Ignition Management: Notes
Ignition Coils: The high voltage supply required to ignite the mixture in the combustion chambers is determined by the stored energy in the ignition coils. The stored energy contributes to the ignition duration, ignition current and rate of high voltage increase. The Coil circuit including primary and secondary components consists of:
- Coil Assembly
- Primary Winding
- Secondary Winding (with diode)
- Resistor (Boot Connector)
- Spark Plug
- ECM Final Stage Transistor
- Secondary Coil Ground
The Coil Assembly contains two copper windings insulated from each other. One winding is the primary winding, formed by a few turns of thick wire. The secondary winding is formed by a great many turns of thin wire.
The primary winding receives battery voltage from the ignition switch (Terminal 15). The ECM provides a ground path for the primary coil (Terminal 1) by activating a Final Stage transistor. The length of time that current flows through the primary winding is the "dwell" which allows the coil to "saturate" or build up a magnetic field. After this storage process, the ECM will interrupt the primary circuit at the point of ignition by deactivating the Final Stage transistor. The magnetic field built up within the primary winding collapses and induces the ignition voltage in the secondary winding.
The voltage generated in the secondary winding is capable of 30,000 volts (30 KV). The high voltage is discharged (Terminal 4) through the secondary ignition cable and resistor (boot connector) to the spark plug.
The primary and secondary windings are uncoupled, therefore, the secondary winding requires a ground supply (Terminal 4a).
The secondary winding connects to a cascade diode which suppresses any unwanted induced voltages as the primary circuit is switched on and off. This permits a clean, high voltage discharge from the secondary winding.
There is an individual ignition circuit and coil for each cylinder on the M1.7.2 system
The four ignition coils are combined into a single component (coil pack) located on the right front strut tower.
The ignition primary circuits are fault monitored by the ECM. If a fault is present, the "CHECK ENGINE" Light will illuminate and the ECM will deactivate the corresponding fuel injector for that cylinder and engine operation will still be possible.
Ignition Leads: The secondary ignition cables (high tension leads) direct the high voltage from the ignition coils to the spark plugs. The ignition lead assembly consists of:
- Connector Socket
- Ignition Cable
- Resistive Adaptive Boot
The ignition cables are routed into a covered cable tray located on the top of the cylinder head, which contains the boot connector removal tool (arrow).
Spark Plugs: The spark plugs introduce the ignition energy into the combustion chamber. The high voltage "arcs" across the air gap in the spark plug from the positive electrode to the negative electrode. This creates a spark which ignites the combustible air/fuel mixture.
The spark plugs are located in the center of the combustion area (on the top of the cylinder head) which is the most suitable point for igniting the compressed air/fuel mixture.
The correct spark plugs for this system are:
- Bosch F7LDCR (dual electrode, non-adjustable gap)
- NGK BKR7EK (dual electrode, non-adjustable gap)
Faults with the Ignition Output Components are not monitored by the ECM, with the exception of the primary ignition circuit. If there are faults with the ignition coil(s) output, ignition leads and/or spark plugs, the following complaints could be encountered:
- "CHECK ENGINE" Light With Mixture Related Fault Codes
- Poor Engine Performance
- Engine Misfire
- No Start / Hard Starting
- Excessive Exhaust Emissions / Black Smoke
The Ignition Output Components must be individually tested (see WORKSHOP HINTS )
Knock Sensors: are required to prevent detonation (pinging) from damaging the engine. The Knock Sensor is a piezoelectric conductor-sound microphone. The ECM will retard the ignition timing (cylinder selective) based on the input of these sensors. Detonation can occur due to:
- High Compression Ratio
- Poor Quality Fuel (Octane Rating)
- High Level of Cylinder Filling
- Maximum Timing Advance Curve
- High Intake Air and Engine Temperature
- Carbon Build-Up (Combustion Chamber)
The Knock Sensor consists of:
- Shielded Wire
- Cup Spring
- Seismic Mass
- Housing
- Inner Sleeve
- Piezo-Ceramic Element
A piezo-ceramic ring is clamped between a seismic mass and the sensor body. When the seismic mass senses vibration (flexing), it exerts a force on the peizo-ceramic element. Opposed electrical charges build up on the upper and lower ceramic surfaces which generates a voltage signal. The acoustic vibrations are converted into electrical signals. These low voltage signals are transmitted to the ECM for processing.
There are two Knock Sensors bolted to the engine block between cylinders 1 & 2 and between cylinders 3 & 4. If the signal value exceeds the threshold, the ECM identifies the "knock" and retards the ignition timing for that cylinder.
If a fault is detected with the sensors, the ECM deactivates Knock Control. The "CHECK ENGINE" Light will be illuminated, the ignition timing will be set to a conservative basic setting and a fault will be stored.
Crankshaft Position/RPM Sensor: This sensor provides the crankshaft position and engine speed (RPM) signal to the ECM for ignition activation and correct timing. For details about the sensor, refer to the FUEL MANAGEMENT section.
A fault with this input will produce the following complaints:
- No Start
- Intermittent Misfire/Driveability
- Engine Stalling
Camshaft Position Sensor (Cylinder Identification): The cylinder ID sensor (inductive pulse) input allows the ECM to determine camshaft position in relation to crankshaft position. It is used by the ECM to establish the "working cycle" of the engine for precise ignition timing. For details about the sensor, refer to the FUEL MANAGEMENT section.
If the ECM detects a fault with the Cylinder ID Sensor, the "CHECK ENGINE" Light will be illuminated and the system will still operate based on the Crankshaft Position/RPM Sensor.
Upon a restart, a slight change in driveability could occur because the ECM will activate "double ignition" . The ignition coils will be activated on both the compression and exhaust strokes to maintain engine operation.
Engine Coolant Temperature: The ECM determines the correct ignition timing required for the engine temperature. For details about the sensor, refer to the FUEL MANAGEMENT section. This sensor is located in the coolant jacket of the cylinder head.
If the Coolant Temperature Sensor input is faulty, the "CHECK ENGINE" Light will be illuminated and the ECM will assume a substitute value (80° C) to maintain engine operation. The ignition timing will be set to a conservative basic setting.
Throttle Position Sensor: This sensor provides the ECM with throttle angle position and rate of movement.
As the throttle plate is opened, this requests acceleration and at what rate. The ECM will advance the ignition timing. The "full throttle" position indicates maximum acceleration to the ECM, the ignition will be advanced for maximum torque.
If the Throttle Position input is defective, a fault code will be set and the "Check Engine" Light will illuminate. The ECM will maintain engine operation based on the Air Flow Volume Sensor and the Engine Speed Sensor, and the ignition timing will be set to a conservative basic setting.
Air Flow Volume Sensor: This signal to the ECM represents the measured amount of intake air volume. This input is used by the ECM to determine the amount of ignition timing advance.
If this input is defective, a fault code will be set and the "Check Engine" Light will illuminate. The ECM will maintain engine operation based on the Throttle Position Sensor and Engine Speed Sensor, and the ignition timing will be set to a conservative basic setting.
Air Temperature: This signal allows the ECM to make a calculation of air density. The sensor is located in front of the measuring flap.
The ECM will adjust the ignition timing based on air temperature. If the intake air is hot the ECM retards the ignition timing to reduce the risk of detonation. If the intake air is cooler, the ignition timing will be advanced.
If this input is defective, a fault code will be set and the "Check Engine" Light will illuminate. The ignition timing will be set to a conservative basic setting.