Brief description of components

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The SMG system comprises the following components:

Gear lever with mounting block and gear lever cover
The gear lever allow gears to be changed in sequence (sequentially) by brief pressing.

The gear lever has 6 positions:
- 3 engaging positions for neutral ("N"), reverse gear ("R") and manual mode ("-" symbol between "N" and "D").
- 3 one-touch positions for "+", "-", "D".
The gear lever lock (shift-lock) is also installed in the mounting block.

The illuminated gear lever position indicator is located in the gear lever cover.
Steering wheel with 2 gearshift paddles
The steering wheel with 2 gearshift paddles allows manual gear changes without the driver having to take his hands from the steering wheel. Pull one or both of the gearshift paddles to shift up a gear. Press one or both of the gearshift paddles to shift down a gear.

The two gearshift paddles are identical in construction and are wired in parallel.

Power is supplied to the gearshift paddles from the steering column switch cluster (SZL).
SPORT button for Dynamic Driving Control
Dynamic Driving Control (FDC) is activated with the SPORT button: Gearshift times are shortened and the shift characteristics are made more sporty. The engine responds more spontaneously to movements of the accelerator pedal. The steering and holding forces of the steering tuned for a sportier feel - for Servotronic or active front steering (AFS). The AFS includes Servotronic.
Hydraulic unit with sensors
Clutch actions and gear changes are automatically executed by the hydraulic unit and the sensors.
- Gearshift and selector angle actuator
  The gearshift and selector angle actuator shifts the selector shaft along its longitudinal axis and turns the selector shaft to change shift gate.
- Shift travel sensor and selector angle sensor
  The gear engaged is recognized from the shift travel sensor and the selector angle sensor:
  
  The shift travel sensor detects the forwards and backwards movements of the selector shaft in the respective shift gate.
  
  The selector angle sensor detects how far the selector shaft turns, and so recognizes a change of shift gate.
- Pressure accumulator
  The pressure accumulator is fitted on the bottom left of the transmission and makes sure that there is always sufficient oil pressure for a gearshift to be executed.
  
  A freely moving piston subdivides the pressure accumulator into 2 chambers. One chamber is charged with nitrogen at approx. 27 bar and -25 °C (lifelong charge). The other chamber takes up oil from the hydraulic pump. Here, the oil exerts pressure on one side of the piston, compressing the nitrogen. The compressed oxygen maintains the oil pressure during shifting.
- Hydraulic pressure sensor
  The hydraulic pressure sensor is installed directly on the hydraulic unit valve block and reports the current oil pressure to the SMG control unit. The hydraulic pump is switched on and off as required.
- Clutch actuator with clutch travel sensor
  The clutch actuator executes the clutch operations. The clutch travel sensor registers the clutch release travel.
- Hydraulic pump with electric motor
  The hydraulic pump with electric motor builds up to 55 bar system pressure for the gearshifts. The hydraulic pump is actuated by the SMG control unit.
- Inductive sensor for transmission input speed
  The inductive sensor measures the transmission input speed. The SMG control unit compares this speed with the engine speed and computes the closing speed of the clutch (i.e. how fast the clutch can establish the optimal power flow between the engine and the transmission).
- Expansion tank
  The expansion tank makes sure that there is always sufficient hydraulic fluid available in the system.
  
  There is a maximum and a minimum mark on the expansion tank for adjusting the oil level.
- Valve block
  The valve block contains the solenoid valves, the hydraulic pressure sensor, the pressure accumulator and a check valve.
- Solenoid valves
  A total of 4 solenoid valves are installed in the valve block:
  - 1 solenoid valve for controlling the clutch actuator
  - 1 solenoid valve for shifting to 1st, 3rd, 5th gear and reverse (shift travel valve)
  - 1 solenoid valve for shifting to 2nd, 4th, and 6th gear
  - 1 solenoid valve for controlling the selector angle brake (for switching the shift gate). At the same time, the solenoid valve serves as a pressure relief valve for the hydraulic system.
SMG control unit
The SMG control unit (GS35) computes the actuation of the solenoid valves for clutch operation and gear change. Moreover, the SMG control unit sends information to various control units, e.g. to the engine speed adjustment and drive program indicator. (The SMG control unit can be programmed and encoded.)
Reversing light
The reverse light is switched on the basis of messages from the SMG control unit to the light module (LM).

Signal path:

Signals from the shift path sensor and from the selector angle sensor -> SMG control unit -> PT-CAN -> Safety and gateway module (SGM) before 09/2005 or body-gateway module (KGM) as from 09/2005 -> K-CAN -> LM
Transmission
The transmission housings of the 6-speed manual transmission GS6 -37BZ and GS6-53BZ have been modified to accommodate the following components:
- The hydraulic unit with the actuators for engaging and disengaging the clutch and for switching gears
- The additional sensors (shift travel sensor, selector angle sensor, inductive sensor for the transmission input speed)
The construction of the transmission and the clutch remain unchanged.
Instrument cluster display
The following information is displayed on the LC display (liquid crystal display) in the instrument cluster:
- Selected drive position
- Drive program
- Check control messages
- Operation in emergency program.
Additional measured variables from the following components:
Brake light switch
The brake light switch signal is transmitted on the PT-CAN (powertrain controller area network) by the DME control unit. For reasons of safety, the signal is also transmitted by a direct wire from the brake light switch to the SMG control unit.

The brake-light switch signal is used for the following operations:
- To release the shift lever lock (shift-lock)
- To detect a brake application
- To shift to "N" when the engine is shut-down
Starter inhibitor
The Car Access System (CAS) enables the starter only if a release signal is received from the SMG control unit. The following conditions must be satisfied for the start to be enabled:
- The gear lever must be in position "N".
  The LC display in the instrument cluster must show "N", indicating that the transmission is also in "N".
- The SMG control unit enables the start, the clutch is opened.