Principle Of Operation
Air flow into the engine is regulated by the Throttle Valves and/or the Idle Air Actuator. Both of these air "passages" are necessary for smooth engine operation from idle to full load. On the MS S54 system, the Throttle Valve and the Idle Air Actuator are electrically controlled. All of the ECM monitoring, processing and output functions are a result of regulated air flow.
The Accelerator Pedal Position (PWG) is monitored by the ECM for pedal angle position and rate of movement. As the accelerator is moved, a rising voltage signal from the Hall sensors requests acceleration and at what rate. The ECM will increase the volume of fuel injected into the engine, advance the ignition timing and open the Throttle Valves and/or Idle Air Actuator. The "full throttle" position indicates maximum acceleration to the ECM, and in addition to the functions just mentioned, this will have an effect on the air conditioning compressor (covered in Performance Controls).
As the accelerator pedal is released (integral springs), a decrease in voltage signals the ECM to activate fuel shut off if the RPM is above idle speed (coasting). The Throttle Valves will be closed and Idle Air Actuator Valve will open to maintain idle speed.
The ECM monitors the engine idle speed in addition to the accelerator pedal position and throttle position voltage. If the voltage values have changed (mechanical wear of throttle plates or linkage), the ECM will adjust the Idle Air Actuator to maintain the correct idle speed.
The pedal position sensor consists of two separate Hall sensors with different voltage characteristics and independent ground and voltage supply. Sensing of the accelerator pedal position is redundant. The pedal position sensor is monitored by checking each individual sensor channel and comparing the two pedal values. Monitoring is active as soon as the sensors receive their voltage supply (KL15). The ECM decides what operating mode the pedal position sensor is to assume.
Mode 0 = Pedal position sensor fully operable
Mode 1 = Failure of one pedal position sensor (maximum engine speed is limited)
Mode 2 = Failure of both pedal position sensors (engine speed limited to 1500 RPM)
The Idle Air Actuator is controlled by the ECM modulating the ground signals (PWM at 100 Hz) to the valve. By varying the duty cycle applied to the windings, the valve can be progressively opened, or held steady to maintain the idle speed. The ECM controls the Idle Air Actuator to supply the necessary air to maintain idle speed. When acceleration is requested and the engine load is low (<15%), the actuator will also supply the required air.
There are additional factors that influence the ECM in regulating idle speed:
- The RPM sensor input allows the ECM to monitor engine speed because of loads that cause idle fluctuations due to drag on the engine: power steering, thick oil (frictional forces), etc.
- Cold engine temperature (coolant NTC) provides higher idle speed to raise temperature sooner.
- Vehicle speed informs ECM when the vehicle is stationary and requires idle maintenance.
- A/C on request from the climate control system (arming the ECM) and compressor engage (stabilize idle speed) acknowledgment.
The Electronic Throttle Actuator (EDR) is operated by the ECM for opening and closing based on the accelerator pedal position, DSC intervention and cruise control functions. For exclusive control, the ECM supplies the voltage and ground for operation. The system requires approx. 110 milliseconds in order to fully open the closed throttle valves.
When the EDR is operated, the ECM monitors a feedback potentiometer located on the actuator shaft for position/plausibility. As the EDR opens the Throttle Valves to accelerate the engine, the position is also monitored by a feedback potentiometer located on the end of the throttle shaft on the number 1 throttle housing. These two sensors operate inversely (voltage values) with throttle actuation.
The EDR actuator will "open" the throttle valves for acceleration when the engine load is >15%. There is a transition during acceleration when the Idle Air Actuator will also be open providing additional air for initial acceleration torque.
With the Idle Actuator System and Electronic Throttle Control (EDR), the S54 is equipped with two independent air systems. The ECM is therefore capable of dividing the air volume of the engine between the idle actuator and/or throttle valves corresponding to the load status.