Engine limp-home mode, function - GF07.10-P-1019MMW

ENGINE 276.9 in MODEL 212.095 as of model year 2014 

The partial function "engine limp-home mode function" is considered and described from the point of view of the engine timing for the internal combustion engine.

Switching off path The switching off path is used for very serious system failures (e.g. a CAN failure) to power down the high-voltage on-board electrical system and the HD. The status "READY" of the hybrid system is retracted after powering down, the internal combustion engine is switched off and the vehicle is no longer ready to drive. If a serious fault is recognized at the hybrid system, the ME-SFI [ME] control unit (N3/10) sends a shut-down signal to power down the hybrid system via the switching off path to the power electronics control unit (N129/1).

Function requirements for engine limp-home mode, general points 

• Circuit 87M (engine timing ON)
• Hybrid system "READY" (yellow or green)
• Internal combustion engine running

The function requirements "internal combustion engine running" includes all function sequences of the fuel low pressure and high-pressure circuit, as described in the function "fuel supply".

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 timing system.

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

• Left and right intake camshaft Hall sensor (B6/4, B6/5), intake camshaft positions
• Left and right exhaust camshaft Hall sensor (B6/6, B6/7), exhaust camshaft positions
• Accelerator pedal sensor (B37), accelerator pedal position
• Crankshaft Hall sensor (B70), engine speed
• Throttle valve actuator (M16/6), throttle valve position
• Regenerative braking system control unit (N30/6), wheel speed via chassis CAN 1 (CAN E1)

Function sequence for engine limp-home mode 

The function sequence is described in the following steps:

• Function sequence for limp-home mode on failure of accelerator pedal sensor 
• Function sequence for limp-home mode in the event of failure of throttle valve actual value potentiometer 1 or 2 (M16/6r1, M16/6r2) 
• Function sequence for limp-home mode in the event of failure of the throttle valve actuator motor (M16/6m1) 
• Function sequence for limp-home position 
• Function sequence for limp-home mode on failure of the crankshaft Hall sensor 

Function sequence for limp-home mode on failure of 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 for failure of throttle valve actual value potentiometer 1 or 2, 

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.

The position feedback of the throttle valve actuator takes place by means of a contact-less angular sensor.

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

The throttle valve actuator is switched in a deenergized state and the throttle valve is set to the limp-home position at an opening angle of approx. 5° by the spring capsule.

Function sequence for limp-home position 

When the throttle valve IS in limp-home position, fuel is shut off/resupplied 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 ME-SFI [ME] control unit activates/deactivates the cylinders by switching the fuel injectors (Y76), on or off, depending on:

• Engine speed
• Accelerator pedal input

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).

Faults recognized on 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 cleared with Xentry diagnosis.

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