Part 1
Design
Control module
Warning! The battery must be disconnected before any work is carried out on the supplemental restraint system (SRS). Otherwise the airbags could accidentally deploy.
The primary task of the supplemental restraint system module (SRS) is to minimize injuries to the driver and passengers in different types of accident. To do this, the control module measures the acceleration signals generated by a collision and then determines the required action. The control module has internal and external sensors. Using the data collected from the sensors, the control module determines which of the following protection systems will be activated:
- seat belt tensioners front seat
- front airbags
- adaptive steering column (USA/CDN only)
- adaptive seatbelt force limiter (USA/CDN only)
- seat belt tensioners rear seat
- side impact protection (SIPS) bags
- door mounted inflatable curtains
- ROPS (Roll Over Protection System).
The collision sensors in the control module and the frontal collision sensor and the side collision sensors register the mechanical stresses that occur in the event of a collision. The stresses are measured by the acceleration and pressure sensors. If the stresses exceed a certain level, the collision pulse starts to be calculated. The calculation evaluates whether the collision pulse was sufficient to activate the protection system. Airbags and seat belt tensioners can be activated individually.
Certain markets also use:
- an occupant weight sensor (OWS) located under the seat cushion on the front passenger seat
- two seat position sensors located on the rail under the driver's seat and front passenger seat.
The driver and passenger airbags and the seat belt tensioners are designed to deploy in the event of a frontal collision as necessary. In USA/CDN, the adaptive steering column and the function for adaptive seatbelt force limiting are also activated as necessary.
In the event of a side impact, the side airbags and the door mounted inflatable curtain on the impact side activate, as well as the seat belt tensioners in the front seat.
In the event of a rear impact, the front seat belt tensioners and the ROPS (Roll Over Protection System) are activated as necessary.
The control module has a local CAN network connected to it (only applies to USA/CDN). The occupant weight sensor (OWS) is connected to the controller area network (CAN). There is also a switch on the passenger side of the dashboard. This switch can be used to activate or deactivate the passenger airbag using the ignition key (does not apply to USA/CDN).
Note! The activating switch, on/off, for the front passenger airbag and occupant weight sensor (OWS) must not be installed in the vehicle together.
The control module also has an accident data recorder that records certain system information in the event of a collision. This information can be analyzed and used after a collision as necessary and is used to develop the supplemental restraint systems of the future. The control module can store information from 3 collisions. Once 3 collisions have occurred, no more information can be stored and the control module must be replaced. A diagnostic trouble code is generated when the control module has stored information from 3 collisions.
In the event of a frontal collision, side on collision, rear collision or a roll, a collision signal is transmitted from the supplemental restraint system module (SRS) to the phone module (PHM) and the central electronic module (CEM) using a directly connected cable. The purpose of this signal is to:
- switch off the fuel pump (FP)
- switch off the engine cooling fan (FC)
- switch off the windshield wipers when the car is stationary
- switch on the interior lighting and the courtesy lighting
- set the vehicle to collision status
- unlock the central locking system and
- if the vehicle is equipped with Volvo On Call (Plus), send an automatic alarm to CSC (Customer Service Center).
A collision signal is also sent to the phone module (PHM) and the central electronic module (CEM) via the Controller area network (CAN).
The supplemental restraint system module (SRS) decides to deploy the airbags and/or the seat belt tensioners and/or adaptive steering column and/or the function for adaptive seatbelt force limiting in the event of a frontal collision. This depends on whether:
- if the driver and front passenger are wearing seat belts or not
- if there is a fault in the seat belt buckles
- if there is a fault in the controller area network (CAN) communication.
As a result of the above conditions, in the event of a low impact collision the seat belt tensioner may deploy on the driver's side (belted driver) whilst the airbag is deployed on the passenger side (non belted passenger).
The SRS indicator lamp lights when the ignition is switched on. If no faults are registered by the control module, either internally or from the sensors, wiring or igniters, a signal is transmitted via the CAN network to the Driver information module (DIM) confirming that the SRS indicator lamp can be switched off. If there is a fault, the supplemental restraint system module (SRS) transmits information to the driver information module (DIM) indicating which lamp should be lit and the text to be displayed in the combined instrument panel:
- The SRS warning lamp
- the general warning lamp or
- the yellow information light
The driver information module (DIM) continuously transmits information to the supplemental restraint system (SRS) control module via the controller area network (CAN) about the status of the SRS warning lamp.
If the SRS indicator lamp stops working, the general red warning lamp is used instead.
Note that after a collision, it may be necessary to replace the cable harness for the components of the protection system which have deployed. This is because the connectors may melt at the moment of deployment. If a connector has melted, the cable harness adjacent to the connector must be replaced.
Warning! The battery must be disconnected before any work is carried out on the supplemental restraint system (SRS). Otherwise the airbags could accidentally deploy.
Side impact sensor
The side impact sensors in the vehicle are used to measure the mechanical stresses that occur in the event of a collision.
The four sensors which are used in the car are:
- 1 side impact sensor in each left and right B post
- 1 side impact sensor in each left and right door.
The side impact sensors in the B-post are acceleration sensors while the side impact sensors in the doors are pressure sensors. The sensors transmit data continuously to the control module. In the event of a collision, the control module uses the acceleration and pressure data from the sensors, and the control module's internal sensors to determine if the collision severity is enough to activate the side impact protection system.
Only the side impact protection system on the collision side is activated.
In the event of a fault in the side impact sensor, the control module stores a diagnostic trouble code (DTC). A diagnostic trouble code (DTC) is also stored if the communication between the control module and one of the side impact sensors does not function. Each side impact sensor has a serial number. The serial number is used to check that the correct side impact sensor is installed.
If one (or several) of the side impact sensors stops functioning, the control module uses the remaining sensor to determine when the collision protection should be activated.
Note! The side impact sensors in the B-posts are the same and can be installed on the left or right side, but cannot be used in the doors. The side impact sensors in the doors are also the same and can be either installed in the left or right door.
Data is transmitted on the same cable used for power supply. There is a signal/power supply cable to each side impact sensor and a ground cable.
Front impact sensor
The front impact sensors are used to provide the supplemental restraint system module (SRS) with advance information about the collision pulse. This enables the control module to more accurately determine which components to deploy.
The front impact sensors have integrated logic which transmits acceleration data to the supplemental restraint system module (SRS) in the same way as the side impact sensors.
In the event of a collision, the control module calculates the acceleration data from the frontal collision sensors and the internal sensors of the control module to check if the impact is sufficiently great to activate the frontal impact protection.
In the event of a fault in a frontal impact sensor, the control module stores a diagnostic trouble code (DTC). A diagnostic trouble code (DTC) is also stored if the communication between the control module and one of the frontal impact sensors does not function. Each frontal impact sensor has a serial number. The serial number is used to check that the correct frontal impact sensor is installed.
If one or both of the frontal impact sensors stops functioning, the control module uses the remaining acceleration sensor to determine when the collision protection should be activated.
Data is transmitted on the same cable used for power supply. There is a signal/power supply cable to each front impact sensor and a ground cable.
The two front impact sensors are behind the left and right-hand headlamps.
Seat position sensors (USA/CDN only)
The seat position sensor is used to determine the size of the person sitting on the seat. There are two seat position sensors in the vehicle, one under the driver's seat and one under the front passenger seat. The seat position sensors are directly connected to the supplemental restraint system module (SRS). The seat position sensors have separate signal cables to the control module and a common ground.
The seat position sensor consists of a Hall sensor which modifies the current depending on the position of the seat. The seat position sensor has two classifications, these are a small person and a large person. A small person means that the seat is approximately 10 cm from the first seat position and forwards. Other positions are classified as a large person.
This information is used by the function for adaptive seatbelt force limiting.
Switch on/off, front passenger airbag (not USA/CDN)
With the switch it is possible to activate or deactivate the airbag for the front seat passenger.
The switch is located on the side of the instrument panel, on the passenger side. The switch is only accessible when the front side door on the passenger side is open.
The ignition key is used to change the position of the switch. When the switch is in position "OFF", both steps for the passenger airbag are deactivated and warning light airbag passenger (PAD) is lit.
When the switch is in position "ON", the passenger airbag is activated and warning light activated airbag passenger (PAE) is lit.
When the switch position is moved between "OFF" and "ON", two Hall sensor sense in which position the switch is. The Hall sensors for the switch are directly connected to the Supplemental restraint system module (SRS).
Each Hall sensor is separately connected to the control module. The signal that is sent from the Hall sensors consists of two different current levels depending on which position the switch is in.
If the signal from the Hall sensor cannot be interpreted correctly, a diagnostic trouble code is generated and the control module may, depending on the problem, choose to deactivate the passenger airbag.
If the passenger airbag is deactivated then warning light airbag passenger (PAD) and SRS-warning light are lit, and a text message is shown in the Driver information module (DIM). If a problem is detected and the passenger airbag is not deactivated, instead warning light activated airbag passenger (PAE) and the SRS-warning light are lit, and a text message is shown in the Driver information module (DIM).
Note! The activating switch, on/off, for the front passenger airbag and occupant weight sensor (OWS) must not be installed in the vehicle together.
Occupant weight sensor (OWS) (USA/CDN only)
The occupant weight sensor (OWS) consists of three interactive components:
- The pressure sensor, installed under the front passenger seat.
- Belt force sensor, mounted in the seat frame on the outside of the front passenger seat.
- Occupant weight sensor module, installed under the front passenger seat.
The pressure sensor and occupant weight sensor are linked via a hose. The pressure sensor registers the pressure the seat cushion is exposed to, for example when someone is sitting in the seat.
Information from the seat belt force sensor is also used to ensure the good performance of the system. The seatbelt force sensor informs the occupant weight sensor module what force the belt is tensioned with. This is done because the pressure measured by the pressure sensor can be affected by the passenger fastening the seatbelt tightly. This means that the passenger exerts greater pressure on the seat than during normal seatbelt use. The passenger is then interpreted as being heavier than he or she is.
Using information from both the pressure sensor and the seat belt force sensor, the occupant weight sensor module carries out calculations and classifications of what is affecting the system. This forms the basis for the recommendation that is communicated to the supplemental restraint system.
The occupant weight sensor (OWS) transmits continuous signals on the local controller area network (CAN), which the supplemental restraint system module (SRS) and occupant weight sensor (OWS) are connected to. If the occupant weight sensor (OWS) does not function correctly the supplemental restraint system module (SRS) will:
- disable the front passenger airbag
- light the passenger airbag warning lamp (PAD) in the roof console
- light the SRS (supplemental restraint system) warning lamp and display the alert message in the driver information module (DIM).
If a fault occurs in any of the components the following should be noted:
- The pressure sensor and occupant weight sensor module must be replaced at the same time
- The seat belt force sensor is replaced separately.
Note! The activating switch, on/off, for the front passenger airbag and occupant weight sensor (OWS) must not be installed in the vehicle together.
Adaptive seatbelt force limiter (USA/CDN only)
Warning! Never use an ohmmeter or similar tool to test airbags, door mounted inflatable curtains, adaptive seat belt tensioners, the adaptive steering column, rollover bar cassette or seat belt tensioners. Disconnect airbags, door mounted inflatable curtains, adaptive seat belt tensioners, adaptive steering column, rollover bar cassette or seat belt tensioners before testing them. Failure to do so may result in accidental deployment. This could result in serious injury or death.
The adaptive seatbelt force limiter improves the seatbelt function in the event of a frontal collision when the airbag has been activated. There are two adaptive seatbelt force limiters in the car. These are integrated in the seat belt reel, positioned at shoulder height on the driver's and front passenger seatbelts.
The adaptive seatbelt force limiters are electrically separated from each other. The adaptive seatbelt force limiters have a pyrotechnic charge which adapts the seatbelt force to a lower level against the chest on activation.
The adaptive seatbelt force limiter, seat belt reel, seat belt (inertia) reel sensor and seat belt tensioner are installed in a single unit. In the event of a fault in any of these the whole unit must be replaced.