ME-SFI starting, function

Starting function requirements, general points

 Start enable drive authorization system stage 3 (DAS 3)

 Selector lever position "P" or "N" with transmission 722.6

 Circuit 50 ON

Starting, general points

The following sub-functions are active when starting:

 Starter control, function

 Start, post-start phase and warm-up enrichment, function

 Synchronizing fuel injection and firing order function

Starter control, function

Starter control which is integrated in the ME-SFI [ME] control unit (N3/29) takes over actuation and shutoff of the starter (M1).

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The ME-SFI control unit manages the starter control according to the following sensors and signals:

- Coolant temperature sensor (B11/18), coolant temperature

- Crankshaft sensor (L5/12), engine speed

- EIS [EZS] control unit (N73) start enable via Controller Area Network bus class C (engine compartment) (CAN C)

- ETC control unit (N15/3), gear range via CAN C, with transmission 722.6

The start control function sequence is divided into the following points:

 Start function sequence

 End of start function sequence, with transmission 722.6

 End of start function sequence, with transmission 711.6

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Start function sequence

When "terminal 50 ON" the ME control unit receives the "Terminal 50 signal" from the EIS control unit.

This control unit actuates the circuit 50 starter relay (K40/9k5) with a ground signal. The power contact on the starter relay terminal 50 closes and the solenoid switch of the starter is supplied with voltage at "terminal 50" from "terminal 30" unfused.

End of start function sequence, with transmission 722.6

Start control is carried out automatically by the ME-SFI [ME] control unit (touch start). This means as soon as the start process is initiated switching off of "terminal 50" has no further influence on the start process. The ME-SFI [ME] control unit interrupts the negative activation to starter relay terminal 50 upon reaching the engine cranking speed of 400 to 700 rpm or a starting time of about 5 to 40 s (depending on the coolant temperature) and thus ends the starting procedure.

 The starting procedure is interrupted only by switching off terminal 15.

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End of start function sequence, with transmission 711.6

On reaching an engine cranking speed of 400 to 700 rpm or on receiving the signal "terminal 50 OFF", the ME-SFI [ME] control unit interrupts negative activation to the starter relay terminal 50 and thus ends the starting procedure.

 The starting procedure can also be interrupted by switching off terminal 15.

Function requirements for starting, the post-start phase and warm-up enrichment

 Circuit 87M ON

 Circuit 61 ON

Start, post-start phase and warm-up enrichment, function

To ensure stable and smooth engine running during a cold start, the ME-SFI [ME] control unit enriches the fuel-air mixture.

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The function sequence for startup, poststart and warmup enrichment is subdivided into the following areas:

 Starting mixture enrichment function sequence

 Post-start enrichment function sequence

 Warm-up enrichment function sequence

Starting mixture enrichment function sequence

To allow easier starting when the engine is cold, more fuel has to be injected during the start procedure.

The ME control unit calculates the injection time for the starting mixture enrichment with the aid of the following sensors and signals:

- Coolant temperature sensor, coolant temperature

- Crankshaft sensor, engine speed

- Engine shutoff time

- EIS control unit, signal terminal 50 via CAN C

- Atmospheric pressure sensor (B18/4) for altitude adaptation

When it receives the "terminal 50 signal" from the EIS control unit over the CAN C, the ME-SFI control unit lengthens the injection time of the fuel injection valves.

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Starting mixture enrichment remains active until the engine speed corresponding to the particular coolant temperature is exceeded. The higher the coolant temperature, the shorter is the injection time calculated by the ME-SFI [ME] control unit.

Post-start enrichment function sequence

After a cold start the fuel-air mixture is enriched for a short time to ensure that the engine runs smoothly.

Fuel condensing in the cold cylinder barrels is compensated by the post-start enrichment.

The ME control unit calculates the injection time for the poststart enrichment with the aid of the following sensors and signals:

 Coolant temperature sensor, coolant temperature at engine start

 Number of combustion cycles after start

 ETC control unit, selection lever position via CAN C, with transmission 722.6

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An additional quantity of fuel is added to the engine by the ME-SFI [ME] control unit in line with the performance map.To do this, the ME-SFI [ME] control unit extends the injection time of the injection valves.

Warm-up enrichment function sequence

After the start and post-start enrichment, the fuel-air mixture is further enriched by the warm-up enrichment. The warm-up control compensates for fuel which precipitates on the still cold engine parts.

The ME-SFI [ME] control unit calculates the injection time for the warm-up enrichment using the following sensors and signals:

- Hot film MAF sensor (B2/13), engine load

- Coolant temperature sensor, coolant temperature

- Charge air temperature sensor (B17/14)

- Crankshaft sensor, engine speed

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An additional quantity of fuel is added to the engine by the ME-SFI [ME] control unit in line with the performance map. To do this, the ME-SFI [ME] control unit extends the injection time of the fuel injection valves.

In the idle speed range, injection time mainly depends on the coolant temperature at engine start.

On partial load, the crucial factor is engine load. Once enabled, the lambda control monitors the injection time.

The ignition angles of the ignition coils during warm-up are determined according to a specific performance map.

Function requirements synchronization of fuel injection and firing order, general points

 Circuit 87M ON

 Circuit 61 ON

Synchronizing fuel injection and firing order function

Synchronization of the injection and firing order is processed by the ME-SFI [ME] control unit for actuation of the ignition coils and the fuel injection valves.

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Furthermore, the synchronization is required for the knock control and the fuel shutoff on the individual cylinders.

The ME-SFI [ME] control unit synchronizes the injection and firing using the signals from the following sensors:

- Crankshaft sensor, engine speed and crankshaft position

- Intake camshaft sensor (B6/21), intake camshaft position

- Exhaust camshaft sensor (B6/22), exhaust camshaft position

When the engine is started, the injection sequence is determined according to the firing order, using the voltage signals from the crankshaft sensor and the intake camshaft sensor or exhaust camshaft sensor. To do this the ignition-top dead center (OT) of cylinder 1 must be recognized.

This is also accomplished by evaluation of the voltage signals from the camshaft sensor and intake camshaft sensor or exhaust camshaft sensor by the ME control unit.

 If no voltage signal is received from the intake camshaft sensor, the voltage signal from the exhaust camshaft sensor is used instead to detect ignition TDC of cylinder 1.

 If there is no voltage signal at all available from a camshaft sensor, ignition and injection take place after 360° crank angle in order to make possible an emergency start.