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 is composed of left and right dash sensors, the SRS unit (including cowl sensors), the cable reel, driver's airbag and, on the LS model, front passenger's airbag.


Seat belt pretensioner

The seat belt pretensioner is linked with the SRS airbag to further increase the effectiveness of the seat belt. In a front- end collision, the pretensioner instantly retracts the belt to firmly 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 sets of sensors are connected in series to each airbag inflator circuit and the car battery. In addition, a back-up power unit is connected in parallel with the car battery. The back-up power unit and the cowl sensors are located inside the SRS unit.


For the SRS to operate:

1. One or both cowl sensors activate and one or both dash sensors activate.

2. Electrical energy is supplied to the airbag inflator by the battery, or the back-up power unit if the battery voltage is too low.

3. Airbags deploy and seat belt pretensioners trigger.


At least one cowl sensor and one dash sensor must be activated simultaneously for at least 0.015 seconds in order for the airbag(s) to be deployed and seat-belt pretensioners to be triggered.






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 out after about 6 seconds if the system is operating normally. If the light does not light, or does not go out after 6 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