per crankshaft revolution. The inner ring and the inner switch provide the ICM with 3X signals or 3 pulses per crankshaft revolution, each having a different duration.

The camshaft position (CMP) sensor signal is a 1X digital ON/OFF pulse, with an output once per revolution of the camshaft. The CMP sensor does not directly affect the operation of the ignition system. The CMP sensor information is used by the PCM to determine the position of the valve train relative to the CKP sensor. By monitoring the CMP and CKP signals the PCM can accurately time the operation of the fuel injectors. The CMP signal circuit is an input to the ICM.

The ICM receives an 18X pulse from the CKP sensor 1 signal circuit and a 3X pulse from the CKP Sensor 2 signal.

The 3X is called the sync pulse. Each sync pulse represents a pair of companion cylinders and is also used by the ICM for a fast start. The 18X reference pulses are passed from the CKP sensor to the ICM on the CKP sensor 1 signal circuit. The ICM uses the CKP Sensor 1 18X and CKP Sensor 2 3X sync pulses to determine the crankshaft position by counting how many ON-OFF 18X pulses occur during a sync pulse. With this dual interrupter ring arrangement the ICM can identify the correct pair of cylinders to fire within as little as 120 degrees of crankshaft rotation for a fast start.

The ICM uses the CKP Sensor 1 and 2 signals to determine the correct coil triggering sequence, based on how many 18X ON-OFF pulses occur during a sync pulse. The ICM will also determine the correct direction of the crankshaft rotation, and will prevent spark to the coils to prevent damage from backfiring if reverse rotation is detected. Coil sequencing occurs at start-up, and is remembered by the ICM. Once the engine is running, the ICM will continue to trigger the coils without the CKP sync pulse.

The 18X CKP Sensor 1 signal is used by the ICM to convert the analog signal to a digital 3X signal for use by the PCM. The ICM divides the 18X signal pulses by 6. This divider circuit will not begin operation without a sync pulse present at start-up.

The 18X CKP Sensor 1 signal is also used by the ICM to convert the analog signal to a digital 18X signal for use by the PCM.

The PCM maintains proper spark and fuel injection timing for all driving conditions. Ignition control (IC) spark timing is the method the PCM uses to control spark advance. To provide optimum driveability and emissions, the PCM uses the following circuits to calculate ignition spark timing:

Low resolution engine speed signal is produced by the ICM. The PCM uses this signal to calculate engine RPM and crankshaft position above 1,200 RPM. The PCM also uses the pulses on this circuit to initiate fuel injector operation. The PCM compares the number of 3X pulses to the number of 18X and cam pulses. If the number of 3X pulses is incorrect while the engine is cranking or running, the PCM will set a DTC. Medium resolution engine speed signal is used to accurately control spark timing at low RPM and allow ignition control (IC) operation during cranking. The ICM calculates the 18X reference signal by filtering the 18X CKP sensor 1 pulses when the engine is running and the sync pulses from the 3X CKP Sensor 2 are being received. The PCM uses the Medium resolution engine speed signal for accurate ignition timing below 1,200 RPM. The PCM compares the number of 18X medium resolution engine speed signal pulses to the number of 3X low resolution engine speed signal pulses and 1X cam pulses. If the number of 18X pulses is incorrect while the engine is cranking or running, the PCM will set a DTC. The engine will continue to start and run normally using the 3X reference signal.

Camshaft position signal is used to determine the position of the cylinder #1 piston during the pistons power stroke. This signal is used by the PCM to calculate true sequential fuel injection (SFI) mode of operation. The PCM compares the number of 1X CMP Sensor signal pulses to the number of 18X medium resolution engine speed signal pulses and 3X low resolution engine speed signal pulses. If the number of medium resolution and low resolution engine speed signal pulses occurring between 1X CMP Sensor signal pulses is incorrect, or if no 1X CMP Sensor signal pulses are received while the engine is running, the PCM will set a DTC. If the 1X CMP sensor signal is lost while the engine is running the fuel injection system will shift to a calculated sequential fuel injection mode based on the last CMP sensor signal pulse, and the engine will continue to run. The engine can be re-started and will run in the calculated sequential mode as long as the condition is present with a 1 in 6 chance of being correct.

Low reference is used to provide a common reference between the ICM and the PCM. This is a low reference for the digital RPM counter inside the PCM; the wire is connected to engine ground only through the ICM. IC timing signal is used to control spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives the 3X low resolution engine speed signal from the ICM, the PCM applies 5 volts to the IC timing signal circuit allowing the ICM to switch spark advance to PCM control.

IC timing control is used to control timing signals to the ICM on this circuit. When in the Bypass Mode, the ICM grounds these signals. When in the IC Mode, the PCM sends the signal to the ICM to control spark timing.

Knock Sensor signals is used to detect engine vibration or noise level.

The PCM has 2 modes of operation:

Bypass mode is used by the ICM to control the triggering of each coil for base spark timing. During bypass mode the timing is fixed at 10° BTDC and the PCM does not apply 5 volts to the IC timing signal circuit. The Bypass mode is used during each of the following conditions:

IC Mode is used by the PCM to accurately control spark timing for all driving conditions. During IC mode the PCM is receiving the 18X and the 3X reference pulses from the ICM and is supplying 5 volts to the IC timing signal circuit. This allows the PCM to control spark timing. Knock Sensor (KS)

The knock sensor (KS) system enables the PCM to control the ignition timing for the best possible performance while protecting the engine from potentially damaging levels of detonation. The PCM uses the KS system to test for abnormal engine noise that may indicate detonation, also known as spark knock.

The KS system uses two flat response two-wire sensors. The sensor uses piezo-electric crystal technology that produces an AC voltage signal of varying amplitude and frequency based on the engine vibration or noise level. The amplitude and frequency is dependant upon the level of knock that the KS detects. The PCM receives the KS signal through a signal circuit. The KS ground is supplied by the PCM through a low reference circuit.

The PCM learns a minimum noise level, or background noise, at idle from the KS and uses calibrated values for the rest of the RPM range. The PCM uses the minimum noise level to calculate a noise channel. A normal KS signal will ride within the noise channel. As engine speed and load change, the noise channel upper and lower parameters will change to accommodate the normal KS signal, keeping the signal within the channel. In order to determine which cylinders are knocking, the PCM only uses KS signal information when each cylinder is near top dead center (TDC) of the firing stroke. If knock is present, the signal will range outside of the noise channel.

If the PCM has determined that knock is present, it will retard the ignition timing to attempt to eliminate the knock. The PCM will always try to work back to a zero compensation level, or no spark retard. An abnormal KS signal will stay outside of the noise channel or will not be present. KS diagnostics are calibrated to detect faults with the KS circuitry inside the PCM, the KS wiring, or the KS voltage output. Some diagnostics are also calibrated to detect constant noise from an outside influence such as a loose/damaged component or excessive engine mechanical noise.