System functions

The SMG comprises the following functions:

• Engine start
• Gearshift
• Driving in gear lever position "D" (automatic mode)
• Launch control (acceleration assist system)
• Rock free, e.g. from snow
• Vehicle standstill
• Gear lever position neutral and ignition off
• Emergency program

Engine start 

To start the engine, the gear lever and transmission must both be in "N" (neutral).

When the engine is started, sufficient system pressure must immediately be built up to open the clutch. For this reason, the SMG control unit is activated by a wake-up signal from the car access system (CAS) when the vehicle is unlocked. The SMG control unit measures the system pressure via the hydraulic pressure sensor. If the system pressure is too low, the hydraulic pump will be actuated. The hydraulic pump will build up pressure. When the system pressure has been reached, the SMG control unit will switch the hydraulic pump off again.

With the "prompt signal", the gear lever position indicator in the center console is activated. If terminal 15 (wake-up wire) has not been activated approx. 15 minutes after the vehicle was unlocked, the gear lever position display will be switched off again.

Gearshift 

If a gearshift is initiated in manual or automatic mode, the SMG control unit will actuate the corresponding solenoid valves in the valve block.

The pressurized hydraulic fluid flows into the clutch actuator and opens/closes the clutch.

At the same time, the SMG control unit sends information to the DME control unit that a gearshift has been initiated and that engine torque must be reduced.

In the next step, the gearshift and selector angle actuator is actuated. At the same time, the gearshift and selector angle actuator executes the forward/backward movement of the selector shaft, and if necessary turns the selector shaft.

During the gearshift, the engine speed may be raised as required to allow the clutch engagement to be completed more quickly and more smoothly.

When the gear is engaged, the clutch will automatically close.

The SMG control unit receives the precise clutch position from the clutch travel sensor.

After the gear change, the solenoid valves are actuated without the use of current.

The course of the entire gearshift depends on the accelerator pedal position, road speed, engine speed and other factors (e.g. engine oil temperature).

Just before the vehicle reaches a standstill, the SMG will shift into 1st gear and open the clutch. The transmission of power between the engine and transmission is broken.

Gearshifts that would result in an excessively high or low engine speed are suppressed.

Driving in gear lever position "D" (automatic mode) 

• Constant-speed travel

  Continuous changes of the accelerator pedal position (opening and closing the throttle) are registered in the SMG control unit. The SMG control unit then suppresses a pendulum shift between adjacent gears (up and down, e.g. on a motorway with constantly changing speed).

• Upshift suppression

  Based on the accelerator pedal position, the SMG control unit recognizes when the throttle is closed rapidly, e.g. before applying the brake.

  The SMG control unit suppresses the upshift. This ensures that the engine braking effect is retained and supports the braking action.

• Driving uphill

  The SMG control unit recognizes when the vehicle is driving uphill from the data from the wheel speed sensors and the accelerator pedal position, and then prevents unnecessary upshifts and irritating pendulum shifts (e.g. at constantly high engine speed and foot taken off the accelerator pedal).

• Driving downhill

  The SMG control unit recognizes when the vehicle is driving uphill from the data from the throttle valve potentiometer (load signal), wheel speed sensors (road speed) and the brake light switch. To maintain and increase the engine's braking effect, the SMG control unit will shift down a gear as road speed increases.

• Braking deceleration and automatic downshift

  The SMG control unit recognizes braking deceleration from the brake light switch, the wheel speed sensors and the accelerator pedal position.

  The SMG control unit shifts down according to road speed to maintain the engine's braking effect. As this happens, several gears may be skipped. The correct gear is automatically selected when the vehicle accelerates again.

• Manual intervention in automatic operation

  When the gear lever or the gearshift paddles are operated in automatic mode, the system will automatically change to manual mode.

  Automatic mode must be selected again.

Launch control (acceleration assist system) 

Preconditions for launch control:

• High-grip road surface
• Dynamic Stability Control (DSC) deactivated (DSC button pressed for longer than 3 seconds)
• SPORT button pressed

The launch control enables optimum vehicle acceleration from standstill on a motor sports level. Upshifts are automatic even in manual mode when the launch control is active. This prevents the engine from reaching critical speeds.

Deactivate launch control:

• By the position of the accelerator pedal (releasing the kickdown position)

  or

• By a manual shift request

  or

• By switching off the Dynamic Driving Control (FDC) via the SPORT button

  or

• By activating the DSC

Rocking free, e.g. from snow 

To rock the vehicle free, e.g. from snow, the gear lever can be moved rapidly from 1st gear, via position "N" to position "R" and vice versa. The gearshift must be completed within the space of 0.8 seconds as the gear lever lock (shift-lock) will otherwise be activated.

Vehicle standstill 

The clutch is disengaged and 1st gear selected when the SMG control unit detects that the vehicle is stationary. Vehicle standstill is detected by means of the accelerator pedal position, the wheel speeds and the engine speed. The clutch id disengaged and 1st gear selected irrespective of the drive program (manual or automatic mode).

An audible warning is triggered immediately if the driver's door is opened while the engine is running. At the same time, the "N" symbol flashes nine times in the LC display in the instrument cluster. This draws attention to the fact that a drive stage is still engaged and the transmission is not in position "N" or "P".

If despite this, neither the pedals, the gearshift paddles nor the gear lever are moved, the gear will automatically be cancelled after approx. 2 seconds.

Gear lever position neutral and ignition off 

When the gear lever is in "Neutral" position and the ignition is switched off, the "N" symbol flashes in the LC display of the instrument cluster. In addition, the multi-audio system controller (M-ASK) will emit an acoustic warning.

Emergency program 

In the emergency program, the vehicle remains operational, albeit with limitations.

Only the gear lever positions "R", "N" and 1st to 3rd gears are available. Depending on the type of transmission fault, the vehicle may no longer be operational after a vehicle standstill.

Subject to change.