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 < 0ºC)
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 fail-safe.
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 (fail-safe).
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
Engine Management Systems
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.
Engine Management Systems
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 if 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.