The inflatable restraint Sensing and Diagnostic Module (SDM), inflatable restraint steering wheel module coil (SIR coil assembly), inflatable restraint steering wheel module (driver inflator module), inflatable restraint instrument panel module (passenger inflator module) and connector wires make up the deployment loops. The function of the deployment loops is to supply current through the inflator modules, which will cause deployment of the air bags. Deployment occurs when the SDM detects vehicle velocity changes severe enough to warrant deployment.
The SDM contains a sensing device (accelerometer) that converts vehicle velocity changes to an electrical signal. The SDM compares this electrical signal to a value stored in memory. When the generated signal exceeds the stored value, the SDM performs additional signal processing and compares the generated signals to values stored in memory. When two of the generated signals exceed the stored values, the SDM will cause current to flow through the inflator modules, deploying the air bags.
The SDM performs the following functions in the SIR system.
The SDN connects to the SIR wiring harness using a 12-way connector. The SDM harness connector uses a shorting bar across certain terminals in the contact area. Removal of the SDM Connector Position Assurance (CPA) or the harness connector itself will connect the AIR BAG warning lamp to ground through the shorting bar.
The AIR BAG warning lamp will come ON steady with power applied to the SDM when either one of the following two conditions exist:
The SDM receives power whenever the ignition switch is at the ON or START positions. In addition, the SDM can maintain sufficient voltage to cause a deployment for up to ten minutes after the ignition switch is turned to the OFF position, the battery is disconnected or the fuse powering the SDM is removed. Many of the SIR service procedures require removal of the "AIR BAG-1" fuse and disconnection of the deployment loops to avoid an accidental deployment.