Spark

Electronic Ignition System Description

The electronic ignition system produces and controls the high energy secondary spark. This spark ignites the compressed air/fuel mixture at precisely the correct time, providing optimal performance, fuel economy, and control of exhaust emissions. The engine control module (ECM) collects information from the crankshaft position sensor and the camshaft position sensor to determine the sequence, dwell, and timing of the spark for each cylinder. The ECM transmits a frequency signal to the ignition coil assembly on the appropriate ignition control circuit to fire the spark plugs.

The crankshaft position (CKP) sensor is a permanent magnet generator known as a variable reluctance sensor. The CKP sensor produces an AC voltage of varying amplitude and frequency. The frequency depends on the velocity of the crankshaft. The AC output depends on the crankshaft position and the battery voltage. The CKP sensor works in conjunction with a 58 tooth encoder wheel attached to the crankshaft. As each encoder wheel tooth rotates past the CKP sensor, the resulting change in the magnetic field creates an ON/OFF pulse 58 times per crankshaft revolution. The engine control module (ECM) processes the pulses to determine the crankshaft position. The ECM can synchronise the ignition timing, the fuel injector timing, and the spark knock control based on the CKP sensor and the camshaft position (CMP) sensor inputs. Using the CKP sensor signals in conjunction with the CMP sensor signals, the ECM determines the engine position with great accuracy. The CKP sensor is also used to detect misfire and for tachometer display. The ECM learns the variations between all 58 teeth under different speed and load conditions to correctly detect misfires. The CKP sensor circuits consist of a signal circuit, a low reference circuit, and a shielded ground circuit. Both CKP sensor circuits are protected from electromagnetic interference by the shielded ground circuit.

The crankshaft encoder wheel is part of the crankshaft. The encoder wheel consists of 58 poles and a reference gap. Each pole on the encoder wheel is spaced 6° apart with a 12° space for the reference gap. The pulse from the reference gap is known as the sync pulse. The sync pulse is used to synchronise the coil firing sequence with the crankshaft position, while the other poles provide cylinder location during a revolution.

The camshaft position sensor is a hall-effect type sensor. The camshaft position sensor signal is a digital ON/OFF pulse, which outputs once per revolution of the camshaft. The camshaft position sensor does not directly affect the operation of the ignition system. The camshaft position sensor information is used by the engine control module (ECM) to determine the position of the valve train relative to the crankshaft position. By monitoring the camshaft position and crankshaft position signals, the ECM can accurately trigger the fuel injectors. This allows the ECM to calculate true sequential fuel injection mode of operation. If the camshaft position 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 fuel injection pulse, and the engine will continue to run. The camshaft position sensor consists of an ignition circuit, a ground circuit, and a signal circuit.

The wheel is a smooth track, half of which is of a lower profile than the other half. This track is read in a radial or axial fashion respectively. This allows the camshaft position sensor to supply a signal as soon as the key is turned ON, since the camshaft position sensor reads the track profile, instead of a notch.

The ignition coil provides the voltage for 2 spark plugs simultaneously. The ignition coil is a dual coil pack, and directly supplies voltage to each spark plug. The engine control module (ECM) will command the ignition coil circuit ON, this allows the current to flow through the primary coil windings for the appropriate time or dwell. When the ECM commands the ignition coil circuit OFF, this will interrupt current flow through the primary coil windings. The magnetic field created by the primary coil windings will collapse across the secondary coil windings, which induces a high voltage. The secondary coil voltage travels from the coil output terminal, through the spark plug wire, and across the spark plug gap to the engine block. The ignition coil is not serviceable and must be replaced as an assembly. The ignition coil consists of an ignition circuit, an ignition coil 1 and 4 control circuits, and an ignition coil 2 and 3 control circuits.

The engine control module (ECM) is responsible for maintaining proper spark and fuel injection timing for all driving conditions. The electronic spark timing is the method the ECM uses to control spark advance. The ignition module is integrated inside the ECM, and the primary coil ON/OFF is directly controlled by the ECM. To provide optimum driveability and emissions, the ECM monitors input signals from the following components in calculating ignition spark timing:

There is one normal mode of operation, with the spark under the engine control module (ECM) control. If the crankshaft position sensor pulses are lost the engine will not run. The loss of the camshaft position signal may result in a longer crank time since the ECM cannot determine which stroke the pistons are on. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

The ignition coils secondary output voltage is more than 40000 V. Avoid body contact with the ignition high voltage secondary components when the engine is running or personal injury may result.

Be careful not to damage the secondary ignition coil boots when servicing the ignition system. Rotate each spark plug wire in order to loosen the boot from the spark plug before removing. Never pierce a secondary ignition boot for any testing purposes. Future ignition system problems are guaranteed if pinpoints or test lights are pushed through the secondary ignition component insulation during testing.