PRE-SAFE rear, function - GF30.30-P-0015FLM

MODEL 212 as of model year 2014 with CODE 253 (Rear impact warning and protection system) 

Fig 1: Rear PRE-SAFE Function Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• No undervoltage or overvoltage
• Engine running or drivetrain operational
• No trailer recognized (with code (550) Trailer hitch)
• Rear end PRE-SAFE function in "Assistance" of instrument cluster switched on

The IC sends the status of the rear end PRE-SAFE is sent via the chassis CAN 2, the chassis gateway control unit and chassis FlexRay to the radar sensors control unit. The CDI control unit or the ME-SFI [ME] control unit sends the "Engine running" or "Drivetrain operational" signal to the radar sensors control unit via the chassis CAN 1, the chassis gateway control unit and chassis FlexRay. The trailer recognition status is sent by the trailer recognition control unit (for code (550) Trailer hitch) via the interior CAN, front SAM control unit, chassis CAN 2, the chassis gateway control unit and chassis FlexRay to the radar sensors control unit.

Rear end PRE-SAFE, general 

The rear end PRE-SAFE function extends the preventative occupant protection system of the vehicle in the event of an imminent rear-end collision. To do this, the traffic area in the rear of the systems vehicle is monitored using the center rear bumper radar sensor. The radar sensors control unit uses the information of the center rear bumper radar sensor to analyze the closing speed and the distance from the vehicle in the rear. Based on this information, various measures are initiated that are designed to lessen the possible consequences of an accident for the passengers.

These are shown chronologically in the following graphic according to the respective escalation stages:

Fig 2: Overview Of Standing Vehicle For Collision

Courtesy of MERCEDES-BENZ USA

Function sequence for the rear end PRE-SAFE 

The rear end PRE-SAFE function consists of the following subfunctions:

• Function sequence for warning of the rearward vehicle (escalation level t  ₁  ) 
• Function sequence for preparing the stationary vehicle for collision (escalation level t  ₂  ) 
• Function sequence for occupants protection measures are activated (escalation level t  ₃  ) 
• Function sequence for securing the vehicle after the collision (escalation level t  ₄  ) 

Function sequence for warning of the rearward vehicle (escalation level t  ₁  ) 

Approx. t = -1400 ms to impact:

If a vehicle is detected in the rearward traffic area by the center rear bumper radar sensor bumper and by the radar sensors control units as accident-critical, the driver of the rearward vehicle is first warned through actuation of the rear turn signal lights on the vehicle.

In vehicles with code (460) Canada version and code (494) USA version, there is no warning of the traffic in the rear by the rear end PRE-SAFE function.

Technical implementation 

To do this, the radar sensors control unit sends a request to issue a warning to the front SAM control unit via the chassis FlexRay, the chassis gateway control unit and chassis CAN 2. This unit evaluates the incoming request from the radar sensors control unit and assigns it a priority based on additional signals and sets the clock frequency of the turn signal lights. The front SAM control unit transmits the signaling request to the rear SAM control unit via the interior CAN. This unit actuates the left and right rear turn signal lights in the respective lamp unit with the clock frequency of f = 3 to 5 Hz set by the front SAM control unit.

A detailed description of this function can be found in the separate description "Turn signal light actuation function".

Additional function requirements for preparing the standing vehicle for collision (escalation level t  ₂  ) 

• Vehicle at rest
• Brake request recognized by:
  • Depressing the brake pedal or
  • Operation of parking brake or
  • Gear range "P" engaged
• Brake request sent by:
  • Hold function or
  • DISTRONIC PLUS

The status of the brake system is sent by the Electronic Stability Program control unit or by the RBS control unit to the radar sensors control unit via chassis CAN 1, the chassis gateway control unit and the chassis FlexRay.

The front SAM control unit detects the status of the parking brake indicator switch directly and sends this via the chassis CAN 1 the chassis gateway control unit and the chassis FlexRay to the radar sensors control unit.

The fully integrated transmission control unit transmits the signal concerning the engaged gear range to the radar sensors control unit via the drive train CAN, CDI control unit or ME-SFI [ME] control unit, chassis CAN 1, chassis gateway control unit and chassis FlexRay. Function sequence for preparing the stationary vehicle for collision (escalation level t  ₂  ) 

Impact in approx. t = 600 ms:

If the radar sensors control unit registers no or only inadequate deceleration of the approaching vehicle via the closing speed, the brake of the systems vehicle is "firmly applied" as a preventive measure by increasing the brake pressure. The purpose is to secure the vehicle to prevent it from accelerating in the forward direction as a result of a possible rear-end collision. The purpose of this measure is to prevent, among other things, that the vehicle is pushed onto a preceding vehicle or for example onto an intersection as a result of a possible rear-end collision. Additionally, these measures can prevent injuries in the area of the cervical vertebrae of the occupants, since the acceleration of the vehicle and thus that of the occupants is reduced.

Technical implementation 

To do this the radar sensors control unit transmits a brake request via the chassis FlexRay, chassis gateway control unit and chassis CAN 1 to the Electronic Stability Program control unit or by the RBS control unit which then actuates the traction system hydraulic unit correspondingly.

Function sequence for occupants protection measures are activated (escalation level t  ₃  ) 

Impact in approx. t = 100 - 50 ms:

If the radar sensors control unit calculates that a rear-end collision is probable using the closing speed of the vehicle in the rear, the front reversible emergency tensioning retractors and NECK-PRO are activated.

Technical implementation 

To do this the radar sensors control unit transmits the following information to the brake system control unit or the Electronic Stability Program control unit via the chassis FlexRay, chassis gateway control unit and chassis CAN 1:

• Closing speed of the rearward vehicle
• Time until the possible impact of the vehicle in the rear
• Deployment recommendation for the front reversible emergency tensioning retractors and NECK-PRO

The Electronic Stability Program control unit or the RBS control unit reacts by transmitting a appropriate signals via chassis CAN 1 to the front left reversible emergency tensioning retractors, the front right reversible emergency tensioning retractors and to the Supplemental Restraint System control unit.

The detailed descriptions for these functions are contained in the separate descriptions "PRE-SAFE function" (front activation reversible emergency tensioning retractor) and "Supplemental Restraint System (SRS) function" (triggering of NECK-PRO).

Function sequence for securing the vehicle after the collision (escalation level t  ₄  ) 

Approx. t = 2000 ms after impact:

After a rear-end collision, the brake pressure is maintained for approx. t = 2000 ms if the brake pressure had previously been increased (escalation level t ₂ ).

After a rear end collision has occurred and been detected, the rear-end PRE-SAFE function is no longer available until it is activated using the diagnostic tester.

Technical implementation 

The technical implementation of "Maintain brake pressure" corresponds to the technical implementation of the escalation level t ₂ .