Engine Limp-Home Mode, Basic Function - GF07.10-P-1047A

Engine all (CAR) 

Overview 

This document contains information on:

• General 
• Function requirements 
• Function 

General 

The engine limp-home mode protects the engine from damage during operation and allows continued operation with restrictions in the event that individual components of the engine management fail.

Function requirements 

• Engine management ON (circuit 87M)
• Engine in operation
• Combustion engine control unit detects a malfunction

Function 

Engine limp-home mode 

To ensure fault-free operation of the engine, the combustion engine control unit constantly monitors the components of the engine management. If a malfunction is detected, the combustion engine control unit activates the engine limp-home mode.

When the engine limp-home mode is active, the engine output is reduced and the engine speed limited.

General causes for engine limp-home mode 

• Failure of the accelerator pedal sensor
• Failure of the crankshaft Hall sensor
• Diesel engine: overloading of the diesel particulate filter
• Malfunction in the exhaust gas turbocharger
• Spark-ignition engine: failure of the throttle valve sensor
• Spark-ignition engine: failure of the throttle valve actuator motor
• Spark-ignition engine: limp-home position of the throttle valve

Failure of the accelerator pedal sensor 

If one accelerator pedal sensor fails, the switch is made to the second accelerator pedal sensor and the engine output is limited. If both accelerator pedal sensors fail, or the plausibility test of the accelerator pedal signals fails, only the idle speed is regulated.

Diesel engine: The combustion engine control unit reduces the fuel pressure in the fuel distributor and, by actuating the fuel injectors, the injection duration for the fuel.

Spark-ignition engine: The combustion engine control unit limits the throttle valve opening to approx. 60 % of the maximum opening. In addition, the throttle valves only open with a delay.

Failure of the crankshaft Hall sensor 

If the signal from the crankshaft Hall sensor fails, the signals from the intake camshaft or exhaust camshaft Hall sensors are used as a replacement for engine limp-home mode.

Diesel engine: overloading of the diesel particulate filter 

If the DPF differential pressure sensor determines that the load condition is too high, the engine output is reduced. The combustion engine control unit reduces the fuel pressure in the fuel distributor and, by actuating the fuel injectors, the injection duration for the fuel.

Malfunction in the exhaust gas turbocharger 

If the exhaust gas turbocharger generates too much boost pressure, the engine output is reduced. The combustion engine control unit reduces the fuel pressure in the fuel distributor and, via the duration of the fuel injector actuation, the injection quantity of the fuel.

Failure of the throttle valve sensor 

If one throttle valve sensor fails, the switch is made to the second throttle valve sensor. If both throttle valve sensors are faulty, the return spring sets the throttle valve to the limp-home position at an opening angle of approx. 5°.

Failure of the throttle valve actuator motor 

If the throttle valve actuator motor fails, it is de-energized and set to the limp-home position at an opening angle of approx. 5° by the return spring.

Limp-home mode position of throttle valve 

When the throttle valve is in limp-home position, the engine speed is regulated to about 1200 to 1400 RPM at idle and about 1800 RPM in driving mode. To do this, the fuel supply to individual cylinders is shut off/activated by the combustion engine control unit. When the driver operates the brake pedal, the engine speed is regulated to the idle speed.

	Function schematics
	Function schematic for engine limp-home mode	Engine 608 in model 118, 177, 247	PE07.10-P-2510-97A
		Engine 282 in model 118, 177, 247	PE07.10-P-2510-97B
		Engine 260 in model 118, 177, 247 Engine 264 in model 167	PE07.10-P-2510-97C
		Engine 654 in model 118, 167, 177, 247 Engine 654 in model 213, 238, 257 as of model year 2021 Engine 656 in model 167	PE07.10-P-2510-97D
		Engine 256 in model 167 Engine 256 in model 213, 238 as of model year 2021 Engine 264 in model 213, 238, 257 as of model year 2021	PE07.10-P-2510-97E
		Engine 176, 177 in model 167	PE07.10-P-2510-97F
		Engine 139 in model 118, 177, 247	PE07.10-P-2510-97G
		Engine 254 in model 206 Engine 256 in model 223	PE07.10-P-2510-97H
		Engine 176 in model 223	PE07.10-P-2510-97I
		Engine 279 in model 223	PE07.10-P-2510-97J
		Engine 654 in model 206 Engine 656 in model 223	PE07.10-P-2510-97L
	Control units
	Combustion engine control unit, basic function	Engine all (CAR)	GF07.08-P-9890A