PRE-SAFE (R), Function - GF91.40-P-0001FR

Model 257, 290 up to Model year 2021

Courtesy of MERCEDES-BENZ USA

Block diagram

Function requirements, general

Circuit 15 ON
No overvoltage or undervoltage

The electronic ignition lock control unit sends the status of circuit 15 over the suspension FlexRay to the Electronic Stability Program control unit.

PRE-SAFE®, general

The preventive occupant protection system PRE-SAFE® detects a potential risk of accident in advance on the basis of critical driving situations. If necessary, functions can be activated as a precaution that reduce the risk of injury to the vehicle occupants. To do this, PRE-SAFE® permanently evaluates sensor data from systems such as the Electronic Stability Program (ESP®) and the driver assistance systems and can therefore, for example, detect excessive understeer or oversteer and emergency braking. If a critical driving situation is recognized, corresponding measures can be started within fractions of a second. These measures allow the restraint systems to provide their full protective effect in the event of a collision.

The PRE-SAFE® function is controlled by the Electronic Stability Program control unit.

The PRE-SAFE function® includes the following subfunctions:

Function sequence for sensing the vehicle condition
Function sequence for reversible tensioning retraction
Function sequence for seat belt adaptation
Function sequence for adjusting seat contour (with code 432 (Left and right dynamic multicontour seat)))
Function sequence for positioning front passenger seat (with code 275 (Memory Package) or with code 278 (Fully electric front seats))
Function sequence for closing sliding roof (with code 414 (Sliding sunroof))
Function sequence for closing side windows
Function sequence for PRE-SAFE® sound
Function sequence for lateral PRE-SAFE® pulse (with code 292 (Lateral PRE-SAFE® pulse))
Function sequence for display of fault messages
Function sequence for terminating PRE-SAFE® functions

Function sequence for sensing the vehicle condition

The current vehicle status is defined by the Electronic Stability Program control unit by evaluating the following variables:

Wheel speed
Natural dynamics of the vehicle (yaw speed, lateral acceleration and longitudinal acceleration)
Steering angle

For vehicles with code 23P (Driving Assistance Package) or code P20 (Driving Assistance Plus Package) or code 239 (Distance Pilot DISTRONIC) or code 258 (Active Brake Assist), the Electronic Stability Program control unit also evaluates active assistance system interventions and collision-critical warnings.

The Mercedes-Benz Intelligent Drive control unit (with code 23P (Driving Assistance Package) or code P20 (Driving Assistance Package)) or the Distance Pilot DISTRONIC controller unit (with code 239 (Distance Pilot DISTRONIC)) sends the corresponding signals over the suspension FlexRay to the Electronic Stability Program control unit.

The Active Brake Assist controller unit (with code 258 (Active Brake Assist)) transmits the appropriate signals to the Electronic Stability Program control unit via the peripherals CAN, electronic ignition lock control unit and suspension FlexRay.

The PRE-SAFE® system is triggered for the following scenarios:

Emergency braking: Fast brake pedal operation, which indicates a startled response by the driver.
Maximum full-stop braking: the driver's deceleration request is significantly higher than the physically possible vehicle deceleration (e.g. aquaplaning, ice or snow). Strong interventions by the anti-lock braking system (ABS)
Strong oversteer: Swerving of vehicle rear area combined with strong ESP® stabilization interventions
Strong understeer: Vehicle pushes for a certain time over the front wheels. Strong stabilization interventions by the ESP®
Crosswind stabilization: Drifting from the specified lane, connected with strong ESP® stabilization interventions
Critical steering movement: rapid steering movements at fast speed that indicate a startled response from the driver and imply that the driver is unable to cope with this situation.
Secondary accident: Occurrence of another accident after an initial accident

Also for vehicles with code 23P (Driving Assistance Package) or code P20 (Driving Assistance Package Plus) or code 239 (Distance Pilot DISTRONIC) or code 258 (Active Brake Assist):

Powerful brake assistance through BAS PLUS: Powerful support of the brake application initiated by the driver upon detection of risk of collision in linear traffic based on radar-assisted evaluation of the surroundings
Autonomous brake application by the PRE-SAFE® brake: Vehicle deceleration with detected risk of collision in linear traffic by the PRE-SAFE® brake based on radar-assisted evaluation of the surroundings
Autonomous course correction by the Active Blind Spot Assist: Significant, course-correcting brake application in the event of a registered danger of a side collision with vehicles in the blind spot based on radar-assisted evaluation of the surroundings
Collision object detection at front/rear/side: detection of an imminent collision based on radar-assisted evaluation of the surroundings

The deployment stage for the PRE-SAFE® functions is modified depending on the following variables:

ESP® mode
Transmission mode

The head unit transmits the request for the selected ESP® mode and the selected transmission mode via the user interface CAN, the electronic ignition lock control unit and the chassis FlexRay to the Electronic Stability Program control unit.

Additional function requirements, reversible emergency tensioning retraction

Front seat belt applied

Function sequence for reversible tensioning retraction

The reversible emergency tensioning retractors reduce any seat belt slack in hazardous situations by tensioning the front seat belts using an electric motor. The driver and the front passenger are held in position better in their seats in order to optimize the protective effect of the conventional restraint equipment. Depending on the situation, the seat belt is tightened with varying force and speed.

The Electronic Stability Program control unit transmits information about the PRE-SAFE® status and the PRE-SAFE® stage via the chassis FlexRay, the electronic ignition lock control unit and the user interface CAN to the reversible emergency tensioning retractors. To tighten the seat belt, the respective reversible belt tensioner actuates an integrated actuator motor.

The reversible emergency tensioning retraction is completed with the end of the PRE-SAFE® situation.

Abort conditions:

Switching off circuit 15
Opening the respective seat belt buckle
Occurrence of a crash event

Once the PRE-SAFE® measure is complete or canceled, the seat belt is slackened again (unless a crash has occurred). The Supplemental Restraint System control unit transmits the crash signal via the user interface CAN to the reversible emergency tensioning retractors.

Additional function requirements for seat belt adjustment

Function activated in head unit
PRE-SAFE® has not been activated
"Engine running" or "Drivetrain operational" signal set

The head unit transmits the status of the seat belt adaptation function via the user interface CAN to the reversible emergency tensioning retractor.

The CDI control unit (diesel engine) or the ME-SFI [ME] control unit (gasoline engine) transmits the "Engine running" signal or the "Drivetrain operational" signal via the drive train CAN, the powertrain control unit, the chassis FlexRay, the electronic ignition lock control unit and the user interface CAN to the reversible emergency tensioning retractor.

Function sequence for seat belt adaptation

The seat belt adaptation function reduces the amount of seat belt slack by tensioning the seat belts of the driver and front passenger seats using an electric motor. The function is triggered when the seat belt is fastened. Insertion of the belt lock tongue of the seat belt into the seat belt buckle is recorded through the status of the respective seat belt buckle switch.

The Supplemental Restraint System control unit reads in the status of the driver seat belt buckle switch and the front passenger seat belt buckle switch directly and transmits this via the user interface CAN to the reversible emergency tensioning retractors. In order to tighten the seat belt, the respective reversible emergency tensioning retractor actuates an integrated actuator motor. The seat belt is thereby tightened at a constant speed until the seat belt slack has been reduced.

Additional function requirements for adjusting the seat contour (with code 432 (Left and right dynamic multicontour seat))

Respective seat belt fastened
Pressure level of side inserts and lumbar support bladders not equal to PRE-SAFE® specified pressure

The Supplemental Restraint System control unit reads in the status of the driver seat belt buckle switch and the front passenger seat belt buckle switch directly and transmits this via the user interface CAN, the electronic ignition lock control unit and the interior CAN to the left front multicontour seat control unit or the right front multicontour seat control unit.

Function sequence for adjusting the seat contour (with code 432 (Left and right dynamic multicontour seat))

In order to optimize the protective effect of the conventional restraint equipment, the seat contour is adjusted before a potential accident using PRE-SAFE®. The Electronic Stability Program control unit transmits information about the PRE-SAFE ® status and the request for PRE-SAFE® adjustment via the chassis FlexRay, the electronic ignition lock control unit and the interior CAN to the multicontour seat control units. An active setting, if given, is interrupted.

The following settings are made:

Inflating the side inserts (vehicle occupants are held in a central position in the seat)
Deflating the lumbar support bladders (vehicle occupants sit lower in the seat)

PRE-SAFE® adjustment is terminated by reaching the PRE-SAFE® specified pressure in the side inserts and lumbar support bladders or when the PRE-SAFE® situation has ended.

Abort conditions:

Switching off circuit 15
Opening the respective seat belt buckle
Manual adjustment request
Memory function adjustment request

After completion or cancellation of the PRE-SAFE® measures, the inflation status of the side inserts and the lumbar support bladders is returned to the respective initial status.

Additional function requirements for positioning front passenger seat (with code 275 (Memory Package) or with code 278 (Fully electric front seats))

Front passenger seat occupied
Seat adjustment normalized
Front passenger seat outside PRE-SAFE® target position

The Supplemental Restraint System control unit transmits the status of the front passenger seat occupied recognition via the user interface CAN, the electronic ignition lock control unit and the interior CAN to the front passenger seat control unit.

Function sequence for positioning front passenger seat (with code 275 (Memory Package) or with code 278 (Fully electric front seats))

In order to optimize the protective effect of the conventional restraint equipment, the front passenger seat is moved into a position that is favorable from a safety point of view before a potential accident. The Electronic Stability Program control unit sends information about the PRE-SAFE® status and the request for PRE-SAFE ® adjustment over the suspension FlexRay, electronic ignition lock control unit and the interior CAN to the front passenger seat control unit. An active setting, if given, is interrupted.

Depending on the current seat position, the following adjustments are made:

Backrest inclination
Seat height
Seat fore/aft position
Seat inclination

The PRE-SAFE® adjustment is completed when the PRE-SAFE® target position of the front passenger seat is reached or at the end of the PRE-SAFE® situation.

Abort conditions:

Switching off circuit 15
Opening or inserting the seat belt buckle
Occurrence of a crash event
Manual adjustment request

After completion or cancellation of PRE-SAFE® measures, the front passenger seat does not automatically return into the default position, but has to be manually adjusted if necessary.

Seat adjustment if the front passenger is not wearing his/her seat belt. The system moves the seat into a position that is adapted to the unsafe conditions. The supplemental restraint system control system reads in the status of the front passenger seat belt buckle switch directly and transmits this via the user interface CAN, the electronic ignition lock control unit and the interior CAN to the front passenger seat control unit.

Additional function requirements for closing sliding roof (with code 414 (Sliding sunroof))

Sliding roof NORMALIZED
Sliding roof opened further than PRE-SAFE® target position

Function sequence for closing sliding roof (with code 414 (Sliding sunroof))

Closing the sliding roof ensures that no objects can penetrate into the vehicle interior and no body parts (e.g. arms) of the vehicle occupants can protrude from the vehicle in the event of a potential accident (rollover). The sliding roof is closed completely by PRE-SAFE® if it was already lifted and open before the PRE-SAFE® measure. If the sliding roof was open in the sliding direction before the PRE-SAFE® measure occurred, it is closed apart from a residual gap.

The Electronic Stability Program control unit sends information about the PRE-SAFE® status and the request for PRE-SAFE® adjustment via the chassis FlexRay, electronic ignition lock control unit and interior CAN to the sliding roof control module. Any automatic movement or manual setting that is active is aborted. The automatic reversing function is active when the sliding roof is being closed. If required, the closing process is reversed.

The PRE-SAFE® adjustment is completed by reaching the PRE-SAFE® target position of the sliding roof.

Abort conditions:

Switching off circuit 15
Manual adjustment request

After completion or cancellation of PRE-SAFE® measure, the sliding roof does not automatically return into the default position, but has to be manually adjusted if necessary.

Additional function requirements for closing the side windows

Power windows normalized
Side windows opened further than PRE-SAFE® target position

Function sequence for closing side windows

Closing the side windows apart from a small gap is intended to ensure that no objects can penetrate into the vehicle interior and no body parts (e.g. arms) of the vehicle occupants can protrude from the vehicle in the event of a potential accident (rollover). This also allows the smoke particles generated by the pyrotechnical restraint equipment to disperse quickly.

Left front door control unit
Right front door control unit
Left rear door control unit
Right rear door control unit

Active window lowering (automatic) or a manual setting is aborted. The automatic reversing function is active when closing the side windows using PRE-SAFE®. If required, the closing process is reversed.

The PRE-SAFE® adjustment is terminated when the PRE-SAFE® target position of the side windows is reached.

Abort conditions:

Switching off circuit 15
Manual adjustment request

After completion or cancellation of the PRE-SAFE® measures, the side windows do not automatically return into the default position, but have to be manually adjusted if necessary.

Additional function requirements for PRE-SAFE® sound

Audio system switched on

Function sequence for PRE-SAFE® sound

In order to reduce the risk of hearing damage to the vehicle occupants caused by the high sound pressure level in the event of a potential accident, the hearing is pre-conditioned by noise generation (pink noise) in the vehicle interior.

The Electronic Stability Program control unit transmits information about the PRE-SAFE® status and the request for triggering the PRE-SAFE® sound function via the chassis FlexRay, the electronic ignition lock control unit and the user interface CAN to the head unit. The head unit implements this request accordingly.

The noise generation stops after a predefined period of time or when the PRE-SAFE ® situation has ended.

Abort conditions:

Switching off circuit 15
Switching off the audio system

Function sequence for lateral PRE-SAFE® pulse (with code 292 (Lateral PRE-SAFE® pulse))

The PRE-SAFE® lateral pulse function is part of the Driving Assistance Package (with code P20 (Driving Assistance Plus Package)) and is triggered before an unavoidable side impact. The PRE-SAFE® lateral pulse function causes a pulse towards the vehicle center to be generated to the vehicle occupant at the impact side (driver or front passenger) by an air cushion in the outer seat side bolster of the backrest. When this takes place, the distance from the door is increased and the consequences of an accident are reduced. At the same time, the reversible emergency tensioning retractor is activated for the vehicle occupant at the opposite side.

If the reversible emergency tensioning retractor was already activated for the vehicle occupant at the impact side (by another PRE-SAFE® scenario), it is deactivated just before the lateral PRE-SAFE® pulse function is activated.

The outer front bumper radar sensors detect objects that are on a collision course with the vehicle and may cause a side impact. If the calculated time to the impact is less than a value of approx. 600 ms, the relevant outer front bumper radar sensor sends information about the detected collision object over the front radar CAN, Intelligent Drive control unit, suspension FlexRay, electronic ignition lock control unit and the user interface CAN to the Supplemental Restraint System control unit, which evaluates and verifies this information. If the speed differential between the collision object and the vehicle is greater than a limit of approx. 20 km/h and the time until the impact drops to a value of approx. 200 ms at the same time, the Supplemental Restraint System control unit actuates the relevant PRE-SAFE® pulse squib directly.

The triggering of the respective PRE-SAFE® pulse squib, the relevant air cushion is abruptly inflated by a gas generator. This causes a pulse in the direction of the vehicle center to be transmitted to the vehicle occupant.

Function sequence for display of fault messages

If the reversible emergency tensioning retractors detect faults, appropriate messages are displayed in the display of the instrument cluster. Faults in other components that are involved in PRE-SAFE® are not displayed as PRE-SAFE® faults.

The reversible emergency tensioning retractors send the request to display a fault message over the user interface CAN to the instrument cluster.

Function sequence for terminating PRE-SAFE® functions

The PRE-SAFE® function is deactivated as soon as the current situation is deemed to be no longer critical. For this purpose, the Electronic Stability Program control unit withdraws the request to trigger the PRE-SAFE® measures from the chassis FlexRay.

The reversible emergency tensioning retractors are equipped with a mechanical acceleration sensor, which locks the belt extension in the event of any vehicle deceleration of more than 0.3 g. For this reason, the belt may remain mechanically locked even when the reversible emergency tensioning retraction has been completed.

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