Airbag Restraint Systems

Steering Wheel Air Bag

The airbag restraint system inflates airbags to protect passengers in case of collision. This system has acquired several names, including SIR Supplemental Inflation Restraint, SRS Supplemental Restraint System, and ACRS Air Cushion Restraint System.

Automotive airbag cross section view.

The airbag module contains the airbag and the inflator assembly incorporated into a module compact enough to fit into a steering wheel, passenger dash, or the side of the front seat. The inflator assembly incorporates an electric match that ignites a solid propellant that turns to nitrogen gas, inflating the airbag in milliseconds, faster than the blink of an eye. The powder that comes from the airbag is usually cornstarch or talcum powder.

Most crash sensors are electromechanical sensors made with a small steel ball held to one side of a tube by a magnet. On the other side of the tube are two open contacts. A crash of sufficient force will jar the steel ball loose from the magnet, connect the two open contacts, and complete the circuit to the airbag primer. Late-model and upper-end vehicles incorporate solid-state piezoelectric crash sensors into their airbag systems. One or several sensors may be located behind the front bumper and inside the passenger compartment.


The clock spring is the moving part of an airbag system. It provides a constant electrical connection to the airbag module, allowing the driver to rotate the steering wheel. They can be sensitive; a free-spinning steering wheel can damage a good clock spring.

The ACU or Airbag Control Unit monitors the readiness of the entire airbag system. It poles the system at each startup, and if a fault is found, it illuminates the amber Airbag warning light. Then, it stores a fault code in the computer's memory. This module will also provide backup power in case the battery is disconnected. Always refer to the manufacturer's documentation before attempting to repair any part of an airbag system. Airbag systems must be disconnected for a certain period before any work is performed.

Laptop scan tool.

An airbag system contains a few basic components. Today's system is typically diagnosed with a scan tool. Some models may require the technician to perform a simple procedure to produce a series of airbag light flashes. These flashes are counted, and a code number is researched in a manual indicating the area of the system to focus diagnostics. The airbag system is considered a supplement to seatbelt restraint systems.