Actuate supplemental restraint system, function

Function requirements, general

 "Circuit 15 ON"

 The circuit 15 status is sent by the electronic ignition lock control unit (EZS) (N73) over the chassis CAN (CAN E) to the supplemental restraint system (SRS) control unit (N2/18).

Actuate supplemental restraint system (SRS), general

The supplemental restraint system (SRS) continuously monitors the occupation status of the front passenger seat and the status of the front seat belts (buckled or not buckled). On vehicles with CODE SK5 (Automatic child seat recognition ACSR in the front passenger seat) the supplemental restraint system (SRS) control unit uses this information to adapt its triggering characteristic.

The SRS deployment function includes the following subfunctions:

 Function sequence system monitoring

 Function sequence for evaluating occupancy status of front passenger seat (with CODE SK5 (Automatic child seat recognition ACSR in front passenger seat))

 Function sequence for detecting status of seat belts

 Function sequence for impact detection

 Function sequence for frontal impact

 Function sequence in the event of a side impact

 Function sequence for rear end collision

 Function sequence for additional functions in the event of a crash

Function sequence system monitoring

When circuit 15 is switched on, the following indicator lamps are actuated:

 Supplemental restraint system indicator lamp (A1/1e15) in the instrument cluster (IC) control unit (A1/1)

 Automatic child seat recognition airbag OFF indicator lamp (N72/2e2) (with CODE SK5 (Automatic child seat recognition ACSR [AKSE] in front passenger seat)) in the upper control panel (UCP [OBF]) control unit (N72/2) (model 447.8)

 Automatic child seat recognition airbag OFF indicator lamp (N72/1e2) (with CODE SK5 (Automatic child seat recognition ACSR [AKSE] in front passenger seat)) in the upper control panel (UCP [OBF]) control unit (N72/1) (model 447.6/7)

 Automatic child seat recognition airbag ON indicator lamp (N72/2e1) (with CODE SK5 (Automatic child seat recognition ACSR [AKSE] in front passenger seat)) (model 447.8)

 Automatic child seat recognition airbag ON indicator lamp (N72/1e1) (with CODE SK5 (Automatic child seat recognition ACSR [AKSE] in front passenger seat)) (model 447.6/7)

Following a successful self-test (t = 4 s) the supplemental restraint system indicator lamp is then switched off.

On vehicles with CODE SK5 (Automatic child seat recognition ACSR in the front passenger seat) the corresponding occupancy status of the front passenger seat is displayed via the automatic child seat recognition airbag OFF indicator lamp.

Model 447.8, 448.8

The request to switch on the supplemental restraint system indicator lamp is sent by the supplemental restraint system (SRS) control unit over the chassis CAN, gateway for the user interface CAN (N93/4), and the user interface CAN (CAN HMI) to the instrument cluster (IC) control unit. The request to switch on the automatic child seat recognition airbag OFF indicator lamp is sent by the supplemental restraint system (SRS) control unit over the chassis CAN, gateway for the user interface CAN, the user interface CAN, radio (Audio/COMAND) (N87/6), telematics CAN (CAN A), central operating unit (COU) control unit (A40) and the telephone keypad-LIN (LIN A2) to the upper control panel (UCP) control unit, which then directly actuates the automatic child seat recognition airbag OFF indicator lamp.

Model 447.6/7, 448.7

The supplemental restraint system (SRS) control unit sends the request to switch on the supplemental restraint system indicator lamp via chassis CAN to the instrument cluster (IC [KI]) control unit. The supplemental restraint system (SRS) control unit sends the request to switch on the automatic child seat recognition airbag OFF indicator lamp via chassis CAN, electronic ignition lock (EZS) control unit, interior CAN (CAN B), signal acquisition and actuation module (SAM) control unit (N10/1) and battery sensor LIN (LIN B15) to the upper control panel (UCP [OBF]) control unit, which directly actuates the automatic child seat recognition airbag OFF indicator lamp.

When driving, the supplemental restraint system (SRS) control unit continuously monitors the SRS and notifies the driver of any faults through the multifunction display (A1/1p13) and the supplemental restraint system indicator lamp in the instrument cluster (IC) control unit.

Model 447.8, 448.8

The request to display a corresponding message is sent by the supplemental restraint system (SRS) control unit over the chassis CAN, gateway for the user interface CAN, and the user interface CAN to the instrument cluster (IC) control unit. The faults are stored in the nonvolatile memory of the supplemental restraint system (SRS) control unit.

Model 447.6/7, 448.7

The supplemental restraint system (SRS) control unit sends the request to display a corresponding message via chassis CAN to the instrument cluster (IC [KI]) control unit. The faults are stored in the nonvolatile memory of the supplemental restraint system (SRS) control unit.

Function sequence for evaluating occupancy status of front passenger seat (with CODE SK5 (Automatic child seat recognition ACSR in front passenger seat))

The deployment behavior of the restraint equipment at the passenger-side is adapted on the basis of the occupancy status of the front passenger seat.

 Detailed information about the seat occupied recognition can be found in the subfunction "Passenger-side seat occupied recognition, function".

Function sequence for detecting status of seat belts

The deployment behavior of the restraint equipment is adapted on basis of the status of the seat belts (worn or not being worn). The status of the respective seat belt is defined via the corresponding switch:

 Driver seat belt buckle restraint system switch (S68/1)

 Front passenger seat belt buckle restraint system switch (S68/2)

The status of the driver seat belt buckle restraint system switch and the status of the front passenger seat belt buckle restraint system switch is read in directly by the supplemental restraint system (SRS) control unit.

Function sequence for impact detection

The supplemental restraint system (SRS) control unit differentiates between two crash variants:

 Head-on collision

 Side-on collision

 Rear end collision

To do so, the signals of the following sensors are evaluated:

 Integrated sensors

 Left B-pillar body acceleration sensor (B164) (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Right B-pillar body acceleration sensor (B164/1) (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Right B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Left C-pillar body acceleration sensor (B164/2) (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear) and with CODE T19 (Left sliding door) or CODE T56 (Electrical actuation for left sliding door)) (model 447.6/7, 447.8)

 Right C-pillar body acceleration sensor (B164/3) (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Left sliding door pressure sensor (B165) (with CODE SH8 (Windowbag for driver, front passenger and rear) and with CODE T19 (Left sliding door) or CODE T56 (Electrical actuation for left sliding door))

 Right sliding door pressure sensor (B165/1) (with CODE SH8 (Windowbag for driver, front passenger and rear))

The sensor signals are read in directly by the supplemental restraint system (SRS) control unit.

Function sequence for frontal impact

The supplemental restraint system (SRS) control unit detects a frontal impact by evaluating the signals from the following sensors:

 Integrated sensors

 Left B-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Right B-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Right B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Left C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear) and with CODE T19 (Left sliding door) or CODE T56 (Electrical actuation for left sliding door)) (model 447.6/7, 448.7)

 Right C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear))

The deployment of the restraint equipment takes place depending on the severity of the impact, the occupancy of the front passenger seat and the status of the seat belts.

The following squibs can be ignited (at most):

 Driver emergency tensioning retractor squib (R12/1) (without CODE JP1 (PRE­SAFE®))

 Front passenger emergency tensioning retractor squib (R12/2) (without CODE JP1 (PRE­SAFE®))

 Driver airbag squib 1 (R12/7r1)

 Driver airbag squib 2 (R12/7r2)

 Front passenger airbag squib 1 (R12/8r1)

 Front passenger airbag squib 2 (R12/8r2)

 Left front windowbag squib (R12/14)

 Right front windowbag squib (R12/15)

 Left center windowbag squib (R12/18) (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Right center windowbag squib (R12/19) (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Left rear windowbag squib (R12/22) (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Right rear windowbag squib (R12/23) (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Driver thorax-pelvis sidebag squib (R12/30) (model 447.8, 448.8)

 Driver thorax-pelvis sidebag squib (with CODE SH1 (Driver thorax­pelvis sidebag)) (model 447.6/7, 448.7)

 Front passenger thorax-pelvis sidebag squib (R12/31) (model 447.8, 448.8)

 Thorax/pelvis sidebag squib, front passenger (with CODE SH2 (Thorax/pelvis sidebag, front passenger))

 Center front passenger emergency tensioning retractor squib (R12/34) (with CODE S23 (Two-seater front passenger seat)) (model 447.6/7, 448.7)

The squibs are actuated directly by the supplemental restraint system (SRS) control unit.

The following situations are taken into account:

 Driver seat position:

The supplemental restraint system (SRS) control unit uses the status of the driver seat belt buckle restraint system switch to decide when to trigger the driver airbag. The supplemental restraint system (SRS) control unit reads in the status of the driver seat belt buckle restraint system switch directly.

 Front passenger seat position:

The supplemental restraint system (SRS) control unit uses the status of the front passenger seat belt buckle restraint system switch to decide when to trigger the front passenger airbag. However, this only applies if an occupied front passenger seat has been detected. The supplemental restraint system (SRS) control unit reads in the status of the front passenger seat belt buckle restraint system switch directly.

 Occupancy status of the front passenger seat:

 If the front passenger seat is not occupied or if it is occupied with a child seat (with CODE SK5 (Automatic child seat recognition ACSR in the front passenger seat)) and the belt tab is not inserted, restraint equipment is not triggered on the passenger side.

 If the front passenger seat is not occupied or if it is occupied with a child seat (with CODE SK5 (Automatic child seat recognition ACSR in front passenger seat)) and the belt tab is inserted, the front passenger airbag is not triggered.

 If the front passenger seat is occupied by a person and the belt tab has not been inserted, the emergency tensioning retractor on the passenger side is not triggered.

 Driver seat belt buckle status:

If the belt tab is not inserted, the driver side emergency tensioning retractor is not triggered .

Function sequence in the event of a side impact

The supplemental restraint system (SRS) control unit detects a side impact by evaluating the signals from the following sensors:

 Integrated sensors

 Left B-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Right B-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Right B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Left C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear) and with CODE T19 (Left sliding door) or CODE T56 (Electrical actuation for left sliding door)) (model 447.6/7, 448.7)

 Right C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Left sliding door pressure sensor (with CODE SH8 (Windowbag for driver, front passenger and rear) and with CODE T19 (Left sliding door) or CODE T56 (Electrical actuation for left sliding door))

 Right sliding door pressure sensor (with CODE SH8 (Windowbag for driver, front passenger and rear))

The deployment of the restraint equipment takes place depending on the severity of the impact, the occupancy of the front passenger seat and the status of the seat belts.

The following squibs can be ignited (at most):

 Driver emergency tensioning retractor squib (without CODE JP1 (PRE­SAFE®))

 Front passenger emergency tensioning retractor squib (without CODE JP1 (PRE­SAFE®))

 Driver airbag ignition squib 1

 Driver airbag ignition squib 2

 Front passenger airbag squib 1

 Passenger airbag squib 2

 Left front windowbag squib

 Right front windowbag squib

 Left center windowbag squib (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Right center windowbag squib (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Left rear windowbag squib (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Right rear windowbag squib (with CODE SH8 (Windowbag for driver, front passenger and rear))

 Driver thorax-pelvis sidebag squib (model 447.8, 448.8)

 Driver thorax-pelvis sidebag squib (with CODE SH1 (Driver thorax­pelvis sidebag)) (model 447.6/7, 448.7)

 Front passenger thorax-pelvis sidebag squib (model 447.8, 448.8)

 Thorax/pelvis sidebag squib, front passenger (with CODE SH2 (Thorax/pelvis sidebag, front passenger))

 Center front passenger emergency tensioning retractor squib (with CODE S23 (Two-seater front passenger seat)) (model 447.6/7, 448.7)

The squibs are actuated directly by the supplemental restraint system (SRS) control unit.

The following situations are taken into account:

 Occupancy status of the front passenger seat:

 If the front passenger seat is not occupied and the belt tab has not been inserted, the emergency tensioning retractor and the sidebag on the passenger side is not triggered.

 If the front passenger seat is occupied (person or child seat) and the belt tab has not been inserted, the emergency tensioning retractor on the passenger side is not triggered.

 Driver seat belt buckle status:

If the belt tab is not inserted, the driver side emergency tensioning retractor is not triggered .

Function sequence for rear end collision

The supplemental restraint system (SRS) control unit detects a rear-end collision by evaluating the signals from the following sensors:

 Integrated sensors

 Left B-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Right B-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Right B-pillar body acceleration sensor (with CODE SH1 (Driver thorax-pelvis sidebag), with CODE SH2 (Front passenger thorax-pelvis sidebag), with CODE SH8 (Windowbag for driver, front passenger and rear), with CODE SH9 (Windowbag for driver and front passenger)) (model 447.6/7, 448.7)

 Left C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear)) (model 447.8, 448.8)

 Left C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear) and with CODE T19 (Left sliding door) or CODE T56 (Electrical actuation for left sliding door)) (model 447.6/7, 448.7)

 Right C-pillar body acceleration sensor (with CODE SH8 (Windowbag for driver, front passenger and rear))

The deployment of the restraint equipment takes place depending on the severity of the impact and the status of the seat belts.

The following squibs can be ignited (at most):

 Driver emergency tensioning retractor squib (without CODE JP1 (PRE­SAFE®))

 Front passenger emergency tensioning retractor squib (without CODE JP1 (PRE­SAFE®))



The squibs are actuated directly by the supplemental restraint system (SRS) control unit.

The following situations are taken into account:

 Driver seat belt buckle status:

If the belt tab is not inserted, the driver side emergency tensioning retractor is not triggered .

 Front passenger seat belt buckle status:

If the belt tab is not inserted, the passenger side emergency tensioning retractor is not triggered.

Function sequence for additional functions in the event of a crash

The additional functions in the event of a crash include the following subfunctions:

 Function sequence for activating the hazard warning flasher function

 Function sequence for switching on interior illumination (IBL)

 Function sequence for opening the central locking (CL [ZV])

 Function sequence for opening the side windows

 Function sequence for shutting off the engine

 Function sequence for triggering the pyrofuse A1 (F1fA1)

 Function sequence for switching off air conditioning system (with CODE HH4 (THERMOTRONIC automatic climate control) or with CODE HH9 (Semi-automatically controlled air conditioning, TEMPMATIC))

 Function sequence for switching off stationary heater (STH) (with CODE H12 (Auxiliary water heater))

Function sequence for activating the hazard warning flasher function

The hazard warning flasher function is activated automatically in order to warn other road users in the event of an accident. The supplemental restraint system (SRS) control unit sends the crash signal over the chassis CAN, electronic ignition lock control unit (EZS) and the interior CAN to the signal acquisition and actuation module (SAM) control unit. The signal acquisition and actuation module (SAM) control unit then activates the hazard warning flasher function.

 The crash signal is also sent over a direct line from the supplemental restraint system (SRS) control unit to the signal acquisition and actuation module (SAM) control unit.

 Detailed information about the hazard warning flasher function can be found in the separate function description "Exterior lights, function" in subfunction "Turn signal light actuation, function".

Function sequence for switching on interior illumination

In order to make it easier for the vehicle occupants or persons giving assistance to orientate themselves inside the vehicle in the event of an accident, the interior illumination (IBL) is automatically activated. The supplemental restraint system (SRS) control unit sends the crash signal over the chassis CAN, electronic ignition lock control unit (EZS) and the interior CAN to the signal acquisition and actuation module (SAM) control unit. The signal acquisition and actuation module (SAM) control unit then activates the interior illumination (IBL).

 The crash signal is also sent over a direct line from the supplemental restraint system (SRS) control unit to the signal acquisition and actuation module (SAM) control unit.

 Detailed information about interior illumination (IBL) is available in the separate function description "Interior illumination (IBL), function" in the subfunction "Automatic interior lights control, function".

Function sequence for opening the central locking (CL [ZV])

In order to make it possible for persons giving assistance to gain access to vehicle occupants who may be injured in the event of an accident, the central locking (CL [ZV]) is automatically unlocked. The supplemental restraint system (SRS) control unit sends the crash signal over the chassis CAN, electronic ignition lock control unit (EZS) and the interior CAN to the signal acquisition and actuation module (SAM) control unit, which then implements the request.

The central locking (CL [ZV]) is unlocked after a waiting time of t = 8 to 11 s. In the event of a rear end collision, the central locking (CL [ZV]) is immediately unlocked.

 The crash signal is also sent over a direct line from the supplemental restraint system (SRS) control unit to the signal acquisition and actuation module (SAM) control unit.

 Detailed information on the CL [ZV] is available in the subfunction "Central locking (CL [ZV]), function" of the separate function description "Theft protection and locking".

Function sequence for opening the side windows

Smoke particles are produced in the vehicle interior when the pyrotechnical restraint equipment is triggered. The side windows at the front are opened automatically leaving a gap in order to allow these smoke particles to quickly disperse. The supplemental restraint system (SRS) control unit sends the crash signal over the chassis CAN, electronic ignition lock control unit (EZS) and the interior CAN to the following control units:

 Left front door control unit (N69/3)

 Right front door control unit (N69/4)

 Detailed information on opening the side windows can be found in the separate function description "Windows and sightproofing function description" of the "Power windows (EFH), function" subfunction.

Function sequence for shutting off the engine

After an accident, the engine is automatically turned off to reduce the risk of fire. The supplemental restraint system (SRS) control unit sends the crash signal via chassis CAN to the CDI control unit (N3/33) (with engine 651.9) or to the ME-SFI [ME] control unit (N3/34) (with engine 274) or to the CDI control unit (N3/36) (with engine 622.9). This then causes the engine to be shut off.

 The crash signal is also sent over a direct line from the supplemental restraint system (SRS) control unit to the CDI control unit.

Function sequence for triggering the pyrofuseA1

The pyrofuse A1 is triggered to avoid any risk of fire following an accident.

P91.60-3611-71

The main line is designed inside the housing as a bus bar with predetermined fracture point. After a corresponding crash event occurs, the pyrofuse A1 is directly actuated by the supplemental restraint system (SRS) control unit. Actuation causes pressure to build up in the chamber. The piston then moves against the predetermined fracture point, thereby disconnecting the bus bar.

 Actuation of the pyrofuse A1 ensures that the alternator (G2) is disconnected quickly and reliably from the on-board electrical system.

Function sequence for switching off air conditioning system (with CODE HH4 (THERMOTRONIC automatic climate control) or with CODE HH9 (Semi-automatically controlled air conditioning, TEMPMATIC))

To prevent toxic gases from entering the vehicle interior after an accident, the automatic air conditioning (AAC [KLA]) control and operating unit (N154) (model 447.8, 448.8) or the air conditioning control and operating unit (KLA-V& KLA-H) (S98) (model 447.6/7, 448.8) switch off the air conditioning and close the air recirculation flap. The supplemental restraint system (SRS) control unit sends the crash signal via chassis CAN, electronic ignition lock (EZS) control unit and interior CAN to the automatic air conditioning (AAC [KLA]) control and operating unit (model 447.8, 448.8) or to the air conditioning control and operating unit (KLA-V&KLA-H) (model 447.6/7, 448.7).

 Detailed information about switching off the air conditioning can be found in the separate function description "Climate control, function" in subfunction "Climate control regulation, function".

Function sequence for switching off stationary heater (with CODE H12 (Auxiliary water heater))

After an accident, the stationary heater is automatically turned off to reduce the risk of fire. The supplemental restraint system (SRS) control unit sends the crash signal via chassis CAN, electronic ignition lock (EZS) control unit and interior CAN to the automatic air conditioning (AAC [KLA]) control and operating unit (model 447.8) or to the air conditioning control and operating unit (front & rear AAC) (model 447.6/7). When the stationary heater is switched on, the automatic air conditioning (AAC [KLA]) control and operating unit (model 447.8) or the air conditioning control and operating unit (front & rear AAC) (model 447.6/7) activate the stationary heater emergency shutoff and sends the heating request (0 %) via climate control LIN (LIN B8) to the warm water stationary heater control unit (STH) (N33/3). The stationary heater (STH) is then permanently switched off.

 Detailed information on switching off the STH can be found in the separate function description "Stationary heater (STH), function".