Principle of Operation
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 (M3 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 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 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 (45%),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 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.
Pre-drive Check
The pre-drive check has following tasks:
- Zero point adaptation of the throttle potentiometers.
- Checking freedom of movement of the throttle valves and electronic accelerator pedal control circuit.
- Checking the safety cutout and the return springs of the electronic accelerator pedal and throttle valves.
This check is conducted every time KL15 is activated. The full load adaptation stop is learned in a new ECM the first time KL15 is recognized.
The pre-drive check is conducted in 3 phases:
- Phase 1: The throttle valves are closed by the EDR actuator. The position of the throttle potentiometer on the EDR is determined.
- Phase 2: The throttle valves are opened 3% by the EDR actuator. The position of the throttle potentiometer on the throttle valve shaft is determined.
- Phase 3: The throttle valves are opened by approx. 20%. The EDR actuator is switched off. The throttle valves are closed by spring force (a mechanical clicking sound can be heard while the throttle valves are closing).
Post Drive Check
- Post Drive Check: 10 seconds after KL15 is switched "OFF" the EDR actuator is opened 102% in order to carry out renewed full load adaptation. Adaptation during the post drive check is only carried out when the engine is turned off before reaching the operating temperature.
EDR Safety Concept - Emergency Running Programs
The safety concept of the throttle valve control system achieves a slow transition to an emergency running (limp-home) program that can still be managed by the driver.
A basic differentiation is made between PWG emergency operation with a PWG sensor and PWG emergency operation without a PWG sensor. There is a total of 4 emergency operation (limp-home) program stages.
In the event of a PWG sensor failing, the system switches to a PWG emergency operation characteristic curve with lower setpoints. "Engine Emergency Program" is indicated to the driver by the EML warning lamp in the instrument cluster.
Stage 1 (Emergency Operation with a Throttle Position Sensor)
The emergency program stage 1 includes limiting the torque and the EDR setpoint. Based on the current engine torque, the maximum torque is limited in the emergency operation stage. The EDR actuator is limited by reducing the pulse duty factor. The plausibility of the throttle position sensors are checked based on the load signal from the hot-film air mass meter. The measured air mass must not exceed a defined limit. This limit is above the value that can be achieved with the idle air actuator.
Stage 2 (Emergency Operation via Idle Air Actuator)
The transition to emergency program stage 2 greatly depends on the type of fault. For example, if there is a defect in EDR actuator operation, the throttle valves are sprung closed without ECM influence.
In the event of implausible signals from the throttle position Hall sensors 1 and 2, immediate deactivation of the EDR actuator may be necessary under certain circumstances.
In cases where feedback of the actual position is still available and the set position can still be controlled, the ECM closes the throttle valves. The EDR actuator is then switched off and engine speed and road speed limitation activated.
Stage 3 (Emergency Operation with Open Throttle Valves)
The stage 3 emergency operation program is activated when the actual throttle position exceeds the set throttle position for a defined period of time despite power being applied to the EDR actuator, the throttle valves cannot be closed. The ECM reduces the amount of fuel injected (fade out) and retards the ignition timing to limit engine torque. If it is necessary to further reduce the torque, individual fuel injectors are deactivated one cylinder at the time.
Stage 4 (Emergency Operation with Internal ECM Fault)
The stage 4 emergency operation program is always activated when an internal ECM fault is detected. In this case, the characteristics of the throttle valve control (EDR) are not predictable, therefore the ECM reduces the amount of fuel injected (fade out) and retards the ignition timing to limit engine torque. If it is necessary to further reduce the torque, individual fuel injectors are deactivated one cylinder at the time.
Emergency Operation Functions
Engine torque limitation In the emergency programs stage 1 - 4 is restricted to a value specified by the emergency operation (limp-home) program.
In the relevant emergency programs, the MS S54 limits the engine characteristics to the values indicated in the table. In emergency programs 3 and 4, in addition to the engine emergency program being indicated in the instrument cluster (EML), all warning elments in the tachometer are activated.
The Hot-Film Air Mass Meter (HFM) varies voltage monitored by the ECM representing the measured amount of intake air volume. This input is used by the ECM to determine the amount of fuel to be injected.
The heated surface of the hot-film in the intake air stream is regulated by the ECM to a constant temperature of 180° above ambient air temperature. The incoming air cools the film and the ECM monitors the changing resistance which affects current flow through the circuit. The hot-film does not require a "clean burn", it is self cleaning due to the high operating temperature for normal operation.
If this input is defective, a fault code will be set and the "Malfunction Indicator Light" will illuminate when the OBD II criteria is achieved. The ECM will maintain engine operation based on the Throttle Position Sensors and Crankshaft Position/Engine Speed Sensor.
The Air Temperature signal allows the ECM to make a calculation of air density. The varying voltage input from the NTC sensor indicates the larger proportion of oxygen found in cold air, as compared to less oxygen found in warmer air. The ECM will adjust the amount of injected fuel because the quality of combustion depends on oxygen sensing ratio.
The ignition timing is also affected by air temperature. If the intake air is hot the ECM retards the base ignition timing to reduce the risk of detonation. If the intake air is cooler, the base ignition timing will be advanced. The ECM uses this input as a determining factor for Secondary Air Injection activation (covered in the Emissions section).
If this input is defective, a fault code will be set and the "Malfunction Indicator Light" will illuminate when the OBD II criteria is achieved. The ECM will maintain engine operation based on the HFM and Engine Coolant Temperature sensor.
The Suction Jet Pump is regulated by the ECM to provide sufficient vacuum for the brake booster under all operating conditions. The ECM controls the Suction Jet Pump Solenoid to allow vacuum flow through.
The additional vacuum compensation is activated by the ECM when the idle air actuator is regulated for:
- A/C compressor "on"
- Vehicle in gear and the clutch is released (driving under load)
- Engine in warm-up phase <70° C
Additional vacuum compensation is applied to the brake booster when the circuit is "deactivated" (Solenoid sprung open). Vacuum enhancement is limited to the brake booster when the control circuit is "activated" (Solenoid powered closed).