Idle Speed: Description and Operation
GF30.22-P-0003M ME-SFI Idle Speed Control, Function
- ENGINE 275.950 in MODEL 215, 220
- ENGINE 275.951 in MODEL 230 up to Model Year 8
- ENGINE 275.953 in MODEL 216, 221 up to Model Year 8
- ENGINE 275.954 in MODEL 230 up to Model Year 8
- ENGINE 275.980 in MODEL 215, 220
- ENGINE 275.981 in MODEL 230 up to Model Year 8
- ENGINE 275.982 in MODEL 216, 221 up to Model Year 8
- ENGINE 285.950 in MODEL 240
- ENGINE 285.980 in MODEL 240
Illustrated model 215, 220, 230, 240
B11/4 Coolant temperature sensor
B37 Accelerator pedal sensor
L5 Crankshaft position sensor
M16/6 Throttle valve actuator
N3/10 ME-SFI control unit
N15/3 ETC [EGS] control unit
N22 AAC [KLA] control and operating unit
N92/1 ECI ignition module, right cylinder bank
N92/2 ECI ignition module, left cylinder bank
X11/4 Data link connector
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 36° crank angle and advanced by as much as 20° crank angle.
The ME-SFI control unit actuates the throttle valve motor in line with the input signals received at the ME-SFI control unit.
The throttle valve actuator controls the idle speed:
- At different engine loads, e.g. 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.
Heating-up speed
In order to warm up the catalytic converters more rapidly to operating temperature, the engine speed is increased in the idle speed range to 1100 up to 1300 rpm for about 20 seconds after each starting procedure at coolant temperatures of less than 40 °C.
Following this period, the idle speed is increased at a coolant temperature less than 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 via the CAN databus to the ME-SFI control unit.
AC compressor
To avoid engine speed dropping in the idle speed range when the refrigerant compressor is engaged, the automatic air conditioning control and operating unit supplies a signal to the ME-SFI control unit which adapts (increases) the idle speed accordingly.
When the air conditioning unit is switched on and the external temperature exceeds 35 °C the idling speed on vehicles delivered from 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 cancelled 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. The SBC control unit stores the fault.
If the SBC control unit (model 230, 240) detects undervoltage, an increased idle speed is requested via a CAN message.
The idle speed can be influenced with the STAR DIAGNOSIS. If the connectors of the pressure sensors downstream of the air filter (B28/4 or B28/5) are interchanged with the pressure sensor upstream of throttle valve actuator (B28/6), the engine switches off after starting.
