The supplemental inflatable restraint (SIR) system supplements the protection offered by the occupant's seat belt system (2). The SIR system may contain several inflator modules located throughout the vehicle, i.e. steering wheel module (1), instrument panel (I/P) module (1) and front seat belt pretensioners. Each inflator module and seat belt pretensioner has a deployment loop that is controlled by the sensing and diagnostic module (SDM) mounted inside the vehicle. The SDM determines the severity of a collision. When the SDM detects a collision of sufficient force it will process the information provided by the sensors to further support air bag deployment. The steering column (1) and knee bolsters (3) are designed to absorb energy and compress during frontal collisions in order to limit leg movement and decrease the chance of injury to the driver and passenger.

A frontal crash of sufficient force up to 30 degrees (a) off the longitudinal centerline of the vehicle will deploy the frontal air bags and the pretensioners.

The frontal SIR system deploys an air bag from the center of the steering wheel and from the right side of the instrument panel.

The seat belt pretensioners remove slack from the driver and passenger front seat belts during a frontal impact. The frontal SIR system may deploy the seat belt pretensioners with or without the frontal air bags.

The SDM performs continuous diagnostic monitoring of the SIR system electrical components. Upon detection of a circuit malfunction, the SDM will set a DTC and inform the driver by requesting the instrument panel cluster (IPC) to turn the AIR BAG indicator ON.

The frontal SIR system consists of the following components:

The inflatable restraint SDM, inflatable restraint steering wheel module coil, inflatable restraint steering wheel module, inflatable restraint I/P module, seat belt pretensioner - LF, seat belt pretensioner - RF, and the connecting wires make up the frontal deployment loops. The function of the frontal deployment loops is to supply current through the frontal inflator modules and the pretensioners to cause deployment in the event of a mild-to-severe frontal collision. 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 allow current to flow through the frontal inflator modules and the pretensioners causing deployment.

The SDM performs the following functions in the SIR system:

The SDM connects to the SIR wiring harness using a 50-way connector.

The SDM receives power whenever the ignition switch is in the ON or START positions.

The AIR BAG indicator is represented by an icon graphic. Ignition voltage is applied to the AIR BAG indicator when the ignition switch is turned to the START or ON positions. The SDM controls the LED by providing voltage  to the AIR BAG indicator control circuit. The SIR system uses the AIR BAG indicator to do the following:

The AIR BAG indicator is the key to driver notification of SIR system malfunctions.

The steering wheel module coil consists of 2 or more current-carrying coils. The steering wheel module coil attaches to the steering column. 2 of the current-carrying coils allow rotation of the steering wheel while maintaining continuous contact of the driver deployment loop to the steering wheel module.

There is a shorting bar on the steering wheel module coil connector. The shorting bar shorts the circuits to the steering wheel module coil and steering wheel module during the disconnection of the connector. The shorting of the steering wheel module coil and steering wheel module circuitry will help prevent unwanted deployment of the air bag when servicing the steering column or other SIR system components.

The inflator modules consist of a housing, an inflatable air bag, the initiator, a canister of gas generating materials, and in some cases, stored compressed gas. The initiator is part of the inflator module deployment loop. When the vehicle is involved in a collision of sufficient force, the SDM causes current to flow through the deployment loops to the initiator. Current passing through the initiator ignites the material in the canister producing a rapid generation of gas and the release of compressed gas, if present. The gas produced from this reaction rapidly inflates the inflator module. Once the inflator module is inflated, it quickly deflates through the vent holes and/or the fabric. Each inflator module is equipped with a shorting bar that is located in the connector of the inflator module. The shorting bar shorts the inflator modules circuitry to prevent unwanted deployment of the inflator module when the connector is disconnected.

The seat belt pretensioners contain a housing, an initiating device, and a canister of gas generating material. The initiator is part of the seat belt pretensioner deployment loop. When the vehicle is involved in a collision of sufficient force, the SDM will cause current to flow through the deployment loops to the initiator. Current passing through the initiator ignites the material in the canister producing a rapid generation of gas and the release of compressed gas, if present. The gas produced from this reaction rapidly shortens the seat belt buckle height.

Each seat belt pretensioner is equipped with a shorting bar located on the connector of the pretensioner. The shorting bar shorts the seat belt pretentioner deployment loop circuitry to prevent unwanted deployment of the pretentioner when it is disconnected.

The wiring harnesses connect the SDM, inflator modules, front end sensors, side impact sensors, seat belt pretensioners, and the class 2 serial data circuit together using weather pack connectors. SIR deployment loop connectors are yellow in color for easy identification. When repairing SIR system wiring harnesses, follow the proper testing and wiring repair procedures outlined in this manual.

The steering wheel and column are designed to absorb energy when driver contact is made with the steering wheel or inflated air bag. In a frontal collision the driver may contact the steering wheel directly or load the steering wheel and column through the inflated air bag. When the driver applies load to the air bag or steering wheel the column will compress downward absorbing some of the impact, helping to reduce bodily injuries to the driver. The steering wheel and column must be inspected for damage after a collision.

The knee bolsters are designed to help restrain the lower torsos of front seat occupants by absorbing energy through the front seat occupants' upper legs. In a frontal collision the front seat occupants legs may come in contact with the knee bolsters. The knee bolsters are designed to crush or deform absorbing some of the impact, which helps to reduce bodily injuries. The driver and passenger knee bolsters are located in the lower part of the instrument panel and must be inspected for damage after a collision.