Basic ignition timing

Basic ignition timing

Crankshaft position sensor

The speed and position of the engine is detected by the crankshaft position sensor which is bolted to, and projects through, the engine adapter plate adjacent to the flywheel.

The crankshaft position sensor is an inductive sensor consisting of a bracket mounted body containing a coil and a permanent magnet which provides a magnetic field. The sensor is situated such that an air gap exists between it and the flywheel. Air gap distance is critical for correct operation.

The flywheel incorporates a reluctor ring which consists of 32 poles spaced at 10° intervals, with 4 missing poles at 0°, 120°, 180° and 310°. The missing poles inform the ECM when to operate the groups of injectors. When the flywheel rotates, as a pole passes the sensor it disturbs the magnetic field inducing a voltage pulse in the coil. This pulse is transmitted to the ECM.

By calculating the number of pulses that occur within a given time, the ECM can determine the engine speed. The output from this sensor when used in conjunction with that from the manifold absolute pressure sensor provides idle stabilisation and reference for injection timing.

Manifold absolute pressure sensor

The manifold absolute pressure (MAP) sensor is located within the ECM and detects manifold pressure via a hose connected to the inlet manifold. The sensor converts pressure variations into graduated electrical signals which can be read by the ECM. Increases and decreases in the manifold pressure provide the ECM with an accurate representation of the load being placed on the engine allowing the ECM to adjust the quantity of fuel being injected and the ignition timing to achieve optimum fuelling of the engine.

Ignition timing compensation

Engine coolant temperature sensor

The engine coolant temperature sensor is a ’thermistor’ (a temperature dependent resistor), i.e. the voltage output varies in proportion to temperature. The sensor is located in the front of the coolant outlet elbow. The ECM constantly monitors this signal and uses the information to provide optimum driveability and emissions by advancing or retarding the ignition timing.

Idle speed control

With the throttle pedal released and the engine at idle, the ECM uses the fast response of ignition timing to maintain idle stabilisation.

When loads are placed on, or removed from the engine the ECM senses the change in engine speed and in conjunction with adjusting the idle air control valve, advances or retards the ignition timing to maintain a specified idle speed. When load is removed from the engine, the idle air control valve returns to its original position and the ignition timing returns to the idle setting.

NOTE: Due to the sensitivity of this system the ignition timing will be constantly changing at idle speed.

Knock sensor - ’T’ Series only

The knock sensor is a capacitive device mounted in the cylinder block between Nos. 2 and 3 cylinders below the inlet manifold. The sensor monitors noise and vibration in the engine and passes this information to the ECM which is able to identify the characteristics of the knocking and make the necessary adjustments to the ignition timing of individual cylinders.