Restraint systems (Airbag): Description and operation

Airbag

When triggered, the fire signal from the diagnostic control unit passes via the airbag harness to the airbag module. Grains of Nitrocellulose and Nitroglycerine inside the airbag module, ignite and combine in a chemical reaction to form a large amount of Nitrogen gas, leading to inflation of the airbag in approximately 30 milli-seconds.

As the driver moves forward into the airbag it rapidly deflates to provide progressive deceleration and reduce the risk of injuries.

Seat belt pre-tensioners

The front seat belt reels are linked by cables to gas generators housed within remote steel tubes. The generators are driven by small quantities of nitrocellulose. On receiving a fire signal from the DCU, the generators deploy to force pistons down the steel tubes, pulling the cables tight. The cables are connected to the seat belt reels which in turn pull the seat belts tight, reducing any slack in the belts.

Sequence of operation:

1. The main sensor and the safing sensor detect crash conditions.
2. The DCU supplies an electrical pulse to the airbag igniter and pre-tensioners.
3. Airbag and pre-tensioners deploy.

It takes about 0.1 seconds from the beginning of the airbag deployment until it is completely deflated.

System check

The SRS warning lamp, located on the fascia, illuminates when the electrical circuits are switched on whilst the system performs a self diagnosis test. If the system finds no fault during self diagnosis the light will extinguish after approximately 5 seconds and remain extinguished.

In the event of a fault in the system, the warning light will either:

1. Illuminate continuously;
2. Illuminate for 0.5 second, extinguish, then illuminate continuously;
3. Fail to illuminate during the self diagnosis test.

Fault diagnosis

Diagnosis is accomplished using TestBook connected to the vehicle diagnostic connector which is located on a bracket by the foot pedals.