General description

Ignition systems / General description

1 The Bosch Mono-Motronic and Digifant systems are self-contained engine management systems, which control both the fuel injection and ignition. This Chapter deals with the ignition system components only - refer to Chapter 4A or B for details of the fuel injection system components.

2 The ignition system comprises four spark plugs, five HT leads, the distributor, an electronic ignition coil, and an Electronic Control Unit (ECU) together with its associated sensors, actuators and wiring. The component layout varies from system to system but the basic operation is the same for all models.

3 The operation is as follows: the ECU supplies a voltage to the input stage of the ignition coil which causes the primary windings in the coil to be energised. As the engine rotates, the ECU is triggered to interrupt the coil primary supply voltage by the distributor-mounted Hall sender. This results in the collapse of a primary magnetic field, which then induces a much larger voltage in the secondary coil, called the HT voltage. This voltage is directed by the distributor, rotor arm and HT leads, to the spark plug in the cylinder on its ignition stroke. The spark plug electrodes form a gap small enough for the HT voltage to arc across, and the resulting spark ignites the fuel/air mixture in the cylinder. The timing of this sequence of events is critical and is regulated solely by the ECU. The basic ignition setting can be adjusted by turning the distributor body on its mountings - refer to Section 5 for greater detail.

4 The ECU calculates and controls the ignition timing and dwell angle primarily according to engine speed, crankshaft position and inlet air volume flow rate information, received from sensors mounted on and around the engine. Other parameters that affect ignition timing are throttle position and rate of opening, inlet air temperature, coolant temperature and on certain systems, engine knock. Again, these are monitored via sensors mounted on the engine.

5 On systems where knock control is employed, the knock sensor is mounted on the cylinder block - this has the ability to detect engine pre-ignition (or ‘pinking’) before performance is affected or engine damage is caused. If pre-ignition occurs, the ECU retards the ignition timing of the cylinder that is pre-igniting in steps until the pre-ignition ceases. The ECU then advances the ignition timing of that cylinder in steps until it is restored to normal, or until pre-ignition occurs again.

6 Idle speed control is achieved partly by an electronic throttle valve positioning module (or auxiliary air valve, depending on system type - see Chapter 4A or B for details) and partly by the ignition system, which gives fine control of the idle speed by altering the ignition timing.

7 On certain systems, the ECU has the ability to perform multiple ignition cycles during cold starting. During cranking, each spark plug fires several times per ignition stroke, until the engine starts. This greatly improves the engines cold starting performance.

8 It should be noted that comprehensive fault diagnosis of all the engine management systems described in this Chapter is only possible with dedicated electronic test equipment. Problems with the systems operation that cannot be pinpointed by following the basic guidelines in Section 2 should therefore be referred to a VW dealer for assessment. Once the fault has been identified, the removal/refitting sequences detailed in the following Sections will then allow the appropriate component(s) to renewed as required.

Note: Throughout this Chapter, vehicles are frequently referred to by their engine code, rather than by engine capacity - refer to Chapter 2A for engine code listings.