ME-SFI ignition system function - GF15.12-P-0001GS

ENGINE 111.951 /952 /955 /956 /957 /958 /982 /983 

Fig 1: ME-SFI Ignition System Function Diagram - Engine 111.951 /952 /955 /956 /957 /958 /982 /983

When the crankshaft is rotating, an alternating voltage is generated in the crankshaft position sensor (L5) by the teeth of the driven plate. In this case, each tooth generates an alternating voltage signal. No voltage is generated as a result of the gap caused by 2 missing teeth. The ME-SFI control module detects the TDC position of the crankshaft with the 2nd negative slope after the gap.

If the signal of the camshaft Hall sensor (B6/1) is at U Batt ("high") at this moment, the ME control module processes this for detecting ignition TDC of cylinder 1. This is used for actuating the ignition coils and the fuel injectors.

The high voltage is distributed rotorlessly. Advantages of the rotorless high voltage distribution are:

• significantly reduced electromagnetic interference level (no naked sparks)
• no rotating parts
• reduction in noise
• reduced high voltage connections.

1. Determining the ignition angle in line with the input signals, actuating ignition coils and triggering ignition sparks. 

   For determining the ignition angle, the ME control module essentially analyzes the following signals from:

   • Hot film mass air flow sensor
   • Crankshaft position sensor
   • Camshaft Hall sensor
   • Coolant temperature sensor

   and interrupts the primary circuit of the ignition coil at the ground side at the moment of the ignition timing point.

   The ignition angle can only be tested with the STAR DIAGNOSIS.

2. Adaptation of ignition angle 
   1. Catalytic converter heating-up (warming-up) 

      The ignition angle is continuously retarded for about 20 seconds in order to more rapidly warm up the catalytic converter to its operating temperature if:

      • coolant temperature at start > 15°C and < 40°C
      • selector lever position P or N

      At the same time idle speed is increased by the idle speed control.

   2. Idle speed 

      To assist the idle speed control, the ignition angle can be retarded by as much as 36° crank angle or advanced by as much as 20° crank angle.

      The control by altering the ignition angle reacts more rapidly than by altering the position of the throttle valve (idle speed control).

   3. Inertia fuel shutoff 

      The ignition angle is briefly retarded in order to prevent a sudden increase in torque when combustion is resumed (actuation of fuel injectors).

   4. Intake air temperature/coolant temperature 

      In order to prevent the knocking tendency at high inlet and coolant temperatures, the ignition angle under load is retarded in line with the intake air temperature and coolant temperature. The ignition angle is retarded if:

      • intake air temperature > 35°C
      • coolant temperature > 105°C

      The values of the retardation of the ignition angle of intake air temperature and coolant temperature are added together.

   5. Transmission overload protection 

      In order to protect the shift elements of the automatic transmission from excessive thermal stresses during powershifts (1-2-1, 2-3-2), the ignition angle is briefly retarded during the gearshift and the engine torque is thus reduced. The ME-SFI control modules are supplied with a signal for this purpose from the ETC control module (N15/3) over the CAN databus.

   6. ESP/ASR control mode 

      In order to reduce the engine torque as rapidly as possible in the ESP/ASR control mode, the ignition angle is retarded by the throttle valve actuator (opening angle reduced) before the commencement of the control cycle. The information flows from the ESP/ASR control module over the CAN databus to the ME control module.

   7. Knock sensor system (KSS) 

      If uncontrolled combustion (knocking) occurs at one or several cylinders, the ignition angle is retarded at the relevant cylinder or cylinders.

   8. Smooth engine running analysis 

      Smooth engine running is constantly monitored in order to protect the catalytic converter from excessive thermal stresses if combustion misfiring exists.

      If the smooth engine running analysis detects combustion misfiring, the appropriate fuel injector or injectors are no longer operated after a certain number of misfirings.