Engine limp-home mode, function - GF07.10-P-1019OMA

ENGINE 651.9 in MODEL 207, 212 (except 212.098/298) as of model year 2014 

ENGINE 651.9 in MODEL 218 as of model year 2014 

Function requirements for engine limp-home mode, general points 

• Circuit 87M ON (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 CDI control unit (N3/9) controls engine limp-home mode based on the following sensors and signals:

• Camshaft Hall sensor (B6/1)
• Accelerator pedal sensor (B37), accelerator pedal position
• Crankshaft Hall sensor (B70), engine speed
• Electronic Stability Program control unit (N30/4) (except code (233) DISTRONIC PLUS) or Electronic Stability Program Premium control unit (N30/7) (with code (233) DISTRONIC PLUS), brake actuation via chassis CAN 1 (CAN E1)
• Fully integrated transmission control unit (Y3/8n4) (with code 427 (7-speed A/T)) or (with code 421 (9-speed automatic transmission)) as of 01.09.14, gear range via the drive train CAN

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

Function sequence for limp-home mode on failure of 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 rail over the pressure regulating valve (Y74) and quantity control valve (Y94) and the injection period by 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).

Diagnosis

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

	Electrical function schematic for engine limp-home mode	MODEL 207	PE07.10-P-2719-97EAN
		MODEL 212	PE07.10-P-2719-97DAR
		MODEL 218	PE07.10-P-2719-97XAH
	Overview of system components for common rail diesel injection (CDI)		GF07.16-P-9997OM