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

ENGINES 651.955/956/957 in MODEL 906 

with CODE (XZ1) Model generation 1 

with CODE (MZ0) Blue EFFICIENCY 

Function requirements 

• Circuit 87M ON (engine control ON)
• Circuit 61 ON

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 control system. The CDI control unit (N3/28) controls engine limp-home mode based on the following sensors and signals:

• Camshaft sensor (B6/23)
• Accelerator pedal module (B37/3) (for model 906), accelerator pedal position
• Crankshaft sensor (L5/10), engine speed
• ESP control unit (N30/4), brake actuation
• EGS control unit (N15/3) (for transmission 722.6), gear range via the engine compartment CAN (CAN C)

Function sequence 

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 on failure of the crankshaft sensor 

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

In a case of failure of a Hall sensor integrated in the accelerator pedal module the system switches to the 2nd integral Hall sensor and the engine power is reduced. To do this the CDI control unit regulates the fuel pressure in the rail over the pressure regulating valve (Y74/5) and performs actuations of the following fuel injectors:

• Cylinder 1 injector (Y76/14)
• Cylinder 2 injector (Y76/15)
• Cylinder 3 injector (Y76/16)
• Cylinder 4 injector (Y76/17)

If the plausibility check is negative or if both Hall sensors fail, only idle speed is then still controlled.

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

If the voltage signal is missing from the crankshaft sensor, the voltage signal from the camshaft sensor is used instead.

Faults detected by the accelerator pedal module are stored in the fault memory and can be read out and deleted using the Diagnosis Assistance System (DAS).