Automatically Shifted Manual Transmission/Transaxle: Description and Operation
23 03 03 (044) Sequential manual gearbox E60/E61/E63/E64
Introduction
The sequential manual transmission (SMG) has its origins in motor racing and has in time been modified for use in series production.
> - E60, E61, E63, E64 before 09/05
> - E60, E61, E63, E64 from 09/05
In detail, SMG means that in contrast to a conventional manual transmission, the clutch control and gear change are automated. An electrohydraulic control system takes over the clutch operation and gearshifts.
The SMG can be driven in manual mode or in automatic mode, as follows:
- Manual mode: sequential, manual gearshifts
Gears are selected in sequence (sequentially) by briefly pressing "+ or "-". Gears can be skipped by rapidly pressing the button repeatedly.
Gears are changed by one of the following means:
- Gear lever in the center console, or
- 2 gearshift paddles on the steering wheel.
The shift pulse is transmitted electrically to the SMG control unit. With this gearshift method ("shift-by-wire") there is no mechanical linkage between the gear lever or gearshift paddles and the transmission.
- Automatic mode = D:
In automatic mode, the shift points are automatically controlled according to characteristic curves and are adapted to the driving situation.
The SMG is available as special equipment (option 206).
So as to be compatible with higher-torque engines, the SMG are designed with different power ratings. The SMGs therefore differ, for example, in terms of more powerful clutch actuators.
The following SMGs are installed depending on the motorization:
- GS6S37BZ (designed for a maximum torque of 370 Nm)
- GS6S53BZ (designed for a maximum torque of 600 Nm)
The SMGs are based on the 6-speed manual transmissions GS6-53BZ and GS6-37BZ.
The principal benefits of the SMG compared to a conventional manual transmission:
- No clutch pedal as the clutch is operated automatically
- Selection of 4 operating modes:
- Manual mode
Sequential, manual gearshift via gear lever or gearshift paddles
- Manual mode with Dynamic Driving Control (FDC) via SPORT button
Sporty driving style by power steering being adapted (only with Servotronic) and intervention in engine and transmission control
- Automatic mode = D ("Drive")
- Automatic mode with Dynamic Driving Control (FDC) via SPORT button
Sporty driving style by power steering being adapted (only with Servotronic) and intervention in engine and transmission control
- Short and precise gearshifts, no incorrect gearshifts possible
(gear change in approx. 0.15 seconds)
- Safety and comfort: because the gearshift paddles are on the steering wheel, you can keep your hands on the steering wheel when you change gear.
Brief description of components
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 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 09/05 or body-gateway module (KGM) as from 09/05 -> 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.
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
Important: Launch control can lead to increased wear of components.
During the running-in phase of a new vehicle, do not accelerate the vehicle using launch control. Frequent use of launch control leads to increased component wear.
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.
Notes for service staff
Service staff should note the following points:
- General information:
- Diagnosis: ---
- Encoding/programming: ---
Subject to change.