Ignition Control

The following components affect ignition control:

^ Engine speed (RPM) sensor (1)
^ Camshaft position (CMP) sensor (2)
^ Mass air flow (MAF) sensor (3)
^ Engine coolant temperature sensor (4)
^ Throttle position (TP) sensor (5)
^ Knock sensors (KS) (6)
^ Automatic transmission control module (TCM) (7).

The engine control module has an ignition coil for each cylinder. This ensures high ignition voltage and allows ignition to be controlled exactly. The firing order is 1-5-3-6-2-4.

The ignition coils are mounted on the spark plugs. Two ignition discharge modules (IDM) (9) control the ignition coils. To ensure that the ignition discharge modules are cooled effectively they are mounted on the intake manifold. The front ignition discharge module is connected to cylinders 1, 3 and 5 and the rear ignition discharge module to cylinders 2, 4 and 6. The ignition discharge modules contain three ignition driver stages each connected to an ignition coil. Using separate ignition coils for each cylinder guarantees the rapid build-up of a high ignition voltage (approx. 49 kV). This results in an effective start to combustion and helps improve cold-starting.

In response to signals from the knock sensors the engine control module calculates ignition advance and operates the ignition discharge modules which interrupt power to the ignition coils (9). The HT pulses generated are distributed to the spark plugs (8).

Timing is fixed during the start-up phase. However, when the engine has been started and the car is driven away, the engine control module computes the optimum timing based on engine speed, load and temperature, and other parameters.

The engine control module analyses signals from the knock sensors when the engine has reached working temperature. If engine knock occurs, ignition is retarded in the affected cylinder until knocking ceases.

The ignition is then advanced to its normal setting or until knock recurs.

Sometimes, before shifting gear, the transmission control module transmits a torque limiting request to the engine control module, which responds by retarding the timing momentarily to reduce torque and relieve the load on the transmission.

Ignition can be retarded to three levels depending on the signals from the transmission control module. A return signal from the engine control module to the transmission control module confirms that the signal has reached the Motronic engine control module.

Together with the ignition discharge module, the engine control module incorporates a function for controlling ignition coil current, ensuring that the coil always has an optimum charge, regardless of engine speed.