Design
Design
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 car-phone 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.
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 (SRS) module.
The sensors used in the car are:
- Two side impact sensors in the B-post
- Two side impact sensors in the C-post.
The side impact sensors have integrated logic which, in the event of a collision, determines whether the impact was sufficient to warrant transmitting a trigger pulse to the control module to deploy the side impact airbag. Only the side impact protection on the side from which the 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.
Front airbags
Driver airbag
Front passenger airbag
The airbags at the front of the car are:
- Driver's airbag - 2 stage deployment (introduced model year 2001)
- Driver's airbag - 1 stage deployment (S80 1999-2000 model year only)
- Front passenger airbag - 2 stage deployment (introduced model year 2001)
- Front passenger airbag - 1 stage deployment (S80 1999-2000 model year only).
1 stage deployment (model year 1999-2000)
The driver's airbag contains a pyrotechnic charge and an igniter. In the event of activation the charge combusts and the gas which develops is blown into the airbag.
2 stage deployment (model year 2001-)
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 pyrotechnic. 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.
Note! Never use an ohmmeter or similar tool to test airbags 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.
Side impact air bags
There are two side impact protection (SIPS) bags connected to the supplemental restraint system module (SRS). These connections are electrically separated from each other. The side impact protection (SIPS) bags are on the outside of the front seats. The side impact protection (SIPS) bags deploy on the side of the impact. Their task is to protect the upper body in the event of a side on collision.
The side impact protection (SIPS) bags and inflatable curtains protect the driver and front passenger in the event of a side impact.
The side impact protection (SIPS) bag 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 airbag.
Note! Never use an ohmmeter or similar tool to test airbags 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.
Seat belt tensioners front and rear
There are five seat belt tensioners in the car, two for the front seats and three for the rear seat. They contain an explosive charge which tensions the seat belts when deployed. The seat belt tensioner tensions the seat belt to reduce the forward movement of the body. This also reduces the forces on the chest from the airbag and seat belt.
The seat belt tensioners are separated from each other electrically. This means that each seat belt tensioner has a separate individual power supply and ground from the control module. Information on whether the seat belt is engaged or not is sent from the seat belt buckle. The way in which this is done varies depending on the model year:
- Model year 1999-2002: The front seat belt buckles transmit a signal to the central electronic module (CEM). The central electronic module (CEM) then sends the data onwards on the control area network (CAN) to the supplemental restraint system (SRS) module
- Model year 2003-: The front seat belt buckles are directly connected to the supplemental restraint system module (SRS).
The supplemental restraint system module (SRS) determines whether to deploy the seat belt tensioners based on whether the seat belts are being used. The seat belt tensioner will not be activated if the seat belt is not engaged. However, if the central electronic module (CEM) reads off incorrect data for the front seat belt buckles, the seat belt tensioners will be activated and the air bag will be deployed. Deployment of the airbags occurs in the same way as when the seat belts are not fastened.
The seat belt tensioner consists of a pyrotechnic charge and a cylinder with a piston. The piston is connected to a steel cable which is secured to the seat belt reel. When the control module activates the seat belt tensioner the piston is forced downwards by the expanding gas. The piston pulls in the steel cable which reels in the belt.
Note! Never use an ohmmeter or similar tool to test airbags 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.
Inflatable curtains
There are two inflatable curtains connected to the supplemental restraint system module (SRS). These connections are electrically separated from each other. The inflatable curtains are located in the roof between the A and C posts. The inflatable curtains have only one ignition component. The inflatable curtain protects the heads and upper bodies of any occupants in the event of a side on collision. The supplemental restraint system (SRS) module deploys the inflatable curtains individually, depending on the signals from the side impact sensors.
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.
Note! Never use an ohmmeter or similar tool to test airbags 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 supplemental restraint system (SRS) 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 which detect whether or not the seat belts are engaged. They are connected differently depending on the model year:
- Model year 1999-2002: The two front seat belt buckles are directly connected to the central electronic module (CEM). The central electronic module (CEM) then sends the data onwards on the controller area network (CAN) to the supplemental restraint system (SRS) module
- Model year 2003-: The two front seat belt buckles are directly connected 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.
Note! Applies to model year 1999-2002. In the event of a fault, either in the Hall sensor or wiring for the front seat belt buckles, a diagnostic trouble code (DTC) is stored locally in the central electronic module (CEM). This diagnostic trouble code (DTC) is also stored in the supplemental restraint system (SRS) module. The diagnostic trouble code (DTC) in the supplemental restraint system (SRS) module cannot be erased before the fault has been completely remedied and the corresponding code erased from the central electronic module (CEM).
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.
1 stage deployment
On model year 1999-2000 cars, the front airbags had 1 stage deployment.
When the supplemental restraint system (SRS) module deploys the airbags, they are deployed at the same rate.
2 stage deployment
From model year 2001, the front airbags have 2 stage deployment.
If the supplemental restraint system (SRS) 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.
Deployment levels for airbags and seat belt tensioners (1999-2000)
The table below shows how the SRS system in model year 1999-2000 reacts in the event of a frontal impact.
Note! The deployment of the driver and passenger airbags is calculated individually.
Deployment levels for airbags and seat belt tensioners (2001-)
The table below shows how the SRS system in model year 2001- reacts in the event of a frontal impact.
Level 1= Low speed collision
Level 5= Heavy collision
Note! The deployment of the driver and passenger airbags is calculated individually.
Deployment of side impact protection (SIPS) bags
The table below shows how the SRS system reacts in the event of a side impact.
