VW Passat & Audi A4 1990-2000

General Information


Years ago, ignition systems consisted of a breaker-point distributor, with mechanical and manifold vacuum operated advance and retardation of the ignition timing. Over time, the development of computerized fuel injection technology allowed the integration of electronically manipulated ignition control, which reduces emissions, allows maximum running efficiency, and maximizes the power from an internal combustion engine.

The 2.0L Passat models use two different engine management systems on their gasoline engines: CIS-E-Motronic and Motronic. Although there are differences between the fuel management components on these systems, the electronic ignition system functions much the same.

To allow the Engine 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 parameters of the engine. These sensors allow variables such as engine coolant temperature, exhaust gas oxygen 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 performance. The main component responsible for this feature is the Knock (KS) sensor. Because the ECM receives a continuous input from the KS, the ignition timing can be advanced (meaning spark is created before each piston reaches Top Dead Center (TDC) just below the threshold of detonation, providing peak performance from the engine.

The KS also allows the ignition system to adapt to the grade of fuel being used. Because the flash point and flame front propagation differs based on the fuel's octane rating, the ignition timing can be retarded to compensate for a low octane fuel and prevent detonation. This will reduce the optimum power output of the engine, however, the engine can operate without damaging its internal components.

The distributor ignition system is composed of the following components:

Engine Control Module (ECM)
Ignition Coil
Knock Sensor (KS)
Camshaft Position (CMP) Sensor
Engine (Crankshaft) speed sensor

The Engine Control Module (ECM) is the "brain" of the ignition system. The ECM controls when the ignition spark occurs, based on the input from the various sensors on the engine. These sensors include the engine (crankshaft) speed sensor, Camshaft Position (CMP) sensor, and the Knock (KS) sensor.

The distributor allows the high voltage from the ignition coil to be distributed to the spark plug in the proper order. The high energy electrical voltage which is generated by the ignition coil flows to the distributor cap via the ignition coil wire. From there, the voltage is transmitted to the distributor rotor via a spring-loaded carbon connector in the center of the distributor cap. The voltage flows through the imbedded resistor in the rotor and transmitted from the rotor to one of the terminals on the inside of the distributor cap. From there, the high energy voltage travels through the spark plug wire to the spark plug where it "jumps" the gap at the spark plug's electrode to ignite the fuel mixture in the engine.

The distributor also contains a Hall-type sensor. The 2.0L 9A engines use this Hall sensor as an engine speed sensor; the 2.0L ABA engines use the Hall sensor as a Camshaft Position (CMP) Sensor. The information from this sensor allows the ECM to precisely calculate when to open the ground circuit of the primary winding of the ignition coil, which creates the high-energy voltage necessary to "jump" the air gap at the spark plug to ignite the fuel in the combustion chamber.

The ignition coil uses a mere 12 volts to generate an electrical voltage in excess of 25,000 volts. This allows a "spark" (actually an electrical arc) to jump the gap at the spark plugs' electrode. The ignition coil actually consists of two separate wire coils: referred to as the Primary and Secondary coil, windings or circuits. The primary coil is supplied with a positive ( + ) 12-volt signal from the ignition switch. This current flows through the primary coil wiring and creates a magnetic field.

When the negative side of the primary coil wiring is opened, or "ungrounded" the magnetic field collapses, inducing an electrical field to the secondary coil winding and that creates a high voltage output from the secondary coil winding. Because the voltage is so powerful, the electricity actually "arcs" or jumps across the electrodes on the spark plug. This grounding and un-grounding of the primary coil wiring is controlled by the ECM.

The Knock (KS) Sensor is a peizo-electric device that senses detonation or pre-ignition from the harmonics in 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.

The Camshaft Position (CMP) Sensor informs the ECM of the exact position of each piston in the engine. The information from the sensor allows the ECM to precisely control when the ignition occurs. This sensor is located around the No. 4 spark plug wire on the 2.0L 9A engines and inside of the distributor on 2.0L ABA engines. This sensor is also used in conjunction with the KS sensor to determine which cylinder(s) is/ are detonating.

The crankshaft speed sensor allows the ECM to monitor the engine's rotational speed. The speed sensor is located in the distributor on the 2.0L 9A engines and on the transaxle bell housing just above the flywheel on the forward part of the engine on the 2.0L ABA engines.

Click image to see an enlarged view

Fig. View of the ignition system components. The ignition timing is controlled by the ECM and varies depending on operating conditions