Design

Design

Control module

Warning! The ignition must be switched off before any work is carried out on the supplemental restraint system (SRS). The airbags could accidentally deploy if the ignition is switched on.

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.
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. Front-rear stresses are gauged by two acceleration sensors. Two frontal collision sensors are also used to register stresses in the event of a frontal 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.
The control module is also able to register when the car 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.
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:
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 curtain 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 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 to 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 supplemental restraint system module (SRS) either deploys the airbags or the seat belt tensioners. This depends on whether:
- if the driver and front passenger are wearing seat belts or not
- if 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.
As a result of the above conditions, in the event of a low speed 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 functioning, the orange or 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 which 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.

Warning! The ignition must be switched off before any work is carried out on the supplemental restraint system (SRS). The airbags could accidentally deploy if the ignition is switched on.

Side impact sensor

The side impact sensors in the car are used to measure and evaluate the collision pulses. An assessment is then transmitted to the supplemental restraint system module (SRS).
The sensors which are used in the car are:
- 2 side impact sensors in the B-post
- 2 side impact sensors in the 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 OK 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 also stored when the control module has not received an OK signal. 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 place of the rear sensors and vice versa.
Data transmission occurs on the same cable used for power supply, i.e. one cable to each side impact sensor.

Frontal impact sensor

The car's frontal impact sensors are used to provide the supplemental restraint system module (SRS) with accurate advanced information about the collision pulse. This enables the supplemental restraint system module (SRS) 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 frontal impact sensors transmit continuous OK 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 when the control module has not received an OK signal. 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 cable used for power supply, i.e. there is one cable for each frontal impact sensor.
The two frontal impact sensors are under the left and right-hand headlamps.

Front airbags

Driver airbag

Front passenger airbag

Warning! Never use an ohmmeter or similar tool to test airbags, inflatable curtains or seat belt tensioners. Disconnect airbags, inflatable curtains 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 airbags at the front of the car are:
- driver airbag - 2 stage deployment
- front passenger airbag - 2 stage deployment.
The front airbags have two ignition components each. 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. Two power supply outputs are required from the control module for each airbag. That is one output for each ignition component. This means that in total four pins on the control module, two power supply pins and two ground pins, are used for each airbag.
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.
The driver's airbag is located in the steering wheel. The passenger airbag is located above the glove compartment in the dashboard.

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

Side impact air bags

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

There are up to four inflatable curtains connected to the supplemental restraint system module (SRS). Their connectors are electrically separated from each other.
The inflatable curtains are located in the roof between the A and D-posts. The front and rear inflatable curtains have only one ignition component each. The task of the inflatable curtain is to protect the head and the upper body in the event of a side impact. The supplemental restraint system module (SRS) deploys the inflatable curtains on the side from which the side impact sensors transmitted an activation signal.
The number of inflatable curtains in the car depends on whether the car is a 5 or 7 seater model. The configuration is as follows:
- 5 seater models: There is a front inflatable curtain on each side, two in total
- 7 seater models: There are front and rear inflatable curtains on each side, four in total.
The inflatable curtain uses 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 inflatable curtain which falls and covers the side windows and B post. If rear inflatable curtains are installed the C post is also covered.
Where there are two inflatable curtains per side, these are always activated simultaneously.

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

SRS indication
The supplemental restraint system module (SRS) checks the system once every second. A diagnostic trouble code (DTC) is stored and the SRS indicator lamp lights if the same fault is detected 10 times in one ignition cycle.
SRS indication is partially via the SRS indicator lamp which changes between red and orange depending on how serious the fault is and partially via a text message display. 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 no faults are detected when the ignition is switched on, the lamp will remain lit for 7 seconds. The lamp goes out when the system has been checked and is operating without faults. If a fault is detected in the supplemental restraint system module (SRS), the SRS indicator lamp lights and a text message is displayed in the text box. Information about which lamp should be lit is transmitted by the supplemental restraint system module (SRS) on the controller area network (CAN).
The control module has an EEPROM memory which retains data about diagnostic trouble codes (DTCs) even when the battery has been disconnected.
The driver information module (DIM) continuously indicates to the supplemental restraint system module (SRS) whether the SRS indicator lamp is working or not. If the lamp stops working, the general warning lamp lights up and a text message is displayed in the text box.

Note! Never take a resistance reading across the airbags or seat belt tensioners if a diagnostic trouble code (DTC) is stored for too high or low igniter resistance. Special tools with the same resistance as the airbag or seat belt tensioner must be used for fault-tracing.

Seat belt buckle

The two front seat belt buckles have hall sensors and are connected to the SRS module. These connections are electrically separated from each other.
The hall sensor detects whether the belt is engaged in the seat belt buckle or not and indicates this to the supplemental restraint system module (SRS).
This data is used to set the activation level of the airbags and to determine whether or not the seat belt tensioners need to be activated. If a fault is detected in the seat belt buckles or in the terminal for the seat belt buckles, the activation level of both the driver and passenger airbags is set as if the occupant is not wearing a seat belt. The seat belt tensioner will still function as if the occupant is wearing a seat belt however.

Deploying airbags / seat belt tensioners
The collision safety system is extremely advanced. It is able to adapt airbag deployment and the activation of the seat belt tensioners to suit the specific circumstances of a collision. In general it is extremely unlikely that all airbags and seat belt tensioners will be deployed in the same accident. It is possible that only one airbag or seat belt tensioners is deployed. This is determined by the supplemental restraint system module (SRS) based on the forces that occur during the collision.
If the control module deploys the front airbags, both stages will always be activated. By varying the time between the two deployment phases, the deployment of the airbag is more gentle for a low speed collision.

Activation levels for airbags and seat belt tensioners
The table below shows how the SRS system reacts in the event of a frontal impact.

Note! The deployment of the driver and passenger airbags is calculated individually.

Level 1 = Low speed collision
Level 5 = Severe collision

Deployment of side impact protection (SIPS) bags
The table below shows how the SRS system reacts in the event of a side impact.