ME-SFI idle speed control function

Idle speed control function requirements, general

 Circuit 87M ON (engine control ON)

 Engine running

 Accelerator not operated

Idle speed control, general

The idle speed control regulates and stabilizes the idle speed according to operating conditions and engine load.

For the idle speed control, the ME-SFI control unit (N3/10) reads the following sensors and signals:

- Hot film MAF sensor (B2/5), engine load

- Coolant temperature sensor (B11/4)

- Crankshaft position sensor (L5), motor speed

- Clutch pedal switch (S40/3) (for manual transmission)

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- ETC control unit (N15/3), gear range over the engine compartment CAN (CAN C) (for automatic transmission)

- Comfort AAC control and operating unit (N22/7), air conditioning status over the interior CAN (CAN B) and engine compartment CAN

Idle speed control function sequence

The ME-SFI control unit controls the idle speed through the throttle valve actuator (M16/6) and adapts the injection time of the fuel injection valves (Y62) according to variations in the air mass.

To assist the idle speed control, the ME-SFI control unit can shift the ignition angles of the ignition coils (T1) towards "retard" by up to 20° crank angle, or towards "advance" by up to 20° crank angle.

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Adjusting the ignition angle enables a faster response than adjusting the throttle valve.

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The ME-SFI control unit adjusts the idle speed in the following operating and engine load conditions:

 Warm-up phase function sequence

 Gear range "D" or "R" engaged function sequence
(for automatic transmission)

 Clutch pedal operated function sequence
(for manual transmission)

 Refrigerant compressor (A9) cut-in function sequence

 Purging function sequence

Warm-up phase function sequence

In the warm-up phase, according to the coolant temperature, the ME-SFI control unit controls the idle speed in order to bring the catalytic converters more quickly up to operating temperature after engine start.

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Additional function requirements in the warm-up phase are:

- Coolant temperature <40°C

- Gear range "N" or "P" engaged

The ME-SFI control unit raises the idle speed to
about 1100 to 1300 rpm for about 20 s.

After this time has elapsed, the idle speed is increased at a coolant temperature of < 70 °C in line with the temperature for approx. 50 seconds by approx. 50 to 100 rpm.

Gear range "D" or "R" engaged function sequence
(for automatic transmission)

When gear range "D" or "R" is engaged, a lower engine speed is set in order to reduce the tendency of the vehicle to creep forward.

To do this, the ME-SFI control unit evaluates the information about the engaged gear range from the ETC control unit.

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Clutch pedal operation function sequence
(for manual transmission)

When actuating the clutch pedal the idle speed is stabilized. Actuation of the clutch pedal is detected by the ME-SFI control unit through the clutch pedal switch.

Refrigerant compressor cut-in function sequence

So that the engine idle speed does not fall when the refrigerant compressor is engaged, the comfort AAC control and operating unit first sends a status signal to the ME-SFI control unit.

The ME-SFI [ME] control unit adapts the idle speed appropriately.

Purging function sequence

The idle speed control prevents any change in the engine speed due to purging.

Diagnosis

The basic setting of the idle speed can be read out and influenced using the DAS.