SRS System Operation

All system operations become active when the ignition switch is turned to position 'II' and remains operational when the ignition switch is in the CRANK position. When the ignition switch is turned on, the SRS warning lamp illuminates for approximately 5 seconds then turns oft, this indicates that the system is functional.

Front Impacts:

The front airbags and the seatbelt pre-tensioners are deployed in the event of a frontal impact of sufficient severity which exceeds the impact trigger threshold.

When the accelerometer and safing sensor in the SRS diagnostic control unit senses the impact, the diagnostic control unit triggers the front airbag modules by firing an igniter. This in turn ignites tablets of sodium azide which generate a large amount of Nitrogen gas causing airbag inflation. For vehicles fitted with a distributed sensing system, an activation signal is also provided to the DCU from one or both of the front crash sensors.

The DCU simultaneously triggers the seatbelt pre-tensioner operation. This is achieved by activating a propellant which acts on the seatbelt inertia reel causing an increase in the tension of the seatbelt to restrain the occupant in a safe and secure position during airbag deployment. The seatbelt pre-tensioners are armed by the safing sensor which have a faster deployment time than the front airbags, so that the occupant is held in the restrained position before the airbag is fully inflated.

The diagnostic control unit (DCU) is able to distinguish between rough road conditions and a frontal collision. If the DCU's main sensor (or front crash sensors for distributed systems) detects a frontal collision of sufficient severity and it is confirmed by the safing sensor, the DCU sends a fire signal to the airbag module and seatbelt pre-tensioner initiators.

The front airbags offer additional protection to the front seat occupants. The front airbags are fully inflated, then as the occupant moves into the airbag it immediately discharges the gas from vent holes to provide progressive deceleration and reduce the risk of injuries.


WARNING: All the SRS system components, including the wiring harness, MUST be renewed after the airbags and seatbelt pre-tensioners have been deployed.


Side impacts
The driver and passenger side airbags are deployed in the event of a side impact of sufficient severity which exceeds the side impact trigger threshold. When the SRS diagnostic control unit senses the impact, the diagnostic control unit activates the seat airbag module on the side of the vehicle suffering the impact. A current from the DCU triggers the module to ignite pellets of nitrocellulose which generate a large amount of Nitrogen gas, causing airbag inflation. The inflating airbag bursts out of the seat cover at the outboard piping and pushes the seat occupant away from the impact force.

When fully deployed, the side airbags offer additional protection to the front seat occupants in the event of a collision acting on the side of the vehicle. Either the driver's side airbag circuit or the passenger's side airbag circuit is activated depending on the side of the vehicle suffering the impact. After the airbag has fully inflated, the airbag progressively deflates the gas from vent holes to reduce the risk of injuries.


WARNING: All the SRS system components, including the wiring harness, MUST be renewed after the side airbags have been deployed.


Front angled impacts
The deployment of airbags and pre-tensioners that occur when a vehicle is involved in a front angled collision is dependent on the speed and angle of the impact. Four possible conditions could apply:
- Impact is below the threshold value for front and side triggers - no response, none of the airbags or seatbelt pre-tensioners are activated.

- The speed of the impact is in excess of the front impact trigger threshold - the driver's airbag, passenger's airbag and seatbelt pre-tensioners are activated.

- The speed of the impact is in excess of the side impact trigger threshold - the driver's side airbag circuit is activated in a driver's side impact and the passenger side airbag circuit is activated in a passenger's side impact.

- Both the front and side impact trigger thresholds are exceeded - the driver's front airbag, passenger's front airbag and seatbelt pre-tensioners are activated, and the side airbag circuit on the side of the vehicle suffering the impact is activated.

Rear impact and roll over
The SRS system does not specifically detect rear impact and roll over conditions, but if as a consequence of the crash situation the system front or side trigger thresholds are exceeded, the relevant airbags and pre-tensioners are deployed.

SRS Warning Lamps And System Diagnostics:

The SRS warning lamp is located in the instrument pack and has two bulbs behind the SRS warning lamp window. If either of the bulbs fail, the DCU will illuminate the other bulb permanently to indicate that a system fault is present (i.e. warning lamp bulb failure).

The SRS warning lamp illuminates after the ignition switch has been turned to position 'II' in order to provide a lamp check. After about 5 seconds, the warning lamp will go out for the remainder of the ignition cycle, providing the SRS system integrity is maintained. The system checks the condition of the SRS DCU, crash sensors and the SRS harnesses.

If one of the following faults are experienced when the ignition switch is turned to position 'II', the SRS warning lamp will illuminate and remain on for the duration of the ignition cycle:
- DCU connector not engaged or faulty
- Harness fault
- Earth connection fault
- Fuse open circuit

If a system fault is detected whilst driving, the warning lamp illuminates, to indicate that there is a fault with the SRS system. With the warning lamp on, the SRS system may not operate in the event of a collision.

While the ignition is on, the diagnostic function of the SRS DCU monitors the SRS system for faults. If a fault is detected, the DCU stores a related fault code in non-volatile memory and switches the earth output to illuminate the SRS warning lamp. A maximum of five faults can be stored in memory along with the timing information associated with each fault. The fault memory is accessible through the use of testbook and the K-line communications bus.

With a supply voltage range fault the warning lamp is illuminated, once the correct system voltage returns within range the lamp will extinguish. The warning lamp will remain lit for a minimum of 5 seconds. With all other faults including intermittent faults, the warning lamp is illuminated for the remainder of the drive cycle. The SRS warning lamp will not illuminate on the next ignition cycle unless the fault re-occurs, but the fault code remains stored in memory.

For a permanent fault, the SRS lamp will stay on after the initial warning lamp test and latch on for every ignition cycle until the cause of the fault has been resolved. In addition, the diagnostic system will record an 'AIR BAG FAULT'.

After detecting a fault, the system may retain some operational capability:
- If a fault is detected in a SRS circuit, only that particular circuit is disabled; the other airbag and pre-tensioner circuits remain operational and will still be deployed in the event of a collision.
- If an internal DCU or power supply fault is detected, the complete system will be disabled.
- If a fault exists in the SRS warning lamp circuit, the lamp will not illuminate during the lamp check at ignition on, but provided there are no other faults, the system will remain fully operational.

Additional information that can be accessed using 'Testbook' via the diagnostics socket include:
- SRS DCU code number
- Evolution number of the hardware, software and diagnostic protocol
- Status of the crashed lock mode
- Vehicle identification number (VIN) data

The SRS DCU logs the fault in internal memory, this can be accessed using TestBook via the diagnostic socket located on the passenger side fascia closing panel.