Idle Speed: Description and Operation

GF30.22-P-0003V ME-SFI Idle Speed Control, Function







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 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, e.g. steering turned to full lock (power steering pump), drive position engaged, clutch pedal actuated, 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 < 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 via the CAN data bus to the ME control unit.

AC compressor
To avoid engine speed dropping in the idle speed range when the AC compressor is engaged, the automatic air conditioning control system issues a signal to the ME control unit via the CAN data bus which is adapted appropriately to the idle speed (increased).

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.