The Supplemental Inflatable Restraint (SIR) system consists of the following deployment loops, along with the sensing and diagnostic module (SDM), an AIR BAG telltale in the instrument cluster, and connecting wires:

The frontal air bags deploy when the vehicle is involved in a frontal crash of sufficient force. The function of the frontal deployment loops is to supply enough current to deploy the frontal inflator modules when the SDM ducts vehicle velocity changes severe enough to warrant deployment.

To detect changes in forward velocity, the SDM contains a sensing device (accelerometer) that converts the vehicle velocity changes to an electrical signal. The electrical signal generated is processed by the SDM and then compared to the values stored in memory (EEPROM). When the proper threshold values are exceeded, the SDM will cause the current to flow through the initiator, which ignites the material in the frontal inflator modules. The gas produced from this reaction rapidly inflates the frontal air bags. The frontal air bags (1) are deployed during certain frontal crashes to supplement the protection offered by the driver and the front passenger seat belts (2). To further absorb crash energy, there are knee bolsters (3) located beneath the instrument panel for both the driver and passenger. Additionally, the steering column is collapsible.

The side curtain air bags deploy when the vehicle is involved in a side crash of sufficient force. The function of the side curtain deployment loops is to supply enough current to deploy the curtain inflator module on the side of impact when the SIS detects a side impact severe enough to warrant deployment. The SIS contains a sensing device (accelerometer) that converts acceleration into an electrical (analog) signal. This signal is converted by a microprocessor in the SIS into acceleration and velocity data. This data is used by a sensing algorithm and compared to various predetermined boundary conditions. When the acceleration value exceeds the predetermined threshold and the velocity boundary is crossed, the SIS signals the SDM to cause current to flow through the initiator, which ignites the material in the curtain inflator module. The gas produced from this reaction rapidly inflates the curtain air bag from the roof rail on the side of impact. The curtain air bag supplements the protection offered by the door and the vehicle structure.

Definitions

Air Bag: An inflatable cloth cushion designed to deploy in certain frontal or side impact crashes. It supplements the protection offered by the sealt belt by distributing the impact load more evenly over the occupant's head and torso.

Air Bag Telltale: When igntion is switched On, the system voltage is applied to terminal A1 of the SDM from the IGN 1 fuse in the UHFB. The SDM self-checks the system voltage and deployment loops while the I/P cluster performs a bulb check by flashing the AIR BAG telltale seven times. If a diagnostic trouble code is detected, the SDM will command the I/P cluster to light the AIR BAG telltale when the bulb check is complete. If no DTC is detected, the telltale will not light. The SDM communicates with the I/P cluster on circuit 1807.

B+ (Battery voltage): The battery voltage avaiable at the time of the indicated measurement. With the Key on and the engine not running, the system voltage will likely be between 12 and 12.5 volts. At idle the voltage may be 14 to 16 volts. The voltage could be as low as 10 volts during engine cranking.

Bulb Check: The seven flashes of the Air Bag Telltale whenever the ignition switch transitions from the OFF position to the Run or Start positions. If no SIR malfunctions are detected the air bag telltale will remain off during normal vehicle operation.

Class II: A serail data communications protocol.

Continuous Monitoring: Tests performed by the SDM on the SIR system every 100 milliseconds while Ignition I voltage is present at the SDM.

Current Codes: Current DTCs are malfunctions that are presently bein detected.

Current Source Diagnostics: Tests performed by the SDM where deployment loop voltages are measured wihin their repsective voltage ranges. This test checks for proper resistance of the deployment loops and for proper operation of the driver and passenger current sources.

Curtain Inflator Module: An assembly located on the C-pillar consisting of an inflatable air bag and and inflator (a canister of gas-generated material and an initiator.)

Data Link Connector (DLC): Connector which is connected by wires to multiple on-board control modules allowing communication with an off-board scan tool.

Deploy: To inflate the air bag.

Deployment Loop: The circuits which supply current to the inflator module to deploy the air bag.

Diagnostic Trouble Code (DTC): An alpha numerical designator used by the SDM to indicate specific SIR system malfunctions.

Driver Inflator Module:: An assembly located in the steering wheel hub consisting of an inflatable bag and an inflator (a canisoter of gas-generated material and an initiator).

EEPROM (Electrically Erasable Programmable Read Only Memeory): Memory which retains its contents when power is removed from the SDM.

Higher Priority Fault: Each DTC is assigned a priority based on the detectablility with other DTCs present. The priority corresponds to the detectablility of the malfunction only, and does not relate to the criticality of the malfunction with respect to deployment or non-deployment under any given condition.

History Codes: History DTCs are malfunctions detected since the last clearing of the SDM memory. History DTCs are stored in the SDMs melory, most history DTCs can be cleared using the scan tool Some history DTCs are latched and cannot be cleared.

Ignition Cycle: the voltage at the SDM Ignition 1 input, with the ignition switch on, is greater than 9 volts for at least 10 seconds before turning the ignition switch off.

Ignition 1: A battery positive (B+) circuit which is only powered with the ignition switch in the RUN or START Position.

Initiator: The electrical component inside the inflator module which, with sufficient current flow, starts the chemical reaction that inlfates the air bag.

Knee Bolsters: The knee bolsters are used to absorb energy and control the forward movement of the driver and front passenger during a frontal crash by limiting leg movement.

Normal Operating Voltage Range: The voltage measured betwen the SDM Ignition 1 terminal and ground is between 8 and 16 volts.

Passenger Inflator Module: An assembly located in the right side of the instrument panel consisting of an inflatable bag and an inflator (a canister of gas-generated material and an initiator).

Portable Scan Tool: An off-board computer used to read diagnostic information from on-board computers via a data link connector.

Steering Wheel Coil Assembly (Roll Connector): An assembly of current-carrying coils (two for air bag operation) in the driver deployment loop. The roll connector is attached to the steering column and allows the rotation of the steering wheel while maintaining continuous electrical contact of the driver deployment loop to the driver inflator module.

Sensing and Diagnostic Module (SDM): The SIR control moudle that provides reserve energy to the depolyment loop(s) to deploy the air bag when required, and performs diagnostic monitoring of all SIR system components.

The SDM is designed to perform the following functions in the SIR system:

1. Energy Reserve: The SDM mainains approximately a 23 volt loop reserve (VLR) energy supply to provide deployment current if ignition voltage is lost in a frontal crash.
2. Frontal Crash Detection: The SDM monitorss vehicle acceleration changes to detect frontal crashes which are severe enough to warrant deployment.
3. Side Crash Detection; The SDM monitors vehicle velocity changes, along with SIS information, to detect side impact crashes which are severe enough to warrant deployment.
4. Inflator Module Deployment: When a crash requiring deployment is detected, the SDM provides enough current to the frontal or a side inflator module(s) to deploy the air bag(s).
5. Frontal Crash Recording: The SDM records information regarding the SIR system status when deployment has been commanded.
6. Side Impact System Malfunction Monitoring: The SDM monitors the SIS. The SIS can communicate the status of the side impact air bag system.
7. Malfunction Detection: The SDM performs diagnostic monitoring of SIR system electrical components and sets diagnostic trouble codes (DTCs) and system status information through a scan tool.
8. Driver Notification: The SDM warns the driver of SIR system malfunctions by controlling the AIR BAG telltale.

The SDM is connected to the SIR wiring harness by an 18-way connector and an 8-way side air bag connector. The SDM receives power whenver the ignition is on.

Serial Data: Information representing the status of the queried system and communicated to an off-board computer (Scan tool) through the DLC.

Shorting Bar: There is a shorting bar on the yellow 2-way connector that connects the inflator module to the SIR wiring harness. The shorting bar shorts the circuits to the inflator module during the disconnection of the yellow 2-way connector. The shorting of the inflator module circuitry will help prevent unwanted deployment of the air bag when servicing the steering wheel steering column, insturment panel, or other SIR system components.

Side Impact Sensor (SIS): The SIS is a crash-sensing device used to detect side impact collsions. An SIS is located in the left and right B-pillars. The SIS connects to the SIR wiring harness using a 2-way connector. The SIS receives power from the SDM when the ignition is on. The SIS is used to perform the following functions.

SIR: Supplemental inflatable restraint.

SIR Wiring Harness: The wires and connectors that electrically connect the components in the SIR system.

Steering Column: The steering column is energy absorbing and is designed to compress in a frontal crash to minmize injury to the driver.

Turn-On-Tests: Tests which the SDM performs on the SIR system once during each ignition cycle immediately after Ignition I voltage is applied to the SDM and before "Continuous Monitoring."

VLR Delay (volt loop reserve): Charging of the VLR power supply is inhibited briefly during ignition turn on. This allows detection of certain malfunctions related to Ignition 1 voltage.