Airbag Restraint Systems

The airbag restraint system is used along with and as a supplement to the seatbelt restraint system to provide passenger protection 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 air bag and the inflator assembly incorporated into a module compact enough to fit into a steering wheel, passengers 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 air bag in milliseconds, faster than the blink of an eye. The powder that comes from the air bag 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 and close the circuit to the air bag primer. Late model and upper end vehicles incorporate solid state piezoelectric crash sensors into their air bag systems. There may be one or several sensors 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 air bag module while allowing the driver to rotate the steering wheel. They can be sensitive; a free spinning steering wheel can damage a perfectly fine 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 will illuminate the Air Bag light and set a 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. Air bag systems must be disconnected for a certain period of time before any work can be performed.

An air bag 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 series of flashes of the instrument panels air bag light. These flashes are counted and a code number is researched in a manual indicating the area of the system to focus diagnostics. The air bag system is designed for front end collisions and is considered a supplement to seatbelt restraint systems.