Ignition, basic function

Idling speed ignition timing
When the idle speed control is active, the ignition timing is adjusted to stabilize idling speed. The value is sent to box 3.

Fig 1: Illustration For Steps 1-3
Courtesy of SAAB-SCANIA OF AMERICA, INC.
Normal ignition timing
When the idle speed control is not active, the ignition timing is obtained from a load- and speed dependent matrix. The matrix value is optimized for the lowest fuel consumption (best torque). The value is sent to box 3.
Selection of ignition timing
One of the ignition timing calculations is selected, depending on which function is active. The value is sent to box 6.
Compensation

Fig 2: Illustration For Steps 4-6
Courtesy of SAAB-SCANIA OF AMERICA, INC.

The timing is corrected according to the coolant temperature and intake air temperature. The value is sent to box 6.
Knock control
If knocking occurs, the amount by which the ignition should be retarded is calculated. The value is sent to box 6.
Totalling
The compensation angle and knock retardation are totalled to give the current ignition timing. The value is sent to box 7.
Selection of ignition timing
Starting ignition is selected when the engine has not yet started. The value is sent to box 9.

Fig 3: Illustration For Steps 7-9
Courtesy of SAAB-SCANIA OF AMERICA, INC.
Starting ignition timing
The ignition timing at start is calculated based on the intake air temperature and coolant temperature. If the engine is running the value is sent via box 7 to box 9.
Activate relevant trigger
At the calculated crankshaft angle, the microprocessor controls the transistor for the trigger that is next in the firing order.