Supplemental restraint system (SRS), function - GF91.60-P-0002RT

MODEL 251.1 with CODE (494) USA version up to Model Year 8 

General information 

The supplemental restraint system (SRS) provides passive passenger protection. Depending on the strength of a detected impact in the event of an accident, the corresponding airbag units, front buckle emergency tensioning retractors and rear emergency tensioning retractors are deployed. An automatic emergency call is also generated when an airbag is deployed.

Function requirements 

• The supplemental restraint system (SRS) must be programmed to the vehicle (left or right-hand drive vehicle, equipment programming and configuration parameters)
• Circuit 15R On
• Circuit 30 ON

Function 

The supplemental restraint system (SRS) can record and differentiate between the following types of accident:

• Head-on collision
• Rear end collision
• Side-on collision
• Roll-over along the longitudinal vehicle axis

The restraint systems control unit (N2/7) has sensors integrated into it. This is why it must not be removed when the power supply is active, as otherwise restraint equipment may be triggered. All work performed on or with the restraint systems control unit may only be conducted when the battery (G1) has been disconnected and after a waiting time of approx. ≥ 60 s.

The following restraint equipment can be triggered depending on the type of accident involved:

• Driver airbag (two-stage)
• Front passenger airbag (two-stage)
• Driver/front passenger sidebag
• Right/left rear sidebag (with code (293) Left and right rear sidebag)
• Left/right windowbag
• Driver/front passenger buckle emergency tensioning retractor
• Driver/front passenger belt force limiter
• 2nd seat row emergency tensioning retractor, left/right
• Front NECK-PRO head restraints (with code (438) NECK-PRO)

Collision detection and ignition time determination 

The restraint systems control unit determines the level of acceleration using its own integrated sensors and the following external sensors:

• Driver-side frontal acceleration sensor (B48/1)
• Passenger-side frontal acceleration sensor (B48/2)
• Drive sidebag sensor (A53)
• front passenger sidebag sensor (A54)

Information from the following components is also evaluated:

• Driver door pressure sensor (B48/7)
• Front passenger door pressure sensor (B48/8)
• Rear door pressure sensor, driver-side (B48/9) (with code (293) Left and right rear sidebag)
• Rear door pressure sensor, passenger-side (B48/10) (with code (293) Left and right rear sidebag)
• Driver seat belt buckle and seat belt warning switch (S68/1)
• Front passenger seat belt buckle and seat belt warning switch (S68/2)
• WSS (Weight Sensing System) control unit (N2/13)

The WSS control unit determines whether the front passenger seat is occupied or not. If a weight is deemed to be low (child seat), then front passenger airbag activation is suppressed.

If a threshold is exceeded, the ignition stages for the corresponding restraint equipment are actuated. Actuation is for at least 2 ms. This serves to ensure that the corresponding restraint system is actuated reliably.

More detailed information is available in the following separate subfunctions:

• Head-on collision, function
• Rear end collision, function
• Side impact, function

Roll-over along the longitudinal vehicle axis 

A rollover is detected by the restraint systems control unit by means of a separate, integrated rollover sensor. In addition to this, the data of the frontal acceleration sensors and the sidebag sensors are also used for evaluation purposes.

The following restraint equipment is triggered accordingly:

• Window airbags
• Front buckle emergency tensioning retractors
• Emergency tensioning retractors in 2nd seat row
• Emergency tensioning retractors in 3rd seat row

The seat belts in the 2nd and 3rd seat rows are not checked. The emergency tensioning retractors for the 2nd and 3rd seat rows will also be triggered even if seat belts are not buckled.

System monitoring 

The system monitors itself permanently automatically and transmits errors immediately to the driver via the multifunction display (A1p13) in the instrument cluster (A1) The airbag indicator and warning lamp (A1e15) in the instrument cluster is also switched on. Faults are stored in the nonvolatile memory of the restraint systems control unit.

Airbag indicator and warning lamp 

The airbag indicator and warning lamp is irreversibly actuated by the restraint systems control unit when malfunctions occur in the following instances:

• Immediately after the entry of the 5th crash telegram.
• Through a malfunction detected in the restraint systems control unit, in the external ignition circuits or the remote sensors (e.g. frontal acceleration sensor)
• If the instrument cluster recognizes an interruption in the flow of data to the restraint systems control unit, the airbag indicator and warning lamp are automatically switched on by the instrument cluster and a fault stored in the memory. In this case a fault is not written to the restraint systems control unit.

The supplemental restraint system has a capacitor in the restraint systems control unit. After a battery disconnection, the system is provided with power for at least 300 ms. The peripheral acceleration sensors (e.g. frontal acceleration sensor) have power supplied to them for at least 30 ms.

Front passenger seat occupied recognition 

The seat occupied and automatic child seat recognition operates by means of weight sensing. The following sensors enable the weight applied to the front passenger seat to be determined:

• Left front WSS sensor (Weight Sensing System) (B48/11)
• Right front WSS sensor (Weight Sensing System) (B48/12)
• Left rear WSS sensor (Weight Sensing System) (B48/13)
• Right rear WSS (Weight Sensing System) sensor (B48/14)

The WSS control unit performs the weight classification process and transmits it via the Controller Area Network bus Class B (interior) (CAN-B) to the central gateway control unit (N93). This routes the data over the Controller Area Network bus Class C (engine compartment) (CAN-C) to the restraint systems control unit. It then decides on the basis of the weight class whether to suppress or to trigger the 1- or 2-stage front passenger airbag. Apart from this, a CAN-B signal with the weight class is sent to the instrument cluster to be displayed in the multifunction display.

If the system recognizes that the front passenger seat is not occupied (person/child seat), the automatic child seat recognition airbag OFF indicator lamp (N71/1e1) is switched on in addition to the multifunction display in the instrument cluster via the upper control panel control unit (N72/1) as long as the system recognizes that no seat is occupied, and the front passenger airbag is switched off by the restraint systems control unit.

When a low weight is recognized, it is considered to be a child seat. The automatic child seat recognition airbag off indicator lamp is switched on and the front passenger airbag is switched off.

If the airbag indicator and warning lamp and the automatic child seat recognition OFF indicator lamp illuminate at the same time, an error is present in the weight determination.

Seat belt warning light (A1e9) 

The seat belt buckle switch and seat belt reminder warning are read in by the restraint systems control unit and shown on the seat belt warning lamp display in the instrument cluster. The seat belt warning lamp is extinguished, when the monitoring conditions are fulfilled.

Special features in event of crash 

Crash telegrams are stored in a designated memory area in the restraint systems control unit when a crash threshold is exceeded. When a crash threshold is reached, it is notified to other control units (e.g. to switch off the engine).

A separate, digital crash output of the restraint systems control unit is used to send a message to the emergency call system control unit (N123/4) to initiate an automatic emergency call.

If the CAN-C malfunctions (e.g. short circuit in the event of a crash) and a crash signal is to be sent to the ME control unit (N3/10) or the rear SAM control unit (N10/8), the restraint systems control unit on vehicles as of 1.6.07 is also connected with these control units by way of a discrete line (redundant crash signal).

Hazard warning flashers 

The hazard warning system is automatically switched on in the event of a crash. The crash signal is generated by the restraint systems control unit and sent via CAN C over the central gateway control unit and made available to the rear SAM control unit, which acts as a turn-signal master. The hazard warning flashing can be switched off at any time via the hazard warning system switch (N72/1s5).

Automatic emergency opening of the central locking (CL) 

If the vehicle is subject to negative acceleration greater than 6 times the normal g force, the "Emergency opening" function is actuated by the restraint systems control unit.

The signal is sent over CAN-C to the central gateway control unit and forwarded from there over CAN-B to the following control units:

• Rear SAM control unit
• Front SAM control unit (N10)
• Left front door control unit (N69/1)
• Right front door control unit (N69/2)

More information on this is available in the separate function description for the central locking.

Crash ventilation 

After a crash, the restraint systems control unit may prompt the side windows and the tilting/sliding roof (with code (414) Power glass tilting/sliding sunroof) or the panoramic sliding roof (with code (413) Panoramic sliding roof) to open to a gap of approx. 50 mm to enable any smoke particles from the pyrotechnical restraint systems to be vented. The signal is sent over CAN-C to the central gateway control unit and forwarded from there over CAN-B to the following control units:

• Front SAM control unit
• Front door control unit
• Overhead control panel (N70)

More detailed information on this is available in the separate function descriptions for the power windows, tilting/sliding roof and panoramic sliding sunroof.

Foresighted accident protection (PRE-SAFE) (with code (299) PRE-SAFE) 

The supplemental restraint system is extended to the extent that any unintended movements of the driver or front passenger under vehicle braking and skidding are reduced and the driver and front passenger are moved to an ideal seating position prior to a possible accident.

	Supplemental restraint system (SRS), location of components		GF91.60-P-0002-01RT
	Supplemental restraint system (SRS), block diagram		GF91.60-P-0002-03RT
	Rear end collision, function		GF91.60-P-2004RT
	Side impact, function		GF91.60-P-2005RT
	Head-on collision, function		GF91.60-P-2006RT
	Foresighted occupant protection function	With code (299) PRE-SAFE and code (242) Electrically adjustable front passenger seat with memory	GF91.60-P-2010RT