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 the event of 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. The longitudinal stresses are measured using two acceleration sensors. 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 driver and passenger airbags and the seat belt tensioners are designed to deploy in the event of a frontal collision. There is an electrical switch in the control module to further increase the safety of the system. The switch must be activated in event of a collision for the front airbags to deploy.
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 normally be activated in the event of a collision from the rear.
For the side impact cushion to deploy the collision required must be powerful enough that it is registered by the side impact sensor, as well as the SRS control module registering a collision. If the side impact occurs at the center of the car, the side impact protection (SIPS) bags and the inflatable curtain are deployed on the side where the collision occurred. If the side impact is towards the rear of the car, normally only the inflatable curtains on that side are deployed. For further information, see Deployment of side impact protection (SIPS).
The control module has a collision recording function. This records certain system information in the event of up to three collisions. If necessary, this information can be analyzed and used after a collision. This information is used in the development of collision safety systems of the future.
There is an internal energy reserve in the control module. This is intended to run the whole system for a short time in the event of a voltage drop. This is to ensure the function of the system, if for example the battery cables should be cut or the battery loses power in some other way during a collision.
In the event of a collision a signal is sent from the SRS control module to the VGLA control module. The intention of the signal is to unlock the doors.
The SRS control module determines whether the airbags or seat belt tensioners should be deployed. This depends on whether:
- if the driver and front passenger are wearing seat belts or not
- if there is a fault in the seat belt buckles.
Information about use of the seat belt buckle for the driver and front passenger comes from the seat belt buckle sensors. If a fault occurs in either the driver or passenger seat belt buckles, a diagnostic trouble code (DTC) is stored locally in the SRS control module.
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 to the combined instrument panel confirming that the SRS indicator lamp can be switched off. If a fault occurs, the SRS control module transmits a signal to the combined instrument panel to light the SRS indicator lamp in the combined instrument panel.
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 closest 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 evaluation 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 can transmit three different signals:
- fault in the sensor (Error)
- the sensor is working (OK)
- deployment signal (Fire).
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 normally 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.
There is a memory in the side impact sensors which stores data for up to three collisions.
This information is used in the development of collision safety systems of the future.
Data transmission occurs on the same cable used for power supply to the side impact sensor. That is a cable for power supply and data transmission and a ground lead.

Front airbags

Warning! 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.

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 (model year 1996-2000)
- front passenger airbag, 2 stage deployment (introduced model year 2001)
- passenger airbag, 1 stage deployment (model year 1996-2000).

1 stage deployment (model year 1996-2000)
The driver's airbag contains a pyrotechnical charge and an igniter. During deployment the charge produces a gas consisting mainly of hydrogen and carbon dioxide which expands and fills the airbag. The passenger airbag uses a smaller pyrotechnical charge (12 grams). This is connected to a container filled with argon, helium and oxygen. When the control module deploys the passenger airbag, the pyrotechnical charge heats up the container which then bursts. The airbag is filled by a gas mixture.

2 stage deployment (model year 2001-)
In 2 stage systems, the deployment mechanism for stage 1 is the same for 1 stage types.
Stage 2 contains a pyrotechnic charge.
The front airbags have two powder charge ignition components. These are electrically separated from each other. This means that they have their own separate power supply and grounding points from the control module. Two outputs are required from the control module to the airbag, one to each ignition component of the airbag. This means that four pins are used on the control module, two power supply pins and two ground pins.
2 stage deployment allows for different speed and duration of inflation, depending on the strength of the collision pulse. For safety reasons both stages are used for each deployment. 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 between stages 1 and 2 is shorter.
The airbags are positioned above the glove compartment and in the steering wheel.
There is an igniter for each stage of the airbags and seat belt tensioners. These igniters consist of two pins, a thin ignition wire and an explosive charge. When the SRS (supplemental restraint system) transmits a current pulse the ignition wire heats up and ignites the explosive powder which ignites a gas generator and deploys the airbag.

Warning! 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

Warning! 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.

There are two seat belt tensioners in the car. The seat belt tensioners are located in the B posts on the passenger and driver's sides. They contain an explosive charge which tensions the seat belts when deployed. 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 to the Supplemental Restraint System Module (SRS).
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 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 control module does not receive a value or reads off incorrect data for the front seat belt buckles, the seat belt tensioners will be activated and the air bag will be deployed. The deployment level of the airbag is the same as if the seatbelt is not used.
The seat belt tensioner consists of a pyrotechnical 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.

Warning! 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 curtain (model year 2001-)

The Supplemental Restraint System Module (SRS) checks the different components of the system twice per second. A diagnostic trouble code (DTC) is stored and the SRS indicator lamp lights if the same fault is detected 5 times over a period of approximately 2.5 seconds.
SRS indication is through a warning lamp in the combined instrument panel. The SRS indicator lamp lights at ignition on while the system is checked. The check takes approximately 5 seconds. If no faults are registered by the control module, either internally or from the sensors, wiring or igniters, a signal is transmitted to the combined instrument panel confirming that the SRS indicator lamp can be switched off. If a fault was found in the Supplemental Restraint System Module (SRS), the SRS indicator lamp remains lit.
The memory in the Supplemental Restraint System Module (SRS) is EEPROM. This retains information about diagnostic trouble codes (DTCs) even after the power supply to the control module has been cut.

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 contain a magnetic sensor, called a hall sensor. 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 a someone was using the seatbelt. The system strives to provide maximum protection for the traveler.

Note! in the event of a fault either in the hall sensor or wiring a diagnostic trouble code (DTC) is stored locally in the Supplemental Restraint System Module (SRS). The diagnostic trouble code (DTC) can be read off using VIDA.

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.
For example a collision at headlamp height can push in the entire upper front section, but the SRS (supplemental restraint system) will still only deploy the seat belt tensioners. This is because the impact is "softer" than if the collision force had been through the chassis where more force would be transferred to the Supplemental Restraint System Module (SRS).

1 stage deployment
On model year 1996-2000 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 Module (SRS) 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 of side impact protection (SIPS)
The deployment of the side impact protection (SIPS) bag and inflatable curtains is achieved by co-operation between the side impact sensors and the Supplemental Restraint System Module (SRS).
The side impact sensors are located on the B and C posts, behind the front and rear doors respectively on both sides. The sensors measure the lateral acceleration of the vehicle. The signals are analyzed. If the calculated impact exceeds a certain level the side impact sensors checks with the Supplemental Restraint System Module (SRS) whether deployment of one or more side impact protection (SIPS) bags or inflatable curtains is required. If the Supplemental Restraint System Module (SRS) determines that the impact has reached that level, the side impact sensors deploy the side impact protection (SIPS) bags and / or inflatable curtains.
The Supplemental Restraint System Module (SRS) can register multiple collisions. The Supplemental Restraint System Module (SRS) determines what action should be taken in the event of each "individual" collision depending on the impact of each collision. The Supplemental Restraint System Module (SRS) and the side impact sensors can therefore deploy one airbag first and then deploy another if it is necessary.
Example:
- If there is a collision directly from the side at driver's door height, the side airbag and the inflatable curtain on the driver's side may be deployed depending on the force of impact
- If a collision from the side occurs at the joint of the right rear door and cargo compartment, the right inflatable curtain may deploy depending on the force of impact
- A collision diagonally from the rear which first hits the rear door and then "continues forward", can deploy both the inflatable curtain and the side airbag even if the impact was first registered at the C post.

Deployment levels for airbags and seat belt tensioners (model years 1996-2000)
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 2 = Severe collision

Deployment levels for airbags and seat belt tensioners (model years 2001-)
The table below shows how the SRS (supplemental restraint 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