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 validates incoming pulsed collision signals (acceleration signals generated by a collision) and then determines the required action. The control module uses internal sensors as well as external collision sensors. Using the data collected from the sensors, the system determines which of the following will be activated:
- front airbags
- side impact protection (SIPS) bags
- inflatable curtains
- seat belt tensioners
- adaptive steering column (XC90 USA/CDN only).
The collision sensor in the control module registers the longitudinal mechanical stress that occurs in a frontal collision. A side impact may also be registered in the collision sensor. The longitudinal stresses are measured using two acceleration sensors. On vehicles with front impact sensors, these are used to register stresses in the event of a head-on collision. If the longitudinal stresses exceed a certain level, the stress will be calculated (the collision pulse). The calculation evaluates whether the collision pulse was sufficient to activate the airbags and/or the seat belt tensioners. Airbags and seat belt tensioners can be activated individually.
On the XC90, the control module is also able to register if the vehicle is about to roll over. If the sideways tilt and rotational energy of the car exceed a certain level, the control module begins a calculation to determine whether the roll over protection should be activated. The critical tilt angle is 50-55 degrees. All seat belt tensioners and inflatable curtains are deployed if the roll over protection is activated.
For certain models, depending on the market, the following are also used:
- 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. For the XC90 (USA/CDN only), the adaptive steering column will also be activated if necessary.
No seat belt tensioners or airbags will be activated in the event of a collision from the rear.
Two requirements need to be fulfilled in order for the side impact protection system to be activated:
1. that the collision pulse is sufficiently high that a trigger signal is sent from the side impact sensor to the control module
2. the control module or another collision sensor transmits a confirmation signal which allows the collision protection system to be deployed.
The side impact protection system is deployed when the control module has received both an activation and a confirmation signal.
If the side impact occurred at the front of the vehicle the side airbags and the inflatable curtains are deployed on the side that received the impact. If the side impact is towards the rear of the car, only the inflatable curtains are deployed on that side.
The control module has a collision recording function. This records certain system information in the event of a collision. If necessary, this information can be analyzed and used after a collision. This data is used in ongoing development of collision safety systems.
In the event of a frontal collision, side on collision or rear collision 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)
- light the interior lighting
- unlock the doors
- if the vehicle is equipped with Volvo On Call (Plus), send an automatic alarm to CSC (Customer Service Center) when the accident has occurred.
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 in the event of a frontal collision. This depends on information such as:
- 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 whilst the airbag is deployed on the passenger side.
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 on the controller area network (CAN) 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 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 indicator lamp.
If the SRS indicator lamp stops working, the general warning lamp is used instead.
Note that after a collision, it may be necessary to replace the cable harness for the components of the collision 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 collision pulses.
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 C post.
The side impact sensors have integrated logic. In the event of a collision, they determine whether the impact was large enough to warrant transmitting an activation signal to the control module to deploy the side impact airbag. Only the side impact protection on the side from which the activation signal was sent can be deployed.
The side impact sensors transmit continuous signals to the control module to indicate that they are working. In the event of a fault in the side impact sensor, a fault signal is transmitted to the control module which then stores a diagnostic trouble code (DTC). A diagnostic trouble code (DTC) is stored when the communication between the control module and one of the side impact sensors does not function. Each side impact sensor has a software ID. The ID is used to check that the correct side impact sensor is installed. This is because the activation level of the signal which is transmitted to the control module is not necessarily the same for the different side impact sensors.
The front (B-post) and rear (C-post) side impact sensors are a different color and have different coding on their connectors to simplify installation.
The front side impact sensors cannot be installed in the position for the rear side impact sensors and vice versa.
Data is transmitted on the same cable used for power supply.

Front impact sensor (certain markets and models only)

The frontal 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 determine more accurately the components to be activated.
The frontal impact sensors have integrated logic which transmits acceleration data to the supplemental restraint system module (SRS).
The front impact sensors transmit continuous signals to the control module to indicate that they are working. In the event of a fault in a frontal impact sensor, a fault signal is transmitted to the control module which then 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 software ID. The ID is used to check that the correct frontal impact sensor is installed.
Data is transmitted on the same cable used for power supply.
The two frontal impact sensors are inside the left and right-hand headlamps.

Seat position sensors (XC90 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 car. One is under the driver's seat and the other under the front passenger seat. The seat position sensors are connected directly to the central electronic module (CEM).
The seat position sensor consists of a Hall sensor which alters 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 sent onwards on the controller area network (CAN) to the supplemental restraint system (SRS) module. The supplemental restraint system module (SRS) uses this information to optimize the function of the front driver and passenger airbags in the event of a collision.

Adaptive steering column (XC90 USA/CDN only)

The occupant weight sensor (OWS) consists of three interactive components:
- The pressure sensor, installed under the front passenger seat
- The seatbelt 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 is exposed to, for example when someone is sitting in the car. 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. The occupant weight sensor module uses the information from both the pressure sensor and seatbelt force sensor to calculate the actual weight of the passenger. The occupant weight sensor (OWS) transmits continuous signals to the supplemental restraint system module (SRS) and occupant weight sensor (OWS) on the LIN bus. If the occupant weight sensor (OWS) does not function correctly the supplemental restraint system module (SRS) will:
- deactivate the front passenger airbag
- light the warning lamp for passenger airbag disconnected (PAD) in the upper electronic module (UEM)
- light the SRS (supplemental restraint system) indicator 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 seatbelt force sensor is replaced separately.

Note! The on/off switch for the front passenger airbag and occupant weight sensor (OWS) must not be installed in the vehicle together.

Switch on/off, front passenger airbag (not USA/CDN)

This switch is used to activate or deactivate the front passenger airbag. The switch is on the side of the dashboard on the passenger side. The switch can only be accessed with the front passenger door open. The ignition key is used to change the position of the switch. When the switch is in the OFF position, all the stages of the passenger airbag are deactivated and the passenger airbag disconnected (PAD) warning lamp lights. When the switch is moved between OFF and ON, the two Hall sensors detect the position of the switch. The Hall sensors for the switch are directly connected to the supplemental restraint system module (SRS). Each Hall sensor is individually connected to the control module. The signal which is transmitted from the Hall sensors has two different current levels, depending on the position of the switch.
A diagnostic trouble code (DTC) is stored if the signal from the Hall sensors cannot be correctly interpreted. The control module may, depending on the fault, deactivate the passenger airbag. The PAD (passenger airbag disconnected) warning lamp lights if the passenger airbag is deactivated. If a fault is detected and the passenger airbag is not deactivated, the SRS warning lamp lights and a text message is displayed in the driver information module (DIM).

Note! The on/off switch for the front passenger airbag and occupant weight sensor (OWS) must not be installed in the vehicle together.

Front airbags

Driver airbag

Front passenger airbag

Warning! Never use an ohmmeter or similar tool to test airbags, inflatable curtains, the adaptive steering column or seat belt tensioners. Disconnect airbags, inflatable curtains, the adaptive steering column or seat belt tensioners before testing them. Failure to do so may result in accidental deployment. This could result in serious injury or death.

For the XC90 in USA/CDN, the front airbags are:
- driver airbag, 3 stage deployment
- front passenger airbag, 3 stage deployment.
For other models and markets, the front airbags are:
- driver airbag, 2 stage deployment
- front passenger airbag, 2 stage deployment.
The driver's airbag is located in the steering wheel. The passenger airbag is located above the glove compartment in the dashboard.
Each airbag stage has a separate ignition section. These are electrically separated from each other. This means that the different ignition components have their own separate power supply and grounding points from the control module.
When activating the airbags electrical signals are transmitted from the supplemental restraint system module (SRS) to the relevant airbag stage.

Stage 1
Stage 1 of the driver's airbag is pyrotechnic. In the event of activation the charge combusts and the gas which develops is blown into the airbag.
Stage 1 on the passenger airbag consists of a pyrotechnic component in combination with a reservoir which stores a gas. On activation the pyrotechnic charge and mixes with the stored gas. The gas mixture is blown into the airbag.

Stage 2
Stage 2 of the driver's and passenger airbags is pyrotechnical. In the event of activation the charge combusts and the gas which develops is blown into the airbag.
The pace of the airbag 2 stage activation is dependent on collision type and seat belt use. Both stages activate but in the event of a lesser impact, there is a greater time delay between stage 1 and stage 2. For more serious impacts the time delay is shorter between stages 1 and 2.

Stage 3 (XC90 USA/CDN only)
Stage 3 of the driver's and passenger airbags is pyrotechnical. A valve opens on the airbag during deactivation.
The control module uses information from the occupant weight sensor (OWS), seat position sensor and collision sensor to determine the size of the driver/passenger and the force of the collision. The control module adapts the volume of the airbag to the force of the collision. This is done using airbag ventilation.

Warning! Never use an ohmmeter or similar tool to test airbags, inflatable curtains, the adaptive steering column or seat belt tensioners. Disconnect airbags, inflatable curtains, the adaptive steering column or seat belt tensioners before testing them. Failure to do so may result in accidental deployment. This could result in serious injury or death.