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

Engine 276.8 in model 172.4 

Function requirements for engine limp-home mode, general points 

• Circuit 87M (Engine management ON)
• Engine runs

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:

• 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 RPM
• Actual value potentiometer 1 and 2 (M16/6r1, M16/6r2), throttle valve position
• Electronic Stability Program control unit (N30/4), brake actuation 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 for failure of the accelerator pedal sensor 
• Function sequence for limp-home mode for failure of throttle valve actual value potentiometer 1 or 2, 
• 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 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 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 return spring brings the throttle valve to the limp-home position of about a 10°opening angle.

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

The actuator motor is de-energized and the throttle valve is set to the limp-home position at an opening angle of approx. 10° by the return spring.

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.

Switching off and switching in of the cylinder takes place by the ME-SFI [ME] control unit through switching off and in of the cylinder 1 to 6 fuel injectors (Y76/1 to Y76/6), dependent on:

• Engine speed
• Brake recognition
• the engaged gear range (for automatic transmission)

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 detected at the accelerator pedal sensor, throttle valve actuator, crankshaft Hall sensor, intake camshaft Hall sensor and exhaust camshaft Hall sensor are stored in the fault memory and can be read out and erased using Xentry Diagnostics.

	Electrical function schematic for engine limp-home mode		PE07.10-P-2719-97TAH
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