See Figure 1
Beginning in 1994, driver's side air bags became standard equipment for W-body vehicles. In 1995, passenger side air bags were implemented on some models. The Supplemental Inflatable Restraint (SIR) system OFF ers protection in addition to that provided by the seat belt by deploying an air bag from the center of the steering wheel or dash panel. The air bag deploys when the vehicle is involved in a frontal crash of sufficient force up to 30º OFF the centerline of the vehicle. To further absorb the crash energy, there is also a knee bolster located beneath the instrument panel in the driver's area and the steering wheel is collapsible.
The system has an energy reserve, which can store a large enough electrical charge to deploy the air bag(s) for up to ten minutes after the battery has been disconnected or damaged. The system MUST be disabled before any service is performed on or around SIR components or SIR wiring.
The SIR system consists of a Sensing and Diagnostic Module (SDM), the driver inflator module, the SIR coil assembly, then passenger inflator module (if equipped) and the AIR BAG warning lamp in the instrument cluster. The SDM, SIR coil assembly (driver side only), driver inflator 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 in the event of a frontal crash of sufficient force, up to 30 degrees OFF the centerline of the vehicle. The inflator modules are only supplied enough current to deploy when the SDM detects vehicle velocity changes severe enough the warrant deployment. The SDM supplies the necessary power, even if the battery has been damaged.
The deployment loop is made up of the arming sensors, coil assembly, inflator module and the discriminating sensors. The inflator module is only supplied sufficient current when the arming sensor and at least one of the two discriminating sensors close simultaneously. The function of the SDM is to supply the deployment loop a 36 Volt Loop Reserve (36VLR) to assure sufficient voltage to deploy the air bag if ignition voltage is lost in a frontal crash.
The SDM, in conjunction with the sensor resistors, makes it possible to detect circuit and component malfunctions within the deployment loop. If the voltages monitored by the SDM fall outside expected limits, the SDM will indicate a malfunction by storing a diagnostic trouble code and illuminating the AIR BAG lamp.
See Figure 2
The SDM is designed to perform five main functions: energy reserve, malfunction detection, malfunction recording, driver notification and frontal crash recording.
The SDM maintains a reserve voltage supply to provide deployment energy for a few seconds when the vehicle voltage is low or lost in a frontal crash. The SDM performs diagnostic monitoring of the SIR system and records malfunctions in the form of diagnostic trouble codes, which can be obtained from a hand scan tool and/or on-board diagnostics. The SDM warns the driver of SIR system malfunctions by controlling the AIR BAG warning lamp and records SIR system status during a frontal crash.Air Bag Warning Lamp
The AIR BAG warning/indicator lamp is used to verify lamp and SDM operation by flashing 7 times when the ignition is first turned ON . It is also used to warn the driver of an SIR system malfunction.SIR Coil Assembly
See Figure 3
The SIR coil assembly consists of two current carrying coils. They are attached to the steering column and allow rotation of the steering wheel while maintaining continuous deployment loop contact through the inflator module.
There is a shorting bar on the lower steering column connector that connects the SIR coil to the SIR wiring harness. The shorting bar shorts the circuit when the connector is disengaged. The circuit to the inflator module is shorted in this way to prevent unwanted air bag deployment when servicing the steering column or other SIR components.Inflator Module
See Figure 4
The inflator module consists of an inflatable bag and an inflator (a canister of gas-generating material and an initiating device). When the vehicle is in a frontal crash of sufficient force to close the arming sensor and at least one discriminating sensor simultaneously, current flows through the deployment loop. Current passing through the initiator ignites the material in the inflator module, causing a reaction which produces a gas that rapidly inflates the air bag.
All vehicles are equipped with a driver's side inflator module located in the steering wheel. If equipped, a passenger side inflator module is located in the dash panel.
See Figure 5
DISABLING THE SYSTEM
See Figures 6 and 7
ON, the AIR BAG warning lamp will be on. The is normal and does not indicate any system malfunction.
- Turn the steering wheel so that the vehicle's wheels are pointing straight ahead.
- Turn the ignition switch to LOCK , remove the key, then disconnect the negative battery cable.
- Remove the AIR BAG fuse from the fuse block.
- Remove the steering column filler panel.
- Disengage the Connector Position Assurance (CPA) and the yellow 2-way connector, located near the base of the steering column.
- If equipped with a passenger side air bag, unfasten the instrument panel compartment door assembly, flip it down to access the connector. Detach the Connector Position Assurance (CPA) and yellow 2-way connector behind the instrument panel compartment door assembly.
- Connect the negative battery cable.
ENABLING THE SYSTEM
See Figures 6 and 7
- Disconnect the negative battery cable.
- Turn the ignition switch to LOCK , then remove the key.
- If equipped with a passenger side air bag, attach the yellow 2-way connector and Connector Position Assurance (CPA) behind the instrument panel compartment door assembly. Secure the instrument panel compartment door assembly.
- Engage the yellow SIR connector and corresponding CPA located near the base of the steering column.
- Install the steering column filler panel/lower trim panel.
- Install the AIR BAG fuse to the fuse block.
- Connect the negative battery cable.
- Turn the ignition switch to RUN and make sure that the AIR BAG warning lamp flashes seven times and then shuts OFF. If the warning lamp does not shut OFF, make sure that the wiring is properly connected. If the light remains on, take the vehicle to a reputable repair facility for service.