ME-SFI starting function

The motor electronics (ME-SFI) fuel injection and ignition system controls the following functions via the ME-SFI control unit (N3/10):

 Starter detection and control

 Start quantity control

 Post-start enrichment

 Synchronization of fuel injection and firing order

 Battery voltage and voltage supply

 Ignition timing at start

 Actuation of fuel pump (M3/3m1)

Starter detection and control

On vehicles with transmission 722.6, the engine can only be started if the selector lever is engaged in position "P" or "N". The selector lever position "P" or "N" message is sent on the Class C Controller Area Network Bus Class C (engine compartment) (CAN C) by the electronic selector lever module control unit (N15/5).

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The ME-SFI control unit (N3/10) receives the message "DAS 3 message MS" and message "tml. 50 ON" over CAN C from the EIS [EZS] control unit (N73) and the selector lever position message "P" or "N" from the electronic selector lever module control unit (N15/5). The ME-SFI control unit (N3/10) actuates the starter (M1) accordingly via the starter relay (K40/9) integrated in the fuse and relay block (K40/9k5). This starter detection is required for the "start quantity control" function, the "post-start enrichment" function and the "ignition timing at start" function.

Start quantity control

In order to facilitate starting a cold engine, in addition fuel must be injected during the starting procedure, depending on the coolant temperature. The increased quantity of fuel injected is achieved by extending the injection time.

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Post-start enrichment

After cold engine start enrichment with additional fuel is necessary for a short time. Post-start enrichment compensates for the fuel which precipitates on the cylinder walls. The engine runs smoothly and without combustion misfires.

Synchronizing fuel injection and firing order

In order to achieve correct synchronization of the fuel injection and firing order to the particular cylinder, the order is synchronized during the first revolutions of the engine. For this step, the ME-SFI control unit (N3/10) requires the signals from the camshaft Hall sensor (B6/1) and the signals from the crankshaft position sensor (L5).

Battery voltage and voltage supply

The quantity of fuel injected is determined by the injection times of the fuel injection valves (Y62). This injection time is dependent on the battery voltage. In order to adjust the injection quantity, for example if the battery voltage is low, the ME-SFI control unit (N3/10) extends the injection timing for the fuel injection valves (Y62).

If the battery voltage < 7 V, an engine start will not be possible.

Ignition timing at start

When the engine is started up to around 500 rpm, ignition timing is controlled only by signals from the camshaft Hall sensor (B6/1). Only when the speed has reached around 600 rpm is a transition made from the established ignition timing to the ignition timing adapted for the respective operating status, which is stored in the performance map of the ME-SFI control unit (N3/10).

Actuation of fuel pump (M3/3m1)

At engine start, the ME-SFI control unit (N3/10) actuates the fuel pump (K40/9k6) via the fuel pump relay (M3/3m1), which delivers fuel to the fuel rail via the fuel filter with pressure regulator.