Control Module

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.