Engine limp-home mode, function

S07.10-0049-79

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 signals:

 Camshaft Hall sensor (B6/1)

 Accelerator pedal module (B37/4), accelerator pedal position

 Crankshaft Hall sensor (B70), engine speed

 Electronic Stability Program (ESP) control unit (N30/4), brake actuation

 Fully integrated transmission control (VGS) electric controller unit (N15/11), drive range

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 Hall 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 (Y74) and quantity control valve (Y94) and the injection period 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.

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

If no voltage signal is received from the crankshaft Hall sensor, the voltage signal from the camshaft Hall sensor will be used instead (for limp-home mode).

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