Manual - Chapter 6 - Ignition system

1. General description

The ignition system is a microprocessor-controlled digital system powered by the battery. The heart of the system is the ignition control unit which receives signals from the trigger assembly mounted on the crankshaft front end and switches off the power supply to the HT coils thus inducing a spark across the spark plug electrodes.

The trigger assembly contains two Hall effect transmitters; on 100 models these are equally spaced, one triggering the spark for cylinders 1 and 4 while the other acts for cylinders 2 and 3. On 75 models the transmitters are positioned at 120° and 240° to serve cylinders 1 and 3 respectively; from this the control unit works out the correct firing position for number 2 cylinder. On both models a 'wasted' spark system is used in which the spark plugs 'fire at every crankshaft revolution.

The impulses transmitted by the trigger assembly are used by the control unit to build up the spark at the HT coils and to advance or retard it according to a preprogrammed curve depending on engine speed and on early 100 models only, intake manifold depression. lt also triggers the control pulses for the injectors and prevents engine damage through excessive speed by first retarding the ignition timing at a set speed and then cutting off the injectors when a maximum set engine speed is reached. A lockout circuit prevents the starter motor from being operated if the engine is still turning over.

On early 100 models a vacuum switch fitted to the intake manifold was used to bring in a second advance curve for part-load operation. However this was found to have so little effect in practice that it was omitted from all models from late 1985 on.


1 Battery 5 Starter relay 9 Fuel pump 13 Vacuum switch - early 100 models
2 Ignition switch 6 Ignition control unit 10 Fuel injection control unit 14 Spark plug
3 Starter button 7 Injector relay 11 Ignition trigger assembly 15 Tachometer
4 Starter motor 8 Fuel tank 12 Ignition HT coil