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

ENGINE 272.9 (except 272.982) in MODEL 204.0/2 as of model year 2009 

ENGINE 272.9 (except 272.983) in MODEL 212 

ENGINE 272.9 (except 272.984) in MODEL 207 

ENGINE 272.9 in MODEL 204.9 

ENGINE 273.9 in MODEL 207, 212 

Function requirements for engine limp-home mode, general points 

• Circuit 87M ON (engine timing ON)
• Engine running

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 (N3/10) controls the 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
• Electronic Stability Program control unit (N30/4) or Electronic Stability Program Premium control unit (N30/7) (vehicles with DISTRONIC PLUS), brake actuation via the chassis CAN (CAN E)
• fully integrated transmission control controller unit (Y3/8), gear range via the drive train CAN (CAN C) (for an automatic transmission)

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 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 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 10° opening angle.

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. 10° 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 injection valves (Y62) on or off, depending on:

• engine speed
• brake recognition
• selected gear range (with 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 by the accelerator pedal sensor and throttle valve actuator are stored in the fault memory, and can be read out and deleted with the Diagnosis Assistance System (DAS).

	Electrical function schematic for engine limp-home mode	MODEL 204	PE07.10-P-2719-97FAB
		MODEL 207	PE07.10-P-2719-97EAB
		MODEL 212	PE07.10-P-2719-97DAB
	Overview of system components motor electronics (ME-SFI) fuel injection and ignition system		GF07.61-P-9997MI