Air Bag Systems: Description and Operation

Supplemental Inflatable Restraint (SIR) System Description and Operation

The main parts of the air bag system are the deployment loop and the diagnostic energy reserve module (DERM). The function of the deployment loop is to supply current through the inflator module in the steering wheel, which will cause deployment of the air bag in the event of a frontal collision.

The arming sensor, air bag coil assembly, inflator module and discriminating sensors make up the deployment loop. The arming sensor switches power to the inflator module on the power side of the deployment loop. The inflator module is only supplied with sufficient current to deploy when the arming sensor and at least one of the two discriminating sensors are closed simultaneously.

A function of the DERM is to supply the deployment loop with a 36 Volt Loop Reserve (36 VLR) to ensure sufficient energy to deploy the air bag if the ignition feed to the arming sensor is lost during a frontal crash.

Another function of the DERM is electrical system diagnostics. The DERM monitors the deployment loop in conjunction with the resistors in the arming sensor and discriminating sensors. These resistors make it possible for the DERM to detect circuit and component malfunctions within the deployment loop. The DERM monitors the voltage drop across each component within the loop during normal non-deployment conditions. If the monitored voltages fall outside of expected limits, the DERM will indicate a malfunction through the storage of a diagnostic trouble code and the illumination of the "AIR BAG" warning lamp.