ME-SFI Idle Speed Control Function

ME-SFI Idle Speed Control Function




B11/4 Coolant temperature sensor
B37 Accelerator pedal sensor
M16/1 EA/CC/ISC [EFP/TPM/LLR] actuator (other designation: M16/6 throttle valve actuator)
N3/10 ME-SFI control unit

Task
Controlling idle speed in the case of
- different engine loads, e.g. power steering turned to full lock, engaging drive mode, switching on AC compressor
- different mixture, e.g. from activated charcoal canister if purging activated (depending on activated charcoal charge)

Function
The actuator EFP/TPM/LLR (additional designation: throttle valve actuator) regulates the idling speed by changing the throttle valve position. It is actuated by the ME-SFI control unit.
The ME-SFI control unit actuates the throttle valve motor (M16/1m1) in line with the input signals received at the ME-SFI control unit (N3/10).

Heating speed of the catalytic converter
In order to warm up the catalytic converter more rapidly to operating temperature, engine speed is increased in the idle speed range to 1100 up to 1500 rpm for about 20 seconds after each starting procedure at coolant temperatures of < 40 �C. When a Drive position is engaged, engine speed is immediately controlled to a lower value.

Automatic transmission
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 ETC control unit (N15/3) over the CAN databus to the ME-SFI control unit.

Ignition angle
To assist the idle speed control, the ignition angle can be retarded by as much as 36° crank angle or advanced by as much as 20° crank angle from the calculated specified value.

Note: The control in the idle speed range by altering the ignition angle reacts more rapidly than altering the position of the throttle valve.
The idling speed can be influenced with HHT or the STAR DIAGNOSIS.

AC compressor
In order to avoid engine speed dropping in the idle speed range when the AC compressor is engaged, the AC pushbutton control unit (N19) or automatic air conditioning pushbutton control unit (N22) supplies a signal to the ME-SFI control unit (as of approx. 6/96 over CAN) and the opening cross-section of the throttle valve is enlarged.
When the air conditioning unit is switched on and the external temperature exceeds 35° C the idling speed on vehicles delivered from 6/00 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.
If the control unit SBC reports an undervoltage to the engine control unit over CAN the idling speed for the driving mode selected is increased to about 750 rpm.