Overview
This document contains information on:
Function requirements
Sub-transmission limp-home mode, even/odd gears
Transmission limp-home mode, power-free state
Transmission limp-home mode, transmission overtemperature
Transmission limp-home mode, no oil pressure at engine standstill
Transmission limp-home mode, park pawl system
Function requirements
Engine management ON (circuit 87M)
Engine running
Sub-transmission limp-home mode, even/odd gears
In the even of a fault in or failure of a gear control actuator, one of the sub-transmissions will no longer be available, depending on the actuator which is defective or cannot be actuated. If sub-transmission 1 fails, e.g. gears 1-3-5-7 are no longer available. If sub-transmission 2 fails, e.g. gears 2-4-6 are no longer available. The design assignment means that reverse gear is no longer available either.
Transmission limp-home mode, power-free state
In the event of any drop in system pressure in the dual-clutch transmission or defective actuation of both clutches the power flow fails. The vehicle is no longer propelled.
Transmission limp-home mode, transmission overtemperature
In drive mode, friction at the clutch plates and gear sets along with transmission control waste heat, generates high transmission oil temperatures. Particularly high waste heat is produced in the area of the clutches. The waste heat is absorbed by the transmission oil and dissipated to the environment through the coolant circuit. The transmission oil temperature can exceed a predefined threshold value, e.g. due to multiple start-off procedures on uphill gradients and at high outside temperatures. The combustion engine control unit reduces the engine output until the transmission oil temperature is back below the threshold value. If the transmission cooling fails, e.g. during an interrupted flow of coolant through the transmission oil heat exchanger, it may be possible to continue driving up to a defined threshold value.
Transmission limp-home mode, no oil pressure at engine standstill
If the electric auxiliary oil pump fails, oil pressure will be insufficient when the engine is switched off. A drive range can then not be selected from drive range "P" or "N".
Transmission limp-home mode, park pawl system
The park pawl prevents the vehicle from moving off by itself when the selector lever is in position "P" in normal operation. The park pawl cannot be engaged if the actuation of the following components is faulty or incorrect:
Park pawl switchover valve
Park pawl lift solenoid
The vehicle cannot be secured against rolling away through the selector lever position "P". Alternatively, it is possible to prevent it from rolling by actuating the electric parking brake.