Motor Driven Throttle Valve
Motor Driven Throttle Valve
The MDK control function has been integrated into the ECM. The purpose is for precision throttle operation, OBD II compliant for fault monitoring, ASC/MSR control, and cruise control. This integration reduces extra control modules, wiring, and sensors.
The MDK control function is integrated into the Siemens MS42.0 ECM. The ECM carries this function out by regulating the engine throttle valve.
The engine throttle valve performs the following functions:
- Precision intake air control
- ASC control
- MSR control
- Cruise control
- Preset position during engine start up (if temperature is < 0 °C)
The new engine throttle valve (MDK) differs from the familiar EML in the following points:
- The accelerator pedal potentiometer (PWG) is now integrated in the MDK housing.
- A throttle cable is used to actuate the throttle potentiometers and also serves as a backup to open the throttle plate (full control) if the MDK system is in failsafe.
The throttle cable (foot pedal controlled) is connected to a pulley on the side of the MDK. The pulley is linked by springs to one end of the throttle shaft, the MDK electric motor is attached to the other end of the throttle shaft.
With the pulley linked by springs to the throttle shaft, this allows ASC intervention to override the driver's set throttle position.
As the pulley and shaft are rotated, the twin potentiometers (integral in the MDK housing, driver's wish) are sensing the requested load. A twin potentiometer is used for back up redundancy (failsafe).
The MS42.0 ECM will actuate the MDK motor pulse width modulated in both directions at a basic frequency of 600 Hz) which positions the throttle plate.
The second twin potentiometers feedback the actual throttle plate position, allowing the ECM to verify correct throttle position. Again, twin potentiometers are used for back up redundancy (failsafe).
MDK Emergency Operation
If a fault is detected in the system, the following modes of operation are:
- Emergency operation 1 - Faults which do not impair safety, but which adversely affect the functioning of the MDK.
- Emergency operation 2 - Applies when faults are encountered which might impair safe driving operation.
- Emergency operation of idle speed actuator.
Emergency Operation 1
- Activation of the EML warning lamp.
- MDK is deactivated, the throttle valve is opened mechanically by the springs and throttle cable.
- To maintain vehicle control, the MDK opening is compensated for by closing the idle speed actuator and retarding the ignition (engine power reduction).
- Engine power is further limited by fuel injector cutout.
Emergency operation 1 limits the dynamic operation if one or more of the potentiometers fail. The engine can slowly reach maximum speed with limited power. The EML light will be illuminated to alert the driver of a fault.
Emergency Operation 2
If another fault is encountered in addition to emergency operation 1 or if the plausibility is affected, emergency operation 2 is activated by the ECM.
An example of plausibility fault would be that the pulley position does not match the MDK position and the associated airflow.
Emergency operation 2 can also be initiated by simultaneously pressing both the accelerator pedal and the brake pedal, or if a fault is encountered in the brake light switch diagnosis.
When in emergency 2 operation mode, there is an engine speed limitation (slightly above idle speed) in addition to the measures for emergency operation 1.
In emergency operation 2, the engine speed is always limited to 1300 RPM it the brake is not applied, and approximately 1000 RPM if the brake is applied.
The vehicle speed is limited to approximately 20 - 25 mph. The reason for limiting the vehicle speed is if the MDK is wide open, the vacuum assist is insufficient for the brakes.
The emergency operation functions are inactive when:
- Ignition is switched off, main relay is deactivated, and engine is started again
- A fault is not detected
- Brake pedal is not depressed
- The throttle valve is in the idle speed setting
Further Safety Concepts
The MDK safety concept can detect a jammed or binding throttle valve as well as a broken link spring. This fault is detected by the ECM monitoring the feedback potentiometers from the MDK in relation to the pulse width modulation to activate the MDK motor.
Emergency operation functions if the throttle valve is jammed:
- Engine speed limitation depending on driver's wish potentiometers and the MDK position.
- Limited vehicle speed if MDK is wide open.
- The ECM will alternate between 0 - 100% sensing ratio to "shake" the MDK loose.
In the event of a fault, the DIS or MoDiC must be used to interrogate the fault memory, and clear the fault once the proper repair has been performed.
Intake Air Flow Control
Under certain engine parameters, the MDK throttle control and the idle speed actuator (ZWD) are operated simultaneously.
The ECM detects the driver's wish from the twin potentiometers monitoring the cable/pulley position.
This value is added to the idle speed control value and the total is what the ECM uses for MDK activation. The ECM then controls the idle speed actuator to satisfy the idle air "fill", in addition, the MDK will also be activated pre-control idle air charge. Both of these functions are utilized to maintain idle RPM.
The MDK is electrically held at the idle speed position, and all of the intake air is drawn through the idle speed actuator. Without a load placed on the engine (< 15% load), the MDK will not open until the extreme upper RPM range.
If the engine is under load (> 15%), the idle speed actuator is open and the MDK will also open.
In the upper PWG range (approximately > 60%), the MDK is switched off. The throttle valve is opened wider exclusively by the pulley via the spring linkage.
At the full throttle position, "kickdown" is obtained by depressing the accelerator pedal fully. This will overwind the pulley, but the spring linkage will not move the throttle plate past 90 degrees of rotation.
NOTE: If the MDK is defective, it is replaced as a unit and is not internally serviceable.