Air Bag Systems: Description and Operation

Description

SRS Airbag System

The SRS is a safety device which, when used in conjunction with the seat belt, is designed to protect the driver in a frontal impact exceeding a certain set limit. The system consists of left and right dash sensors, the SRS unit (including cowl sensor), the cable reel, driver's airbag, and front passenger's airbag.


Seat Belt Pretensioner

The seat belt pretensioner is linked with the SRS airbags to further increase the effectiveness of the seat belt. In a front- end collision, the pretensioner instantly retracts the belt firmly to secure the occupants in their seats.






Operation

As shown in the diagram below, the left and right dash sensors are connected in parallel. The parallel set of sensors is connected in series to each airbag inflator circuit and the car battery. In addition, a back-up power circuit is connected in parallel with the car battery. The back-up power circuit and the cowl sensor are located inside the SRS unit


For the SRS to operate:

(1) One or both cowl sensor contacts and one or both dash sensors must activate.

(2) Electrical energy must be supplied to the airbag inflators by the battery, or the back-up power circuit if the battery voltage is too low.

(3) Airbag and seat belt pretensioner charges must be released.
Then the airbags will deploy and the pretensioners will activate.

It takes about 0.1 second from the beginning of the airbags' deployment until they are completely deflated.






Self-diagnosis system

A self-diagnosis circuit is built into the SRS unit; when the ignition switch is turned ON, the SRS indicator light comes on and goes off after about six seconds if the system is operating normally. If the light does not come on, or does not go off after six seconds, or if it comes on while driving, it indicates an abnormality in the system. The system must be inspected and repaired as soon as possible.






Circuit Diagram