Restraint systems (Operation): Description and operation

OPERATION

The supplementary restraint system (SRS) is a safety device which, when used in conjunction with the front seat belt pre-tensioners, is designed to protect the driver and front seat passenger by operating when the vehicle receives a frontal impact (in the area shown shaded) exceeding a certain set speed.

Airbag

In the event of a frontal impact, when the SRS diagnostic control unit senses the impact, the control unit fires igniters. This in turn ignites tablets of sodium azide which generate a large amount of Nitrogen gas leading to airbag inflation in approximately 30 milli-seconds.

When fully deployed the airbag offers additional protection to the occupants face and upper torso. As the occupant moves into the airbag it immediately discharges the gas to provide progressive occupant deceleration, reducing the risk of injuries. The whole process is completed in approximately 0.3 seconds.

Front seat belt pre-tensioners

The front seat belt reel is linked by a cable to a gas generator housed within a remote steel tube. The generator is driven by a small quantity of nitrocellulose. On deployment the generator reacts, retracting the seat belts via the seat belt reels, reducing any slack in the belts.

The pre-tensioners are linked to the SRS diagnostic control unit and in the event of a frontal impact will fire simultaneously with the airbags.

SRS warning light

The warning light in the instrument pack illuminates after the electrical circuits are switched on whilst a system check is carried out. After about 8 seconds the warning light will go out. The system checks the SRS diagnostic control unit, front seat belt pre-tensioners and SRS harness.

In the event of a fault in the system the warning light will illuminate and begin modulating. The SRS diagnostic control unit logs the fault which can only be accessed using TestBook.