Engine Limp-Home Mode, Function - GF07.10-D-1019TSA

Engine 651 in model 907

Block diagram

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 CDI control unit controls the engine limp-home mode based on the following signals:

Brake application ESP® control unit via suspension FlexRay (FlexE)
Rpm sensor
Accelerator pedal module, accelerator pedal position
Crankshaft position sensor, engine speed
Fully integrated transmission control (VGS) electric controller unit (N15/11), with code G42 (7G-TRONIC PLUS), drive range via drive CAN (CAN C)

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 for failure of the crankshaft position sensor

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

In case of failure of a Hall sensor the system switches to a second Hall sensor and the engine power is reduced.

For this purpose, the CDI control unit regulates the fuel pressure in the rail via the pressure regulating valve and the quantity control valve and the injection duration via the actuation of the fuel injectors.

If the plausibility check made by the CDI control unit is negative or if both Hall sensors fail, the system will only work to regulate the idle speed.

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

If no voltage signal is received from the crankshaft position sensor, the voltage signal from the RPM sensor is used instead (for the limp-home mode).

Faults detected by the accelerator pedal module are stored in the fault memory and can be read out and cleared using Xentry Diagnostics.

Electrical function schematic for engine limp-home mode	PE07.10-D-2719-97TSA
Overview of system components for common rail diesel injection (CDI)	GF07.16-D-9997TSA