Electronic Ignition (EI) System Description

The Electronic Ignition (EI) system is responsible for producing and controlling 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. Each pair of cylinders that are at Top Dead Center (TDC) at the same time are known as companion cylinders. 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:

Crankshaft Position (CKP) Sensors
The Crankshaft Position (CKP) sensor contains two hall-effect switches in one housing. 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 sensors magnetic field. The rotating element is called an interrupter ring or blade. In this case there are two interrupter rings built into the crankshaft balancer. The outer ring and outer switch provides the Ignition Control Module (ICM) with 18 X signals, or 18 identical pulses per crankshaft revolution. The inner ring and inner switch provides the ICM with three pulses per revolution, each one of different duration. This is called the sync pulse, each pulse represents a pair of companion cylinders. The ICM supplies a 12-volt and low reference circuit to the CKP sensor, and uses the 18 X and sync pulses to determine the crankshaft position, by counting how many ON-OFF 18 X pulses occur during a given 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.

Camshaft Position (CMP) Sensor
The Camshaft Position (CMP) sensor signal is a digital ON/OFF pulse, 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 Powertrain Control Module (PCM) to determine the position of the valve train relative to the CKP. By monitoring the CMP and CKP signals the PCM can accurately time the operation of the fuel injectors. The CMP sensor shares 12-volt and low reference circuits with the CKP sensor. The CMP signal circuit is input to the ICM.

Ignition Control Module (ICM) and Ignition Coils
Three dual tower ignition coils are mounted to the ICM, and are serviced individually. The ICM performs the following functions:
^ The ICM supplies a power and low reference circuit to the CMP and CKP sensors.
^ The ICM determines the correct direction of the crankshaft rotation, and cuts spark and fuel delivery to prevent damage from backfiring if reverse rotation is detected.
^ The ICM determines the correct coil triggering sequence, based on how many 18 X ON-OFF pulses occur during a sync pulse. This coil sequencing occurs at start-up, and is remembered by the ICM. After the engine is running, the ICM will continue to trigger the coils without the CKP input.
^ The ICM inputs 18 X and 3 X reference signals to the PCM.
^ The 3 X reference signal is also known as the low resolution engine speed signal. This signal is generated by the ICM using an internal divide-by-six circuit. This circuit divides the 18 X signal pulses by six. This divider circuit will not begin operation without a sync pulse present at start-up, and without 18 X and 3 X reference signals no fuel injection will occur.

Powertrain Control Module (PCM)
The PCM is responsible for maintaining 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 monitors input signals from the following components in calculating ignition spark timing:
^ The Ignition Control Module (ICM)
^ The Throttle Position (TP) sensor
^ The Engine Coolant Temperature (ECT) sensor
^ The Mass Air Flow (MAF) sensor
^ The Intake Air Temperature (IAT) sensor
^ The Vehicle Speed Sensor (VSS)
^ The transmission gear position or range information sensors
^ The engine Knock Sensors (KS)

The following describes the PCM to ICM circuits:
^ Low resolution engine speed - 3 X reference - PCM input - from the ICM. The PCM uses this signal to calculate engine RPM and crankshaft position above 1200 RPM. The PCM also uses the pulses on this circuit to initiate injector operation.
^ Medium resolution engine speed signal - 18 X reference - PCM input - from the ICM. The 18 X reference signal is used to accurately control spark timing at low RPM and allow ignition control (IC) operation during cranking. Below 1200 RPM, the PCM is monitoring the 18 X reference signal and using the 18 X signal as the reference for ignition timing advance. When engine speed exceeds 1200 RPM, the PCM begins using the 3 X reference signal to control spark timing.
^ Camshaft position - PCM input - from the ICM. The PCM uses this signal 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 CAM pulses to the number of 18 X and 3 X reference pulses. If the number of 18 X and 3 X reference pulses occurring between CAM pulses is incorrect, or if no CAM pulses are received while the engine is running, the PCM will set a DTC. If the CAM 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 CAM 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 - PCM input - this is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ICM. This circuit assures there is no ground drop between the PCM and ICM.
^ IC timing signal - PCM output - to the ICM. The ICM controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives 3 X reference signals 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 - PCM output - to the ICM. The IC output circuitry of the PCM sends out timing signals to the ICM on this circuit. When in the Bypass Mode, the ICM grounds these signals. When in the IC Mode, the signals are sent to the ICM to control spark timing.

Modes of Operation
Anytime the PCM does not apply 5 volts to the IC timing signal circuit, the ICM controls ignition by triggering each coil in the proper sequence at a pre-calibrated timing advance. This is called Bypass Mode ignition used during cranking or running below a certain RPM, or during a default mode due to a system failure.

When the PCM begins receiving 18 X reference and 3 X reference pulses, the PCM applies 5 volts to the IC timing signal circuit. This signals the ICM to allow the PCM to control the spark timing. This is IC mode ignition. During IC mode, the PCM compensates for all driving conditions. If the IC mode changes due to a system fault, the system will stay in default until the ignition is cycled OFF to ON, or the fault is no longer present. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.