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PRE-SAFE function - GF91.40-P-0001CE

MODEL 204.0/2 with CODE (299) PRE-SAFE as of model year 2009/YoM 08 

MODEL 204.3 with CODE (299) PRE-SAFE 

MODEL 204.9 with CODE (299) PRE-SAFE 

Fig 1: Pre-Safe Block Diagram
G10107492Courtesy of MERCEDES-BENZ USA

Function requirements, general 

IMPORTANT The status of circuit 15 is provided by the electronic ignition lock control unit over chassis CAN.

The "PT_Rdy" signal is sent by the CDI control unit (N3/9) (with diesel engine) or by the ME-SFI [ME] control unit (N3/10) (with gasoline engine) over the chassis CAN.

PRE-SAFE, general 

The PRE-SAFE system is activated during a potential accident. The goal is to create the most favorable conditions possible for the occupants during the expected accident by activating the functions before the actual impact.

The Electronic Stability Program control unit data is evaluated for triggering the PRE-SAFE functions.

The PRE-SAFE system consists of the following subfunctions:

Additional function requirement for seat belt slack reduction (as of 1.6.09) 

Function sequence for seat belt slack reduction (as of 1.6.09) 

The seat belt slack reduction function reduces the seat belt slack that exists after the seat belt is fastened.

The insertion of the seat belt tab into the belt buckle triggers the seat belt slack reduction function.

Insertion is detected via the driver seat belt buckle restraint system switch and front passenger seat belt buckle restraint system switch by the supplemental restraint system control unit.

The left front reversible emergency tensioning retractor and right front reversible emergency tensioning retractor now pretension the belt strap at a speed of v = 50 mm/s (± 10 %) until the seat belt slack is reduced. The belt force generated here increases at the end of the pretensioning process to F = 20 N (± 20 %). Once the belt force has been reached, the seat belt is pretensioned for another t = 400 ms.

Following expiration of this time, the seat belt slack reduction function is ended.

Function sequence for sensing the vehicle status 

The vehicle status is sensed by the electronic stability program control unit. For this purpose information is evaluated by the following components in addition to the internal data:

The signals from the steering angle sensor are read in by the steering column tube module control unit (N80) and made available via the chassis CAN to the Electronic Stability Program control unit.

The signals from the yaw rate sensor for lateral and longitudinal acceleration are read in by Electronic Stability Program control unit over the vehicle dynamics CAN.

The signals of the following control units are also read in over chassis CAN:

IMPORTANT The triggering threshold for the PRE-SAFE functions is increased when the ESP® is switched passive manually via a menu in the instrument cluster.

The electronic stability program control unit differentiates the following statuses:

The following measures can be initiated in the event of a recognized hazard situation to improve occupant protection:

Function sequence when the ESP® is switched off 

The triggering threshold for the PRE-SAFE functions is increased when the ESP is switched off manually using the ESP button (model 204.9, model 204.0/2 up to 31.5.10) in instrument panel switch group (S6/1).

The signals from the ESP button are read in via instrument panel LIN by the front SAM control unit. The front SAM control unit sends the signals over the chassis CAN to the Electronic Stability Program control unit.

In model 204.0/2/3, the previous functionalities of the ESP button can be selected via a menu in the instrument cluster as of 1.6.10.

Function sequence for reversible emergency tensioning retractors for driver and front passenger 

The left front reversible emergency tensioning retractor and the right front reversible emergency tensioning retractor reduce the existing seat belt slack in dangerous situations. This secures the occupants in the seats at an early stage.

The occupants are therefore held in a favorable seat position in the event of an accident, allowing the conventional restraint systems to have their optimum protective effect. In the event of a rollover the reversible emergency tensioning retractors are also triggered.

The reversible emergency tensioning retractors receive the signal for emergency tensioning retraction via the chassis CAN from the Electronic Stability Program control unit.

Depending on the situation the belt is tightened with varying force or speed.

At the end of the tensioning request, the belt is released again.

IMPORTANT Following a rollover the belt strap is no longer released.

The reversible emergency tensioning retractors are only activated when the seat belt latches are inserted into the seat belt buckles. This is detected through the seat belt buckle restraint system switch by the supplemental restraint system control unit.

The supplemental restraint system control unit sends the status of the seat belt buckle restraint system switch over chassis CAN to the reversible emergency tensioning retractors.

Function sequence for adjustment of front passenger seat (with code (242) Electrically adjustable front passenger seat with memory) 

Adjustment of the front passenger seat is controlled by the front passenger seat control unit.

Depending on the setting at any given time, the following adjustments can be made:

IMPORTANT The automatic adjustment of the front passenger seat can be interrupted at any given time by a manual adjust request.

If a PRE-SAFE situation continues to be present, the front passenger seat is adjusted again via PRE-SAFE after the manual adjustment has been completed.

The adjustment time takes until:

The front passenger seat does not automatically return into the position before the PRE-SAFE situation, but has to be manually adjusted.

IMPORTANT Seat adjustment if occupants are not buckled up

The system moves the seat into a position that is suitable for situations that are unfavorable with regard to safety technology.

The status of the front passenger seat belt buckle restraint system switch is sent by the supplemental restraint system control unit over the chassis CAN to the front SAM control unit and forwarded there over the interior CAN to the front passenger seat control unit.

IMPORTANT Detailed information on the front passenger seat adjustment is available in the separate function description for electrical seat adjustment (ESE).

Function sequence for closing the side windows 

In the event of a PRE-SAFE situation, the front SAM control unit sends the signal to close the side windows via the interior CAN to the following control units:

The side windows are closed until only a gap is left. This serves to ensure that in the event of the possible subsequent impact, smoke particles that are produced as a result of the pyrotechnical restraint equipment can be exhausted.

When the side windows are closed via PRE-SAFE, the obstruction sensor is active. If required, the closing process is reversed.

IMPORTANT The closing of each side window can be interrupted by a manual adjustment request at any given time.

If closing is activated via PRE-SAFE during an automatic lowering process, the automatic lowering process is aborted and closing is carried out via PRE-SAFE.

Following the PRE-SAFE situation, the windows must be manually adjusted into the desired position.

IMPORTANT Detailed information on controlling the side windows is available in the separate function description for the power windows (EFH).

Function sequence for automatic closing of tilting/sliding roof (SHD) (with code (414) Electric glass tilting/sliding roof) or automatic closing of panoramic sliding sunroof (with code (413) Panoramic glass sunroof with top sliding sunroof) 

Closing the roof via PRE-SAFE is intended to prevent objects from penetrating the vehicle interior and stop occupant body parts (such as arms) from protruding from the vehicle if an accident occurs.

The overhead control panel control unit (with code (414) Electric glass tilting/sliding roof) or panoramic sliding sunroof control unit (with code (413) Panoramic glass sunroof with top sliding sunroof) receives the request from the Electronic Stability Program control unit over the interior CAN from the front SAM control unit and then authorizes the roof to be closed.

The roof is closed leaving a gap of approx. s = 50 mm in order to ensure that, in the event of any subsequent impact the smoke particulates generated by the pyrotechnical restraint equipment are then extracted.

Closing can be aborted at any time by means of a manual adjustment request.

The anti-pinch protection is active when the roof is being closed. If required, the closing process is reversed.

If closing is activated via PRE-SAFE during a manual adjustment, the manual adjustment request is completed first and then closing via PRE-SAFE.

If closing is activated via PRE-SAFE during an automatic process, the automatic process is aborted and closing is carried out via PRE-SAFE.

Following the PRE-SAFE situation, the roof must be manually adjusted back into the desired position.

IMPORTANT Detailed information on controlling the roof is available in the separate function descriptions for the power tilting/sliding roof (SHD) or the electric panoramic sliding roof.

Function sequence for display of fault messages 

If a fault is recognized with reversible emergency tensioning retractors, this is shown in the travel distance display (A1p7) (model 204.0/2/9 (except 204.077/277/375) without code (442) Comfort multifunction steering wheel) or in the multifunction display (A1p13) (with code (442) Comfort multifunction steering wheel, model 204.077/277/375) of the instrument cluster. Faults of other components that are part of PRE-SAFE are not displayed as PRE-SAFE faults.

Function sequence for ending the PRE-SAFE functions 

PRE-SAFE is deactivated as soon as a PRE-SAFE longitudinal dynamics situation or PRE-SAFE lateral dynamics situation is no longer present.

The Electronic Stability Program control unit sends a signal for shutting off the PRE-SAFE functions over the chassis CAN.

The front SAM control unit sends the signal via interior CAN to the involved control units.

Adjustment of the front passenger seat is canceled insofar as the PRE-SAFE position has not been reached.

Closing of the side windows and the roof is not stopped. The automatic closing function continues until the PRE-SAFE position has been reached.

The reversible emergency tensioning retraction is released as soon as the vehicle acceleration has dropped below a value of a = 0.3 g.

IMPORTANT The reversible emergency tensioning retractors are equipped with a mechanical acceleration sensor, which locks the belt extension in the event of any vehicle acceleration of a > 0.3 g.

For this reason, the belt may remain mechanically locked even when the reversible emergency tensioning retraction has been completed.

If the supplemental restraint system control unit detects an impact, then all adjustments are immediately stopped.

REFER TO SYSTEM WIRING DIAGRAMS Electrical function schematic, pre-safe   PE91.40-P-2051-97FAB
  Overview of PRE-SAFE system components   GF91.40-P-9997CE 
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