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

Engine 642.8 in model 907

Block diagram

Function requirements for engine limp-home mode, general points

Circuit 87 (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 (N3/28) controls engine limp-home mode based on the following sensors and signals:

Crankshaft position sensor (B70/1), engine speed
Accelerator pedal module (A68), accelerator pedal position via powertrain control unit (N127) and drive CAN (CAN C)
ESP® control unit (N30/4) brake actuation via suspension FlexRay (Flex E), powertrain control unit, and drive CAN
Fully integrated transmission control (VGS) electric controller unit (N1511) (with code G42 (7G-TRONIC PLUS)), drive range via drive CAN

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 on failure of the crankshaft position sensor

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

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

To do this, the CDI control unit regulates the fuel pressure in the rails via the pressure regulating valve (Y746) and quantity control valve (Y944) and the injection duration through the actuation of the fuel injectors (Y76).

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.

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

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

If no voltage signal is received from the crankshaft position sensor, the voltage signal from the RPM sensor (B6/24) is used instead (for limp-home mode).

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