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

Engine 157.9, 276.8, 278.9 in model 217, 222 (except 222.163/173) 

Function requirements for engine limp-home mode, general points 

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 (N3/10) controls the engine limp-home mode based on the following signals:

Function sequence for engine limp-home mode 

The function sequence is described in the following steps:

Function sequence for limp-home mode for failure of the accelerator pedal sensor 

If a Hall sensor fails, the system switches over to the second Hall sensor. The throttle valve opening is limited to about 60% of the maximum opening. In addition, the speed at the which the throttle valve opens is dynamically limited (throttle valve opens with a delay). If the brake pedal is operated, this always means idle speed input. If the plausibility check for the Hall sensor signals is negative or if both Hall sensors fail, only idle speed is then still controlled.

Function sequence for limp-home mode in the event of failure of actual value potentiometer 1 or 2 of throttle valve actuator 

If one potentiometer fails, the other potentiometer is used. Air mass is used as an additional comparison value. If both potentiometers are faulty, the spring capsule brings the throttle valve to the limp-home position of about 5° opening angle by means of a return spring.

Function sequence for limp-home mode due to failure of the actuator motor of the throttle valve actuator 

The throttle valve actuator is de-energized, and the throttle valve is set to the emergency operation position of an opening angle of approx. 5° by the return spring.

Function sequence for limp-home position 

When the throttle valve is in limp-home position, fuel is shut off/switched-in again to the cylinders to limit engine speed to about 1200 to 1400 rpm at idle and about 1800 rpm in driving mode. If the brake pedal is operated, this always means idle speed input.

The cylinder shut off and switch-in takes place over the ME-SFI [ME] control unit through shut off and switch-in of the fuel injectors 1 to 6 (Y76/1 to Y76/6) (for engine 276.8) or the fuel injectors 1 to 8 (Y76/1 to Y76/8) (for engines 157.9, 278.9), dependent on:

Function sequence for limp-home mode on failure of the crankshaft Hall sensor 

If the voltage signal from the crankshaft Hall sensor fails the voltage signals from the intake camshaft or exhaust camshaft Hall sensor are used as replacement (for limp-home mode).

IMPORTANT Diagnosis

Faults recognized at the accelerator pedal sensor, throttle valve actuator, crankshaft Hall sensor, left and right intake camshaft Hall sensor and left and right exhaust camshaft Hall sensor are stored in the fault memory and can be read out and deleted with Xentry Diagnostics.

  Electrical function schematic for engine limp-home mode Engine 276.8 in model 217, 222 (except 222.163/173) PE07.10-P-2719-97SEC
    Engine 157.9, 278.9 in model 217, 222 PE07.10-P-2719-97SED
  Overview of system components for gasoline injection and ignition system with direct injection Engine 157.9, 278.9 in model 222 GF07.70-P-9998MML
    Engine 276.8 in model 222 (except 222.163/173) GF07.70-P-9998MMT 
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