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 in 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.

The collision sensor in the supplemental restraint system (SRS) 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. When the longitudinal stresses exceed a certain level a calculation of the stress (the collision pulse) starts. This assesses if the impact was great enough to activate the airbags and/or the seat belt tensioners. The seat belt tensioners can be activated without the air bags being activated and vice versa. Airbags and seat belt tensioners can be activated individually.

The driver and passenger airbags and the seat belt tensioners are designed to deploy in the event of a frontal collision.

When the collision protection system is activated, the control module transmits a pulsed current to the ignition mechanisms in the airbags and/or the seat belt tensioners. 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. The collision pulse is high enough for the side impact sensor to send a trigger signal to the control module. The control module or another collision sensor transmits a confirmation signal which allows the collision protection system to deploy. The side impact protection system deploys when the control module has received a trigger signal and a confirmation signal.

If the side impact occurred at the front of the vehicle the side airbags and the inflatable curtain deploy on the side that received the impact. If the side impact occurred at the rear of the vehicle, only the inflatable curtain deploys.

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 collision a collision signal is transmitted from the supplemental restraint system module (SRS) to the carphone module (PHM) and the central electronic module (CEM) using a directly connected cable. The purpose of the signal is to switch off the fuel pump, light the interior lighting, unlock the doors and transmit an automatic alarm to the Customer Service Center (if the car is equipped with Volvo On Call (Plus) in the event of a collision. This also applies in the event of a collision from the rear.

A collision signal is also sent to the phone module (PHM) and the rear electronic module (REM) via the controller area network (CAN).

The control module determines whether the airbags or seat belt tensioners should be deployed. This depends on whether:

• The driver and front passenger are wearing seat belts or not
• There is a fault in the side impact sensor or the connection to the side impact sensor
• If there is a fault in the seat belt buckles
• If Controller area network (CAN) communication is not working.

In a low speed collision the above can mean that the seat belt tensioner deploys on the driver's side while the airbag deploys on the passenger side.

The information about the use of seat belt buckles varies depending on the model year. The differences are:

• For model year 1999-2002:  Information about the use of the driver and front passenger seat belt buckles comes from central electronic module (CEM) (4/56) on the controller area network (CAN). If a fault occurs in either of the front seat belt buckles, a diagnostic trouble code (DTC) is stored locally in the central electronic module (CEM). The central electronic module (CEM) transmits this data to the supplemental restraint system (SRS) module which stores the corresponding diagnostic trouble code (DTC)
• For model year 2003-:  The front seat belt buckles are directly connected to the supplemental restraint system module (SRS).

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 functioning the red light in the general warning lamp is used instead.

Note that after a collision, it may be necessary to replace the wiring for the airbags and seat belt tensioners that deployed. This is because the connectors may melt at the moment of deployment. If a connector has melted, the wiring adjacent to it must be replaced.