For 1990-2009 cars

### Inflatable Restraint Sensing and Diagnostic Module (SDM)

The Sensing and Diagnostic Module (SDM) performs the following functions within the SIR system:

• | Energy Reserve -- The SDM maintains a 23 Volt Loop Reserve (23 VLR) energy supplies in order to provide deployment energy. If ignition voltage is lost during a frontal crash, the 23 VLR supply will deploy the air bags.

• | Frontal Crash Detection -- The SDM monitors the vehicle velocity changes in order to detect frontal crashes which are severe enough to warrant deployment.

• | Air Bag Deployment -- When a frontal crash of sufficient force is detected, the SDM will cause enough current to flow through the inflator modules in order to deploy the air bags.

• | Frontal Crash Recording -- The SDM records information regarding the SIR system status during a frontal crash.

• | Malfunction Detection -- The SDM performs diagnostic monitoring of the SIR system electrical components. The SDM sets a diagnostic trouble code when the SDM detects a malfunction.

• | Malfunction Diagnosis -- The SDM displays SIR diagnostic trouble codes and system status information through the use of a scan tool.

• | Driver Notification -- The SDM warns the vehicle driver of SIR system malfunctions by controlling the AIR BAG warning lamp.

The SDM 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. If you remove of the SDM Connector Position Assurance (CPA) or the harness connector itself, the AIR BAG warning lamp will connect to ground through the shorting bar. The AIR BAG warning lamp will come ON steady with the power applied to the SDM when either one of the following two conditions exists:

• | You remove the SDM CPA

• | You disconnect the SDM harness connector

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

### AIR BAG Warning Lamp

The ignition switch applies ignition voltage to the AIR BAG warning lamp. The AIR BAG warning lamp receives power whenever the ignition switch is at the ON or the START positions. The inflatable restraint SDM controls the lamp by providing ground with a lamp driver. The SIR system uses the AIR BAG warning lamp to do the following:

• | Verify lamp and SDM operation by flashing the lamp seven times when the ignition switch is first turned to the ON position

• | Warn the vehicle driver of any SIR electrical system malfunctions which could potentially affect the operation of the SIR system. These malfunctions could result in non-deployment in case of a frontal crash or deployment for conditions less severe than intended.

The AIR BAG warning lamp notifies the driver of SIR system malfunctions. For proper lamp operation, refer to SIR Diagnostic System Check .

### Inflatable Restraint Steering Wheel Module Coil

The inflatable restraint steering wheel module coil (1) consists of two or more current-carrying coils. These coils are attached to the steering column. Two of the current-carrying coils allow rotation of the steering wheel while maintaining continuous contact (2) of the driver deployment loop to the inflatable restraint steering wheel module.

A shorting bar (5) is placed on the yellow 2-way connector (4) near the base of the steering column. This connector joins the SIR coil to the SIR wiring harness. The shorting bar shorts the circuits to the SIR coil and the inflatable restraint steering wheel module whenever you disconnect the yellow 2-way connector. The circuit to the inflatable restraint steering wheel module is shorted in this way in order to help prevent an unwanted deployment of the air bag when servicing the steering column or other SIR system components.

### Inflator Modules

The inflator modules consist of an inflatable bag (1) and an inflator (4) (a canister of gas-generating material and an initiating device). These components are concealed behind a trim vinyl cover (2), positioned in the center of the steering wheel (3).

When the vehicle is in a frontal crash of sufficient force, the inflatable restraint SDM causes a current to flow through the deployment loops. Current passing through the initiator ignites the material in the inflator module. The gas produced from this reaction rapidly inflates the air bag.

A shorting bar is placed on the inflatable restraint steering wheel module side of the upper steering column connector which connects the inflatable restraint steering wheel module coil to the inflatable restraint steering wheel module. The shorting bar shorts across the inflatable restraint steering wheel module circuits when the upper steering column connector disconnects. The circuit to the inflatable restraint steering wheel module is shorted in this way in order to help prevent unwanted deployment of the air bag while you are servicing the inflatable restraint steering wheel module, the steering column (3), or the other SIR system components.

A shorting bar is placed on the inflatable restraint IP module connector which connects to the SIR wiring harness. This shorting bar shorts across the inflatable restraint IP module circuits when the inflatable restraint IP module connector disconnects. The circuit to the inflatable restraint IP module is shorted in this way in order to help prevent unwanted deployment of the air bag while you are servicing the inflatable restraint IP module, the instrument panel (2), or the other SIR system components.

### Steering Column

The steering column is energy absorbing. The wheel is designed to compress in a frontal crash, decreasing the chance of injury to the driver. A steering column reinforcement is also used for additional support.

### Knee Bolsters

The knee bolsters absorb energy. These bolsters limit leg movement by controlling the forward movement of the vehicle's front seat occupants during a frontal crash.

### Definitions

23 VLR: The 23 volt loop reserve is an energy supply internal to the inflatable restraint SDM.
Air Bag: The air bag is an inflatable cloth cushion designed to deploy in certain frontal crashes. This cushion supplements the seat belt protection by distributing the impact load more evenly over the vehicle occupant's head and torso.
Asynchronous: An operation that is performed in a non-periodic fashion, that is, no defined time or interval.
B+: The voltage available at the battery during the indicated measurement. With the key turned to the ON position and the engine not running, the system voltage will likely be between 12-12.5 volts. At idle the voltage may be 14-16 volts. The voltage could be as low as 10 volts during engine cranking.
Bulb Test: The instrument cluster will flash the AIR BAG warning lamp seven times when the ignition switch is turned from OFF to ON. The inflatable restraint SDM then commands the instrument cluster to turn the warning lamp OFF through serial data communication. This indicates that no malfunctions exist in the SIR system.
Continuous Monitoring Tests: These tests are performed by the inflatable restraint SDM on the SIR system every 100 milliseconds while IGNITION 1 voltage is in the normal operating voltage range at the SDM.
Data Link Connector (DLC): A connector which is connected by wires to multiple on-board computers allowing communication with an off-board computer, such as a scan tool.
Datum Line: A base line parallel to the plane of the underbody or frame. All vertical measurements originate from this line.
Deploy: To inflate the air bag.
Deployment Loop Continuity: This test is performed by the inflatable restraint SDM after IGNITION 1 and 23 VLR are measured within their respective normal voltage ranges. This test checks voltage differences between the Driver Side High/Low and the Passenger Side High/Low circuits.
Deployment Loops: These circuits supply current to the inflator modules in order to deploy the air bags.
Diagnostic Trouble Code (DTC): An alpha-numerical designator which is used by the inflatable restraint SDM in order to indicate specific SIR system malfunctions.
Driver Current Sink: An output of the inflatable restraint SDM which supplies a low resistance path to ground for the inflatable restraint steering wheel module circuit.
Driver Current Source: An output of the inflatable restraint SDM which supplies current into the inflatable restraint steering wheel module circuit.
EEPROM: Electronically Erasable Programmable Read Only Memory is memory which retains its contents when power is removed from the inflatable restraint SDM.
Higher Priority Fault: Each diagnostic trouble code is assigned a priority based on its detectability with other DTCs present. The priority corresponds to the detectability of the malfunction . The priority does not relate to the critical nature of the malfunction with respect to deployment or non-deployment under any given condition.
Ignition Cycle: The voltage at the inflatable restraint SDM IGNITION 1 input, with the ignition switch set to the ON position, is greater that 8.2 volts for at least 10 seconds before turning the ignition switch to the LOCK position.
IGNITION 1: A battery voltage (B+) circuit which is only powered with the ignition switch in the ON or the START positions.
Inflatable Restraint IP Module: An assembly located in the right side of the Instrument Panel (IP) consists of an inflatable bag, an inflator, and an initiator.
Inflatable Restraint Steering Wheel Module Coil: An assembly of two current-carrying coils in the driver deployment loop allows rotation of the steering wheel while maintaining the continuous contact of the driver deployment loop to the inflatable restraint steering wheel module.
Inflatable Restraint Steering Wheel Module: An assembly located in the steering wheel hub consists of an inflatable bag, and inflator, and an initiator.
Initiator: The electrical component inside the inflator module. When sufficient current flows, this component sets off the chemical reaction that inflates the air bag.
Normal Operating Voltage Range: The voltage measured between the inflatable restraint SDM IGNITION 1 terminal and the Ground terminal is between 8.2-17.2 volts.
Passenger Current Sink: An output of the inflatable restraint SDM which supplies a low resistance path to ground for the inflatable restraint IP module circuit.
Passenger Current Source: An output of the inflatable restraint SDM which supplies current into the inflatable restraint IP module circuit.
Resistance Measurement Test: This test is performed by the inflatable restraint SDM after the IGNITION 1, 23 VLR, and deployment loop voltages have been measured within their repetitive normal voltage ranges. This test checks for proper resistance of the deployment loops and for proper operation of the driver and passenger current sources. If an open is detected, this test is aborted before resistances are calculated. However if the open clears, the test will be completed and resistances will be checked.
Scan Tool: An off-board computer used to read diagnostic information from on-board computers via the data link connector.
Serial Data: Information representing the status of the SIR system.
SIR: Supplemental Inflatable Restraint
SIR Wiring Harness: The wires and connectors that electrically connect the components in the SIR system.
Turn-on Tests: The inflatable restraint SDM performs these tests on the SIR system once during each ignition cycle. The tests are performed immediately after the IGNITION 1 voltage is applied to the SDM, and before the Continuous Monitoring Tests.