Crash sensors can be fitted in various positions throughout the vehicle. Their location depends upon the direction of deceleration they are designed to detect.
Some manufacturers place the sensors within the electronic control unit. Others are located behind the front bumper, headlights and dash.
Side impact sensors are located in the doorsills or “B” Pillar. They will inform the SRS control unit of a side impact and whether to deploy the left or right side air bags.
When the sensors indicate that a predetermined deceleration rate has been exceeded and it is from the appropriate direction, the SRS control unit deploys the relevant air bags.
If the collision is from the front, the driver and passenger air bags will deploy.
If the collision is from the side, the sensor determines whether the seat mounted air bag, or curtain air bags for one side of the vehicle will deploy.
With more refined designs, the passenger air bag deploys only if there is an occupant in the seat. Deployment can also depend on the weight of the occupant and whether the passenger air bag switch, if fitted, is turned on.
To prevent incorrect and unnecessary deployment, systems include a safing sensor mounted within the SRS control unit. The SRS control unit will only pass current through the squib if both the safing sensor and a crash sensor indicate simultaneously that a predetermined deceleration rate has been exceeded.
The SRS warning light is illuminated, and stays on, if a fault is detected in the system.
Capacitors within the SRS control unit are used to store electricity and act as a back up power supply. If a vehicle has its battery destroyed or disconnected in an accident, the capacitors supply the electricity required to keep the SRS system operational.
Some seat mounted side impact air bags also operate without electricity. When the side of a vehicle is crushed inwards, a detonator mounted on the lower outside edge of the seat is detonated. Pyrotechnic tubes connect the detonator to the air bags, which in turn ignite the squib. Many vehicles use two stage side impact bags. This provides protection to the upper torso over a more extended time.