Ignition System: Description and Operation

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 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 CKP sensor B is a variable reluctance sensor. The magnetic field of the sensor is altered by a crankshaft mounted reluctor wheel that has seven machined slots, six of which are equally spaced 60 degrees apart. The seventh slot is spaced 10 degrees after one of the 60 degree slots. This sensor provides the ignition control module (ICM) with 7X signals, or seven pulses for each revolution of the crankshaft. The pulse from the 10 degree slot is known as the sync pulse. Both of the sensor circuits are connected to the ignition control module (ICM). A signal converter within the ICM produces digital 3X output pulse to the powertrain control module (PCM), the 3X reference is known as the low resolution engine speed signal.
The CKP sensor A contains a hall-effect switch. A hall-effect switch is a solid state switching device that produces a digital ON/OFF pulse when a rotating element passes between the sensor tip and a magnet. This rotating element is called an interrupter ring or blade. In this case the interrupter ring has 24 evenly spaced blades and windows and is part of the crankshaft damper assembly. This sensor provides the PCM with 24X signals, or 24 identical pulses per crankshaft revolution. The 24X signal is used for enhanced smoothness and idle stability at a lower calibrated RPM. The PCM supplies the sensor with a 12-volt reference, low reference, and signal circuit.
- Camshaft Position (CMP) Sensor
The 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 PCM to determine the position of the valve train relative to the crankshaft position. By monitoring the CMP and CKP signals the PCM can accurately time the operation of the fuel injectors. The PCM supplies the sensor with a 12-volt reference, low reference, and signal circuit.
- 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 receives and processes the signals from the CKP sensor B.
- 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 the 7X CKP signal. 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 in the correct sequence.
- The ICM produces and inputs 3X reference signals to the PCM.
- The ICM contains the coil driver circuits that command the coils to operate.
- The 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, 3X reference-PCM input-From the ICM, the PCM uses this signal to calculate engine RPM and CKP. The PCM also uses the pulses on this circuit to initiate injector operation.
- 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 IC module. This circuit creates a common ground plane and assures there is no ground drop between the PCM and IC module.
- IC timing signal -PCM output-ICM controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives 3X 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-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 24X reference and 3X reference pulses, the PCM applies 5 volts to the IC timing signal circuit. This signals the IC module 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/ON, or the fault is no longer present. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.