Part 2

Twin-Clutch Transmission

Cooling of the twin clutch
To cool the twin clutch, hydraulic fluid from the oil sump is sprayed onto the twin clutch. The sensor for the cooling oil temperature on the twin clutch is hit by the jet spray of the cooling oil. The temperature signal is sent to the DKG control unit.
The heat management determines the current cooling requirement and regulates the cooling system accordingly. To achieve this, the DKG control unit activates a solenoid valve.

The solenoid valve has 3 positions:
- Position with minimum flow to twin clutch (no current)
- Opened position
- Closed position

If the oil sump temperature is implausible in relation to the temperature of the cooling oil in the twin clutch, a Check Control message is displayed.

Engine start
Regardless of the drive position and gear in which the engine is shut down, the engine can only be started when the brake is operated. The parking lock is always engaged when the engine is started. If the engine is started in gear lever position "R", the symbol for the monostable position in the functional display in the gear lever flashes. When the vehicle is unlocked, the DKG control unit is wakened by the Car Access System (CAS).

Overlap control
When a gear shift is triggered in the sequential mode "S" or in the automated drive mode "D", the DKG control unit activates the corresponding solenoid valves. Simultaneously, the DKG control unit sends a message to the DME control unit indicating that a gear shift operation is being initiated. The engine torque is reduced. During the gear shift, the engine speed is raised if required to make clutch engagement faster and more comfortable.
The DKG control unit activates the twin clutch, the solenoid valves and the hydraulic shift cylinders. The shift cylinders execute the longitudinal movement (forward or backward movement) of the corresponding gearshift rods. The desired gear is engaged.
The overlap control ensures optimized shift characteristics as follows: in the overlap control, both clutches are used simultaneously. In the engaged clutch the pressure is reduced and simultaneously pressure is built up in the clutch to be engaged. The pressure reduction or pressure build-up is maintained until a synchronized speed is reached. Then, the hydraulic pressure for the clutch to be engaged can be built up in full.
After the gear shift operation, the current to the solenoid valve is switched off.
The entire gear shift operation runs depending on the pedal sensor position, engine speed, driving speed and other control variables (e.g. engine oil temperature, selected drive program).
Gear shifts that would lead to impermissibly high or low engine speeds are not executed.

Drivelogic (motorsport engineering only)
"Drivelogic" provides a total of 11 different drive programs.
- In the automated drive mode "D" (automatic operation), there is a choice of 5 drive programs (from "relaxed" to "sports"). In drive program 1, for example, 2nd gear is selected for driving off. The clutch works particularly sensitively. This makes driving off easier on winter roads.
- In the sequential mode "S" (manual operation), there is a choice of 6 drive programs (from "balanced dynamic" to "very sporty"). Drive program 6 is only possible when DSC is switched off and it is a precondition for launch control (acceleration assistant).
As a general principle, the drive programs differ with regard to the preselected shifting time: the higher the drive program, engine speed and load, the shorter the shifting time.

Driving in gear lever position "D" (automated drive mode)
- Constant-speed driving
The DKG control unit registers continuous changes to the pedal sensor position (accelerate and decelerate). The DKG control unit the continuous change between neighbouring gears that this causes (back and forth, e.g. on a journey on the motorway with continuously changing speed).
- Suppressing upshifts
The DKG control unit uses the pedal sensor position to detect rapid closure of the throttle, e.g. before braking. The DKG control unit suppresses the upshift. This means that the engine braking effect is retained and it supports braking.
- Manual intervention in the automated drive mode "D"
If the gear lever is operated towards upshift or downshift or if the shift paddles are operated in the automated drive mode "D", there is an automatic switch into the sequential mode "S". The automated drive mode "D" must be reselected.
- Hill ascent
The DKG control unit uses the signals of the following components to detect a hill ascent:
- Longitudinal acceleration sensor
- Wheel-speed sensors
- Pedal sensor position
The DKG control unit prevents unwanted upshifts and disruptive pendulum shifts.
- Driving downhill
The DKG control unit uses the signals of the following components to detect a hill descent:
- Throttle potentiometers (load signal)
- Wheel-speed sensors (driving speed)
- Brake light switch.
If driving speed increases, the DKG control unit to the next-lowest gear. This means that the engine braking effect is retained or increased.
- Braking deceleration and automatic downshifting
The DKG control unit uses the signals of the following components to detect braking deceleration:
- Brake light switch
- Wheel-speed sensors
- Pedal sensor position
To maintain the engine braking effect, the DKG control unit shifts down according to the driving speed. Here, a number of gears can be skipped. For renewed acceleration, the right gear is selected automatically.

Launch control (motorsport engineering only)
Launch control enables maximum acceleration from a vehicle standstill on a par with motor racing standards. When launch control is active, the vehicle automatically shifts up even in sequential mode "S" (manual operation) as long as the accelerator pedal is in kick-down position. This prevents the engine from reaching critical speeds. After launch control, the vehicle must be moved over a certain distance before another launch control is permitted.

CAUTION: Launch control leads to increased wear of components.

When running in a new car, do not accelerate the car with launch control. Frequent use of launch control leads to increased wear of components.

Wheel-slip monitoring
While the clutch is being engaged or on load change, excessive wheel slip can arise (e.g. on wet or icy roads). To stabilize the vehicle, the clutch is disengaged again. This procedure takes place as follows:
- Opening of the corresponding clutch after detected wheel slip between the wheels on the same side on the rear axle and front axle. This counteracts vehicle swerving.
- Subsequently, gentle engagement of the clutch with continued analysis of the wheel slip.
- If required, periodical opening and closing of the clutch until no more wheel slip is detected with the clutch closed.

Driving off on inclines
To make it easier to drive off on inclines, the Dynamic Stability Control (DSC) is equipped with special software. (On an upward stretch of road, the vehicle would roll away backwards on releasing the brake.) Driving off on inclines is made easier by an automatic brake intervention. The function can be used both in the sequential mode "S" and in the automated drive mode "D" (driving forwards and reversing).
All that needs to be done is to press the brake pedal with the vehicle stationary. After releasing the brake, it is possible to drive off within approx. 0.8 seconds without rolling away in an uncontrolled manner.

Low Speed Assistant
When the Low Speed Assistant is activated, the vehicle moves at minimum speed. The engine speed is regulated automatically.
- To activate the Low Speed Assistant: Engage a drive position and briefly touch the accelerator pedal
- To deactivate the Low Speed Assistant: Brake the vehicle to a full stop.

Vehicle at a standstill
The DKG control unit detects vehicle standstill via the pedal sensor position, the wheel speeds as well as the engine speed. The clutches detach. There is an automatic shift to 1st gear (in the sequential mode "S" and in the automated drive mode "D").
If the driver's door is opened with the engine running, an acoustic signal draws attention to the position of the transmission. At the same time, a visual warning appears in the LC display of the instrument panel.

Automatic parking lock
The twin-clutch transmission is equipped with an automatic parking lock. The automatic parking lock is required, as with the engine OFF there is no power transmission between the engine and transmission or the drive wheels (reason: The hydraulic pump is driven mechanically and at static pressure the clutches do not adhere).
The automatic parking lock is always engaged automatically after engine OFF. Exception: gear lever position "N". In gear lever position "N", the automatic parking lock is engaged after engine OFF under the following conditions:
- Withdrawal of the remote key from the slot (vehicles without Comfort Access)
- No ID transmitter in the slot (vehicles with Comfort Access)
- 30 minutes after engine OFF
The Car Access System (CAS) uses terminal 30g to effect a time-controlled shutdown of the control unit and components with power supply as follows: Approximately 30 minutes after terminal R is switched OFF, the power supply for this component is switched off. This prevents the battery from being subjected to excess load by the electrical consumers for too long when the vehicle is out of use.

CAUTION: Stopping the engine in a car wash!

In a car wash, gear lever position "N" must be engaged actively before stopping the engine. The remote key must remain in the slot. If the vehicle is equipped with the optional equipment "Comfort Access", the ID transmitter must remain in the slot or be inserted in the slot.

Limp-home program
In the emergency program, the vehicle can still be driven with certain restrictions. If the power supply fails, the currently engaged gear remains active and the corresponding clutch is closed. The clutch only opens when a certain minimum speed of the transmission input shaft is not reached.
Depending on the type of transmission fault, the vehicle can no longer be driven after coming to a standstill.

Notes for Service department

General notes

Emergency release of the automatic parking lock
If the parking lock can no longer be released, mechanical emergency release is possible. This allows the vehicle to be pushed or pulled if necessary. The procedure for towing is described in the Owner's Handbook. Releasing the automatic parking lock using the emergency release process can lead to a fault code memory entry in the DKG control unit. The procedure for releasing the automatic parking lock is described in the Owner's Handbook.

NOTE: Engine start by tow-starting is not possible.

Tow-starting the vehicle is not possible. With the engine off, no hydraulic pressure is built up in the transmission (clutches not adherent).
No liability can be accepted for printing or other faults. Subject to changes of a technical nature