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

Engine 276.8 in model 222.163/173 

Engine 276.9 in model 222 

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

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 hybrid drive. The status "READY" of the hybrid drive system is retracted after powering down, the internal combustion engine is switched off and the vehicle is no longer ready to drive.

If a major fault is detected in the hybrid drive system, the ME-SFI [ME] control unit (N3/10) transmits a shut-down signal for shutting down the hybrid drive system to the power electronics control unit (N129/1) via the shut-down path.

Function requirements for engine limp-home mode, general points 

• Circuit 87M (Engine management ON)
• Hybrid drive system "READY"
• Internal combustion engine running

The function requirement "internal combustion engine running" includes all function sequences of the fuel low-pressure and high-pressure circuits.

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 sensors and signals:

• Left and right intake camshaft Hall sensors (B6/4, B6/5), intake camshaft positions
• Left and right exhaust camshaft Hall sensors (B6/6, B6/7), exhaust camshaft positions
• Accelerator pedal sensor (B37), accelerator pedal position
• Crankshaft Hall sensor (B70), engine rpm
• Throttle valve actuator (M16/6), throttle valve position
• Electronic Stability Program control unit (N30/4), brake actuation via chassis FlexRay (Flex E) to the powertrain control unit (N127) and the drive train CAN (CAN C1)

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 
• Function sequence for limp-home mode in the event of failure of actual value potentiometer 1 or 2 (M16/6r1, M16/6r2) of throttle valve actuator 
• Function sequence for limp-home mode due to failure of the actuator motor (M16/6m1) of the throttle valve actuator 
• 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 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 defective, the spring capsule brings the throttle valve to the limp-home position of about 5°opening angle.

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 limp-home position of an opening angle of approx. 5° by the spring capsule.

Function sequence for limp-home position 

When the throttle valve is in the emergency operation position, fuel is shut off and cut in again to the cylinders to regulate the engine speed to approx. 1, 200 to 1, 400 rpm in idle and to approx. 1, 800 rpm during vehicle operation. If the brake pedal is operated, this always means idle speed input.

The ME-SFI [ME] control unit activates/deactivates the cylinders by switching in 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).

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		PE07.10-P-2719-97SEC
	Overview of system components for gasoline injection and ignition system with direct injection	Engine 276.9 in model 222	GF07.70-P-9998MMK
		Engine 276.8 in model 222.163	GF07.70-P-9998MMT