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

Engine 274.9 in model 205 (except 205.047/053/054/147/247/253)

Engine 274.9 in model 253 (except 253.354/954)

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:

intake camshaft Hall sensor (B6/15), intake camshaft setting
Exhaust camshaft Hall sensor (B6/16), exhaust camshaft setting
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 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 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 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. 5° 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.

Switching on and off of the cylinder takes place by the ME-SFI [ME] control unit through switching off and switching-in of cylinder 1 injection nozzle to cylinder 4 injection nozzle (Y76/1 to Y76/4), dependent on:

Engine speed
Accelerator pedal input
the engaged gear range (for automatic transmission)

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

If the voltage signal fails from the crankshaft position sensor, the voltage signals from the intake camshaft Hall sensor 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-2719-97FBA
	Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-P-9998MRA