Years ago, ignition systems consisted of a breaker-point distributor, with mechanical and manifold vacuum operated advance and retardation of the timing. Over time, the development of computerized fuel injection technology allowed the integration of electronically manipulated ignition control, which allows maximum efficiency and power from an internal combustion engine.
Volkswagen uses three different engine management systems on their gasoline engines: CIS-E-Motronic, Digifant, and Motronic. Although there are differences between the fuel management components on all three systems, the electronic ignition system is of the same basic design.
To allow the Electronic Control Module (ECM) to accurately control the ignition timing, various sensors are placed on the engine, and connected into the ECM to provide information regarding the operational state of the engine. These sensors allow variables such as engine coolant temperature, exhaust gas content, crankshaft position, throttle position, and air temperature to determine the amount of ignition timing advance or retardation the ECM can safely provide.
Because the ECM controls the advance and retardation of the ignition timing, the distributor is not equipped with any type of mechanical or vacuum operated advance mechanism.
Since the ECM can closely monitor the performance of the engine based on the sensor inputs, the ignition timing is continually adjusted to maintain peak horsepower. The main component responsible for this feature is the knock sensor. Because the ECM receives a continuous input from the knock sensor, the ignition timing can be advanced (meaning spark is created before each piston reaches top dead center) just below the threshold of detonation, providing peak performance from the engine.
The distributor ignition system is composed of the following components:
Electronic Control Module (ECM)
-The ECM is the "brain" of the ignition system. The ECM controls when the spark occurs, based on the input from the various sensors on the engine, which include the crankshaft and/or camshaft position sensor, the knock sensor, coolant temperature sensor, and the oxygen (02) sensor.
-The distributor allows the high voltage from the ignition coil to be distributed to the spark plug in the proper order. The distributor also contains a Hall-type sensor, which is sometimes referred to as the camshaft position sensor. The information from this sensor, working together with the ignition control unit, allows the ECM to precisely calculate when to open the ground circuit of the primary ignition coil winding, which creates the high voltage necessary to create an electrical arc at the spark plugs.
-The ignition consists of a coil uses a mere 12 volts to generate over 25,000 volts to allow a "spark" (actually an electric arc) from the spark plugs. An ignition coil actually consists of two separate wire coils: Primary and Secondary. The primary coil is supplied with a 12 volt signal from the ignition switch. This current flow through the primary coil wiring creates a magnetic field. When the negative side of the primary coil wiring is opened, or "ungrounded" the magnetic field collapses, inducing a high voltage in the secondary coil winding. This high voltage travels through the coil wire to the distributor, through the spark plug wire, and finally, to the spark plug. Because the voltage is so powerful, the electricity actually "arcs" or jumps across the electrodes on the spark plug. This grounding and ungrounding of the primary coil wiring is controlled by the ignition control unit, or on later systems, the ECM. The Hall Sender (or crankshaft/camshaft position sensor) supplies the ignition control unit or ECM with a signal for proper coil operation.
Ignition Control Unit
-The ignition control unit is essentially a solid state switch that controls the ground circuit for the primary winding in the ignition coil. The ignition control unit works together with the Hall sensor in the distributor (or crankshaft position sensor) to control the ignition coil. On later systems, this component is integrated into the ECM, with a "power stage" mounted on the coil which essentially performs the same function.
-A knock sensor is a peizo-electric device that senses detonation or pre-ignition from the engine block. When the engine begins to knock, the sensor produces a small amount of voltage; when detected by the ECM, the ignition timing is retarded slightly to prevent detonation.
Crankshaft/Camshaft Position Sensor
-This sensor determines the exact position of each piston in the engine. The information from the crankshaft/camshaft position sensor allows the ECM to precisely control when the ignition occurs. Typically, this sensor is located inside of the distributor itself. On later systems, this sensor is located on the engine block.