Engine Limp-Home Mode, Function - GF07.10-S-1019TRJ

Engine 274 in model 447.6/7 

with code XM0 (Facelift) 

Function requirements for engine limp-home mode, general points 

• Circuit 87M ON (Engine management 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 management.

The ME-SFI [ME] control unit (N3/34) controls the engine limp-home mode based on the following sensors and signals:

• Intake camshaft Hall sensor (B6/29), intake camshaft position
• Exhaust camshaft Hall sensor (B6/30), exhaust camshaft position
• Accelerator pedal module (B37/4), accelerator pedal position
• Crankshaft Hall sensor (B70/2), engine rpm
• Throttle valve actuator (M16/50), throttle valve position
• Electronic Stability Program (ESP) control unit (N30/4), brake actuation
• Fully integrated transmission control (VGS) electric controller unit (N15/11) (Automatic transmission with code G42 (7G-TRONIC PLUS)), drive range
• Fully integrated transmission control (VGS) electric controller unit (N15/15) (Automatic transmission with code G43 (9G-TRONIC)), drive range

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 the accelerator pedal module
• Function sequence for limp-home mode in the case of the failure of an actual value potentiometer of the throttle valve
• Function sequence for limp-home mode for failure of the throttle valve actuator motor
• Function sequence for limp-home mode position of the throttle valve
• Function sequence for limp-home mode on failure of the crankshaft Hall sensor

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

In the case of the failure of a Hall sensor of the accelerator pedal module (B37/4), a switchover to the second Hall sensor will take place. The throttle valve opening is limited to approx. 60% of the max. 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 case of the failure of an actual value potentiometer of the throttle valve 

In the case of the failure of the actual value potentiometer 1 (M16/50r1) or the actual value potentiometer 2 (M16/50r2), a switchover takes place to the respective other potentiometer. Air mass is used as an additional comparison value. If both potentiometers have failed, the throttle valve is set by the spring capsule to the limp-home mode position with an opening angle of approx. 5°.

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

The throttle valve actuator motor (M16/50m1) is de-energized and the throttle valve is set by the spring capsule to the limp-home mode position with an opening angle of approx. 5°.

Function sequence for limp-home mode position of the throttle valve 

If the throttle valve is in limp-home mode position, the ME-SFI [ME] control unit (N3/34) regulates the engine speed by switching off and switching-in the fuel injectors (Y62) in idle to approx. 1200 up to 1400 rpm and during vehicle operation to approx. 1800 rpm.

This takes place depending on the following factors:

• Engine speed
• Accelerator pedal position
• Gear range (for an A/T)

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

If the voltage signal of the crankshaft Hall sensor (B70/2) fails, the voltage signals of the intake camshaft Hall sensor (B6/29) or the exhaust camshaft Hall sensor (B6/30) is used instead.

Diagnosis

Detected faults of the accelerator pedal module, throttle valve actuator, crankshaft Hall sensor, intake camshaft Hall sensor or exhaust camshaft Hall sensor are stored in the fault memory and can be read out and cleared with XENTRY Diagnosis.

	Function schematic for engine limp-home mode		PE07.10-S-2510-97TRM
	Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-S-9998TRJ