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Engine Limp-Home Mode, Function - GF07.10-P-1019MRS

Engine 264.9 in Model 205, 253 

Function requirements, general 

IMPORTANT The ME-SFI [ME] control unit (N3/10) detects the Engine running via the evaluation of the crankshaft Hall sensor signals (B70). The ME-SFI [ME] control unit directly reads in the signals of the crankshaft Hall sensor.

Engine limp-home mode, general 

The task of the Engine limp-home mode consists of ensuring continued safe driving in the case of malfunctions on individual components of the Engine management.

The ME-SFI [ME] control unit controls Engine limp-home mode based on the following signals:

The ME-SFI [ME] control unit also evaluates the status of the service brake. The Electronic Stability Program control unit (N30/4) transmits this via the suspension FlexRay (Flex E), powertrain control unit and drive CAN to the ME-SFI [ME] control unit.

Depending on the fault, a differentiation is made between the following Engine limp-home mode variants:

Limp-home mode upon failure of accelerator pedal sensor 

The accelerator pedal sensor detects the position of the accelerator pedal via two fixed Hall elements. In the case of the failure of a Hall element, a switchover to the second Hall element takes place. The throttle valve opening is limited to about 60 % of the maximum opening. The throttle valve opening speed is also delayed. The ME-SFI [ME] control unit actuates the throttle valve actuator motor directly.

If the brake pedal is operated, this always means idle speed input. If the plausibility check of the Hall signals is negative or if both Hall sensors fail, only idle speed is controlled.

Limp-home mode upon failure of actual value potentiometers 

In the case of the failure of an actual value potentiometer, a switchover to the second actual value potentiometer takes place. Air mass is used as an additional comparison value. If both actual value potentiometers are defective, the throttle valve is set to the limp-home mode position of an approx. 5° opening angle via the return springs.

Limp-home mode in the event of failure of the throttle valve actuator 

In the case of the failure of the throttle valve actuator motor, this is de-energized. The throttle valve is set to the limp-home mode position of an approx. 5° opening angle by a return spring.

Limp-home mode upon failure of crankshaft Hall sensor 

In the case of the failure of the crankshaft Hall sensor, the signals of the intake camshaft Hall sensor or those of the exhaust camshaft Hall sensor are used as a substitute.

IMPORTANT Detected failures or faults are stored in the fault memory of the ME-SFI [ME] control unit and can be read out and deleted with the diagnostic tester.

Limp-home mode position of throttle valve 

If the throttle valve is in the limp-home mode position, the Engine speed is regulated by the deactivation/activation of individual cylinders to approx. 1200 up to 1400 RPM in idle and to approx. 1800 RPM during vehicle operation. If the brake pedal is operated, this always means idle speed input.

The ME-SFI [ME] control unit controls the shutting off and switching in of the cylinders, depending on:

The ME-SFI [ME] control unit actuates the following components for this purpose:

  Electrical function schematic for Engine limp-home mode   PE07.10-P-2719-97FBI 
  Overview of system components for gasoline injection and ignition system with direct injection   GF07.70-P-9998MRS 
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