Limp-Home Mode, Detailed Information - GF27.60-P-2500A

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Transmission 724.033/044/045

Overview

This document contains information on:

General
Function requirements
Function
- 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

General

In order to ensure safe driving and to prevent damage to the dual-clutch transmission, the fully integrated transmission control electrohydraulic controller unit switches to limp-home mode in the event of critical faults.

Function requirements

Engine management ON (circuit 87M)
Engine running.

Function

Depending on the type of fault, the fully integrated transmission control electrohydraulic controller unit switches to an adapted transmission limp-home mode.

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 subtransmissions will no longer be available, depending on the actuator which is defective or cannot be actuated.

If sub-transmission 1 fails, gears 1-3-5-7 are no longer available, for example.

If sub-transmission 2 fails, gears 2-4-6 are no longer available, for example.

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 force flux 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. in the event of 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 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.