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

Engine 274 in model 907 

Block diagram 

Function requirements for engine limp-home mode, general points 

• Circuit 87M (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 ME-SFI [ME] control unit (N3/36) controls the engine limp-home mode based on the following signals:

• Intake camshaft Hall sensor (B6/29), intake camshaft setting
• Exhaust camshaft Hall sensor (B6/30), exhaust camshaft setting
• Crankshaft Hall sensor (B70/2), engine RPM
• Throttle valve actuator (M16/50), throttle valve position
• 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 the suspension FlexRay (Flex E) to the powertrain control unit and the 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 upon failure of actual value potentiometers of throttle valve actuator
• Function sequence for limp-home mode for failure of the throttle valve actuator motor

• Function sequence for limp-home position
• 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 

If a Hall sensor fails, the system switches over to the second Hall sensor. The throttle valve opening is limited to about 60 % of the maximum opening. In addition, the speed at the which the throttle valve opens is dynamically limited (throttle valve opens with a delay). If the brake pedal is operated, this always means idle speed input. If the plausibility check for the Hall sensor signals is negative or if both Hall sensors fail, only idle speed is then still controlled.

Function sequence for limp-home mode upon failure of actual value potentiometers of throttle valve actuator 

In the case of the failure of an actual value potentiometer, a switchover to the second intact actual value potentiometer takes place. Air mass is used as an additional comparison value. If both actual value potentiometers are defective, the throttle valve is set via the spring capsule to the limp-home mode position with an opening angle of approx. 5°.

Function sequence for limp-home mode for failure of the throttle valve actuator motor 

The throttle valve actuator motor is deenergized and the throttle valve is set by the spring capsule to the limp-home mode position with an opening angle of approx. 5°.

Function sequence for limp-home position 

When the throttle valve is in limp-home position, fuel is shut off/switched-in again to the cylinders to limit engine speed to about 1200 to 1400 RPM at idle and about 1800 RPM in driving mode. If the brake pedal is operated, this always means idle speed input.

The cylinders are activated/deactivated via the ME-SFI [ME] control unit by deactivating/activating the fuel injectors (Y62), depending on:

• Engine speed
• Accelerator pedal input
• the engaged gear range (for automatic transmission)

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

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

Electrical function schematic for engine limp-home mode	PE07.10-D-2719-97TSB
Overview of system components for gasoline injection and ignition system with direct injection	GF07.70-D-9998TSB