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 depending on the operating conditions and engine load.

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

- Crankshaft Hall sensor (B70), engine speed

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

- Coolant temperature sensor ()B11/4

- Electric controller unit (VGS) (Y3/8), gear range via the drive train CAN (CAN C)

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- AAC control unit (N22/1), air conditioning status over the interior CAN (CAN B) and chassis CAN (CAN E)

Idle speed control function sequence

The ME-SFI [ME] control unit regulates 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 [ME] control unit can adjust the ignition angles of the ignition coils (T1) by up to 36° CKA (CranK Angle) toward "retard" or 20° CKA toward "advance".

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

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

 Warm-up phase function sequence

 Selection of gear range "D" or "R" function sequence

 Refrigerant compressor (A9) cut-in function sequence

 Undervoltage in on-board electrical system battery (G1) function sequence

 Purging function sequence

Warm-up phase function sequence

In the warm-up phase, the ME-SFI [ME] control unit controls the idle speed according to coolant temperature, 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 [ME] 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.

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Gear range selection "D" or "R" function sequence

When gear range "D" or "R" is selected, the idle speed is set to a lower value to reduce the tendency of the vehicle to creep forward.

To do this, the ME-SFI [ME] control unit evaluates the "gear range engaged" CAN signal from the electric controller unit (VGS).

Refrigerant compressor cut-in function sequence

In order that the engine idle speed does not drop when the refrigerant compressor engages, the AAC [KLA] control unit first sends a status signal to the ME-SFI [ME] control unit.

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

Undervoltage in on-board electrical system battery function sequence

If the ME-SFI [ME] control unit detects an undervoltage through "circuit 30", the idle speed is raised by about 100 rpm when gear range "D" or "R" is selected, until the ignition is switched off.

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 adjusted with the Diagnosis Assistance System (DAS.