ME-SFI idle speed control function

P30.22-2023-06

Shows model 219

B11/4 Coolant temperature sensor

B37 Accelerator pedal sensor

L5 Crankshaft position sensor

M16/6 Throttle valve actuator

N3/10 ME control unit

N22 AAC [KLA] control and operating unit

T1/1... Cylinder 1 to 8 ignition coil

T1/8

X11/4 Data link connector

Y3/8n4 Fully integrated transmission control (VGS) control unit

CAN Data bus

The throttle valve actuator controls the idle speed by altering the position of the throttle valve. It is actuated by the ME-SFI control unit.

To assist the idle speed control, the ignition angle can be retarded by as much as a 36° crank angle and advanced by as much as a 20° crank angle.

The ME control unit actuates the throttle valve motor in line with the input signals received at the ME control unit .

The throttle valve actuator controls the idle speed:

 at different engine loads, for example with the steering turned to full lock (power steering pump), drive position engaged, refrigerant compressor engaged

 in the warming-up phase

 at undervoltage

 at different mixture composition e.g. through lambda control or fuel evaporation control system.

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Heating-up speed

In order to warm up the catalytic converters more rapidly to operating temperature, engine speed is increased in the idle speed range to 1100 up to 1300 rpm for about 20 seconds after each engine start at coolant temperatures of < 40 °C. Following this period, the idle speed is increased at a coolant temperature < 70 °C for around 50 seconds by approx. 50 to 100/rpm. When a drive position is selected, this is immediately reduced.

Automatic transmission (AT)

When a drive position is engaged, a lower engine speed is controlled in order to reduce the creeping tendency of the vehicle.

The information "drive position engaged/not engaged" is transmitted by the transmission control unit to the motor electronics control unit via the CAN databus.

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AC compressor

To avoid engine speed dropping in the idle speed range when the AC compressor is engaged, the automatic air conditioning control and operating module supplies a signal via the CAN databus to the ME control unit which adapts (increases) idle speed accordingly.

When the air conditioning unit is switched on and the external temperature exceeds 35°C, the idling speed is increased for the driving mode selected by about 100 rpm when the switch-on threshold of about 900 rpm is exceeded. This increase is canceled again when the external temperature drops below 32° C.

Undervoltage

If the engine control unit detects an undervoltage the idling speed is increased until the ignition is switched off for the driving mode selected by about 100 rpm.

Vehicles with SBC: If the control unit SBC registers a low voltage to the engine control unit over the CAN then the idling speed for the engaged drive position is increased to 750 rpm.