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:

- Coolant temperature sensor (B11/4)

- Charge air temperature sensor (B17/8) and pressure sensor downstream of throttle valve actuator (B28/7), engine load

- Crankshaft position sensor (L5), motor speed

-----------------------------------------------------------------------------------------

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

- AAC [KLA] control and operating unit (N22), 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 injection valves (Y62) according to the variations in air mass.

To support the idle speed control, the ME-SFI control unit can adjust the ignition angle of the ECI ignition modules of the RH and LH cylinder banks (N92/1, N92/2) by up to 36° crank angle towards "retard" or 20° crank angle towards "advance".



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

-----------------------------------------------------------------------------------------

The ME-SFI control unit adjusts 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 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.

-----------------------------------------------------------------------------------------

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 selection "D" or "R" function sequence

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 "gear range engaged" CAN signal from the ETC control unit.

Refrigerant compressor cut-in function sequence

So that the engine speed at idle does not fall when the refrigerant compressor is engaged, the AAC [KLA] control and operating unit sends a CAN signal to the ME-SFI control unit before the compressor cut-in. The ME-SFI [ME] control unit adapts the idle speed appropriately.

Undervoltage in on-board electrical system battery function sequence

If the ME-SFI 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 influenced using the DAS.