Principle of Operation: Notes

Air flow into the engine is regulated by the Throttle Valve and/or the Idle Air Actuator. Both of these air "passages" are necessary for smooth engine operation from idle to full load. On the MS42/MS43 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.

Fig 1: Identifying Air Management System

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 potentiometers/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 Valve 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 Valve 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 plate or linkage), the ECM will adjust the Idle Air Actuator to maintain the correct idle speed.

The potentiometers/Hall sensors are non-adjustable because the ECM "learns" the throttle angle voltage at idle speed. If the throttle housing/accelerator pedal module is replaced, the ADAPTATIONS MUST BE CLEARED and ADAPTATION PROCEDURE MUST BE PERFORMED using the DISplus/MoDIC. If this is not performed, the vehicle will not start, or run in "fail-safe" mode.

If this input is defective, a fault code will be stored and the "Malfunction Indicator and/or EML" Light will be illuminated. Limited engine operation will be possible.

The MS42 PWG (Driver's Wish) pedal position sensor is integrated in the MDK assembly. The PWG section of the MDK consists of two separate potentiometers with independent voltage supply and monitor circuits (provided by the ECM).

The pedal position sensor is monitored by checking each individual sensor circuit and comparing the two values. Monitoring is active as soon as the sensors receive voltage (KL15).

Fig 2: Identifying MS42 PWG Diagram

The MS43 PWG pedal position sensor consists of two separate Hall sensors with different voltage characteristics and independent power supply (located in the accelerator pedal module).

The pedal position sensor is monitored by checking each individual sensor circuit and comparing the two pedal values. Monitoring is active as soon as the sensors receive voltage (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)

Fig 3: Identifying MS43 PWG Diagram

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.

The basic functions of the idle speed control are:

Control the initial air quantity (air temp >0° C, MDK/EDK is simultaneously opened).
Variable preset idle based on load and inputs.
Monitor RPM Feedback for each preset position.
Lower RPM range intake air flow (even while driving)
Vacuum Limitation
Smooth out the transition from acceleration to deceleration.

Fig 4: Identifying Idle Air Actuator

Under certain engine operating parameters, the MDK/EDK throttle control and the Idle Air Actuator are operated simultaneously. This includes all idling conditions and the transition from off idle. As the request for load increases, the idle valve will remain open and the MDK/EDK will supply any additional air volume required to meet the demand.

Emergency Operation of Idle Air Actuator:

If a fault is detected with the Idle Air Actuator, the ECM will initiate failsafe measures depending on the effect of the fault (increased air flow or decreased air flow). If there is a fault in the Idle Air Actuator/circuit, the MDK/EDK will compensate to maintain idle speed. The "Malfunction Indicator and/or EML" Light will be illuminated to inform the driver of a fault.

If the fault causes increased air flow (actuator failed open), VANOS and Knock Control are deactivated which noticeably reduces engine performance.

The MS42 MDK Feedback Signal Monitoring/Emergency Operation when a fault is detected in the system is as follows:

Emergency operation 1 - Faults which do not impair actuator control, but which adversely affect the functioning of the MDK.
Emergency operation 2 - Applies when faults are encountered which might affect the performance of the MDK.
Emergency operation of idle Air Actuator.