Supplemental Restraint System (SIR) is a generic term for all air bag inflators/modules including driver, passenger, side impact, curtain bag and also includes pyrotechnic seat belt buckle and all associated hardware and wiring.

Air bags are supplemental restraint systems that are designed to work in conjunction with seat belts to reduce the severity and extent of injuries to the occupants of a vehicle in the event of an accident. When a condition or impact of sufficient force occurs, a sensor or sensors send an electrical signal to the diagnostic unit. The diagnostic unit (SDM) determines the type and severity of the collision and which of the SIR components are to be deployed. An electrical signal is then sent to the SIR's, which ignites the energetic materials and deploys the SIR.

Each inflator module has a deployment loop that is controlled by the inflatable restraint Sensing and Diagnostic Module (SDM), which is mounted inside of the vehicle. The SDM determines the severity of a collision and commands deployment of each inflator module. The SDM performs continuous diagnostic monitoring of the SIR system electrical components. Upon detection of a circuit malfunction, the SDM will set a diagnostic trouble code (DTC) and inform the driver by commanding the instrument panel cluster (IPC) to turn the AIR BAG indicator ON. The steering column is designed to absorb energy and compress during frontal collisions in order to decrease the chance of injury to the driver.

Dual stage inflator modules contain a housing, inflatable air bag, 2 initiating devices, canister of gas generating material and, in some cases, stored compressed gas. The 2 initiators are part of the frontal deployment loop. The function of the frontal deployment loops are to supply current through the steering wheel and instrument panel (I/P) inflator modules to deploy the air bags. The inflator modules have 2 stages of deployment, which varies the amount of restraint to the occupant according to the collision severity. For moderate frontal collisions, the inflator modules deploy at less than full deployment (low deployment) which consists of stage 1 of the inflator module. For more severe frontal collisions, a full deployment is initiated which consists of stage 1 and stage 2 of the inflator module. The current passing through the initiators ignites the material in the canister producing a rapid generation of gas and is some cases, the release of compressed gas. The gas produced from this reaction rapidly inflates the air bag. Once the air bag is inflated, it quickly deflates through the air bag vent holes and/or the bag fabric.

Each dual stage inflator module is equipped with a shorting bar located in the connectors of the module. The shorting bar shorts the inflator module deployment loop circuitry to prevent unwanted deployment of the air bag when it is disconnected.

The frontal supplemental restraint system (SIR) consists of the following components:

A frontal collision of sufficient force will deploy the frontal air bags. The SDM contains a sensing device that converts vehicle velocity changes to an electrical signal. In the event of a frontal collision, the SDM receives a signal from the electronic frontal sensor which assists the SDM in determining the severity of the collision. The SDM compares these signals to a value stored in memory. When the generated signals exceed the stored value, the SDM will cause current to flow through the frontal deployment loops simultaneously deploying the frontal air bags. The SDM, I/P module, steering wheel module, steering wheel module coil, seat belt pretensioner and the connecting wires make up the frontal deployment loops. The SDM continuously monitors the deployment loops for malfunctions and turns the AIR BAG indicator ON if a fault is detected.

The sensing and diagnostic module (SDM) is a microprocessor and the control centre for the supplemental restraint system (SIR) . This SDM has 2 fused power inputs; one fuse is for the battery voltage and the other fuse is for the ignition voltage.

The SDM uses vehicle battery voltage as its main power input. The SDM then uses the vehicles GMLAN Serial Data Communication line and the ignition voltage logic input for enabling or disabling the SIR deployment loops. The SDM communicates with internal sensors and several external sensors. In the event of a collision, the SDM compares the signals from the internal and external sensors to values stored in memory. When the generated signals exceed the stored value, the SDM will cause current to flow through the appropriate deployment loops to deploy the seat belt pretensioner's and air bag(s) if required.

If the force of the impact is not sufficient to warrant inflator module deployment, the SDM may still deploy the seat belt pretensioners. The SDM records the SIR system status when a deployment occurs and turns the SIR warning lamp indicator located in the instrument panel cluster (IPC) ON. As soon as 3 distinct deployment commands (representing different events) have been issued to all belt pretensioner, or the SDM commands any front and/or side air bag to deploy once, the SDM shall be considered to not be reusable. The SDM performs continuous diagnostic monitoring of the SIR system electrical components and circuitry when the ignition is turned ON. If the SDM detects a malfunction, a DTC will be stored and the SDM will command the SIR warning lamp indicator ON, notifying the driver that a malfunction exist.

The steering wheel module coil is attached to the steering column and is located under the steering wheel. The steering wheel module coil consists of 2 or more current-carrying coils. The coils allow the rotation of the steering wheel while maintaining continuous electrical contact between the steering wheel module deployment loop and the steering wheel module. Four coil wires are used for the steering wheel module deployment loops. Additional coil wires are used for accessories that are attached to the steering wheel, depending on the vehicle model. The steering wheel module coil connector is located near the base of the steering column. The connector contains a shorting bar that shorts the steering wheel module coil deployment loop circuitry to prevent unwanted deployment of the steering wheel module when the connector is disconnected.

The front end sensor's, also known as the electronic frontal sensor's (EFS), are equipped on vehicles to supplement the Supplemental Restraint System (SIR) performance. The front end sensor's are an electronic sensor and is not part of the deployment loops, but instead provides an input to the sensing and diagnostic module (SDM). The front end sensor can assist in determining the severity of some frontal collisions. The SDM contains a microprocessor which performs calculations using the measured acceleration and compares these calculations to a value stored in memory. When the generated calculations exceed the stored value, the SDM will cause current to flow through the frontal deployment loops deploying the frontal air bags.

The side impact sensor (SIS) contains a sensing device which monitors vehicle acceleration and velocity changes to detect side collisions that are severe enough to warrant air bag deployment. The SIS is not part of the deployment loop, but instead provides an input to the sensing and diagnostic module (SDM). The SDM contains a microprocessor that performs calculations using the measured acceleration and compares these calculations to a value stored in memory. When the generated calculations exceed the stored value, the SDM will cause current to flow through the deployment loops deploying the side air bags and curtain airbag's (if fitted).

The seat belt pretensioner is a device that tightens a seat belt in the event of an accident. When a frontal, side (if side airbag's are fitted) or rear impact of sufficient force occurs, sensors send an activation signal to the pretensioning device. Upon deployment the pretensioning device tightens the seat belt webbing by reducing the length of the anchor line of the buckle. The intended purpose is to couple the occupant to the vehicle at an early stage, thereby reducing the load on the occupant and the severity and extent of injuries to the occupant.

During deployment the energetic materials pyrotechnic pretensioners are consumed in the reaction, therefore these devices can not be reused or repaired and must be replaced after a deployment.

The "Driver air bag" or "Driver SIR" is an inflatable crash protection device deployed from the steering wheel. This system is activated when a frontal impact, up to 30 degrees off the centerline of the vehicle, of sufficient force occurs. The bag itself is shaped like a round pillow and inflates to the size of a large donut.

The "Passenger air bag" or "Passenger SIR" is an inflatable crash protection device deployed from the passenger side instrument panel. This system is activated when a frontal impact, up to 30 degrees off the centerline of the vehicle, of sufficient force occurs. The bag itself is shaped like a rectangular pillow.

The side impact air bag is located in the sides of the front seats. It deploys in a side impact collision to protect the occupant. Side impact air bags offer protection to the occupant, with bag fill timed to provide protection during an impact event.

Important: A side impact airbag only deploys on the side of the vehicle that is struck.

The "roof rail air bag" or "Curtain air bag stows unseen along the vehicle's interior roof rail. During a side impact the air bag deploys downwards from the vehicle's roof rail, providing protection for the front and rear occupants head.

Important: A curtain air bag only deploys on the side of the vehicle that is struck.

The inflatable restraint wiring harness connects the inflator modules, the sensing and diagnostic module (SDM), the deployment loops, and the serial data circuit together using weather-packed connectors (where required). Supplemental restraint system (SIR) connectors are yellow for easy identification. When repairing SIR wiring harnesses, follow the proper SIR/SRS Wiring Repairs.

The SIR warning lamp indicator, located in the instrument panel cluster (IPC), is used to notify the driver of supplemental inflatable restraint (SIR) system malfunctions and to verify that the sensing and diagnostic module (SDM) is communicating with the IPC. When the ignition is turned ON, the SDM is supplied with ignition positive voltage. The SDM requests the IPC to flash the SIR warning lamp indicator seven times. While flashing the indicator, the SDM conducts tests on all SIR system components and circuits. If no malfunctions are detected, the SDM will communicate with the IPC through the GMLAN serial data circuit and command the SIR warning lamp indicator OFF. The SDM provides continuous monitoring of the air bag circuits by conducting a sequence of checks. If a malfunction is detected the SDM will store a diagnostic trouble code (DTC) and command the IPC to turn the SIR warning lamp indicator ON via GMLAN serial data. The presence of a SIR system malfunction could result in non-deployment of the air bags or deployment in conditions less severe than intended. The SIR warning lamp indicator will remain ON until the malfunction has been repaired.

The steering wheel and column are designed to absorb energy when driver contact is made with steering wheel or inflated module. When the driver applies load to the inflator module or the steering wheel, the column will compress downward, absorbing some of the impact and helping to reduce bodily injuries to the driver. The steering wheel and column must be inspected for damages after a collision.