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

Engine 274 in model 447, 448

except code XM0 (Facelift)

Function requirements for engine limp-home mode, general points

Circuit 87M ON (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 (N3/34) controls the engine limp-home mode based on the following sensors and signals:

Intake camshaft Hall sensor (B6/29), intake camshafts position over direct line
Exhaust camshaft Hall sensor (B6/30), exhaust camshafts position over direct line
Accelerator pedal module (B37/4), accelerator pedal position over direct line
Crankshaft Hall sensor (B70/2), engine rpm over direct line
Throttle valve actuator (M16/50), throttle valve position over direct line
Electronic Stability Program control unit (ESP) (N30/4), brake actuation via the chassis CAN (CAN E)
Fully integrated transmission control electric controller unit (VGS) (N15/11), 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 the accelerator pedal module (B37/4)
Function sequence for limp-home mode in the event of failure of actual value potentiometer 1 (M16/50r1) or actual value potentiometer 2 (M16/50r2) of the throttle valve
Function sequence for limp-home mode in the event of failure of throttle valve actuator motor (M16/50m1)
Function sequence for limp-home mode position of the throttle valve
Function sequence for limp-home mode for failure of the crankshaft Hall sensor (B702)

Function sequence for limp-home mode on failure of the accelerator pedal module (B37/4)

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 (M16/50r1) or actual value potentiometer 2 (M16/50r2) of the throttle valve

If one potentiometer fails, the second potentiometer is switched over. 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 an about 5°opening angle.

Function sequence for limp-home mode in the event of failure of throttle valve actuator motor (M16/50m1)

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

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 rpm through switching off and switching-in the fuel injectors (Y62) in neutral at about 1, 200 to 1, 400 rpm and in driving mode at about 1, 800 rpm.

This occurs dependent on:

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

Function sequence for limp-home mode for failure of the crankshaft Hall sensor (B702)

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

Recognized faults of the accelerator pedal module (B37/4), throttle valve actuator (M16/50), crankshaft Hall sensor (B70/2), intake camshaft Hall sensor (B6/29) or exhaust camshaft Hall sensor (B6/30) are stored in the fault memory and can be read out and also deleted using XD.

Electrical function schematic for engine limp-home mode	Engine 274 in model 448 except code XM0 (Facelift)	PE07.10-S-2719-97VDC
	Engine 274 in model 447 except code XM0 (Facelift)	PE07.10-S-2719-97TRC
Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-S-9998TRC