See Figure 1
The Supplemental Inflatable Restraint (SIR) system offers protection in addition to that provided by the seat belt by deploying an air bag from the center of the steering wheel and dash panel (passenger side air bag, if equipped). The air bag deploys when the vehicle is involved in a frontal crash of sufficient force up to 30° 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 column 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 contains a deployment loop for each air bag and a Diagnostic Energy Reserve Module (DERM). The deployment loop supplies current through the inflator module which will cause air bag deployment in the event of a frontal collision of sufficient force. The DERM 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 DERM 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 DERM, 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 DERM fall outside expected limits, the DERM will indicate a malfunction by storing a diagnostic trouble code and illuminating the AIR BAG lamp.
See Figures 2 and 3
Diagnostic Energy Reserve Module (DERM)/Sensing and Diagnostic Module (SDM)
This component is known as the DERM on 1993-95 vehicles and the SDM on 1996 vehicles. The term DERM will be used in this section. The DERM is designed to perform five main functions: energy reserve, malfunction detection, malfunction recording, driver notification and frontal crash recording.
The DERM 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 DERM 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 DERM 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 DERM 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.Discriminating Sensors
There are two discriminating sensors in the SIR system, the LH forward and RH forward discriminating sensors. These sensors are located on the right frame rail for the RH sensor and the left front frame rail for the LH sensor.
The discriminating sensor consists of a sensing element, diagnostic resistor and normally open switch contacts. The sensing element closes the switch contact when vehicle velocity changes are severe enough to warrant air bag deployment.Arming Sensor
The arming sensor is located on the left hand frame rail. The arming sensor is a switch located in the power side of the deployment loop. It is calibrated to close at low level velocity changes (lower than the discriminating sensors), assuring that the inflator module is connected directly to the 36VLR output of the DERM or Ignition 1 voltage when any discriminating sensor closes.SIR Coil Assembly
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
The inflator module is located on the steering wheel hub. 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.
DISABLING THE SYSTEM
See Figures 4, 5 and 6
ON, the AIR BAG warning lamp will be on. This 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 two way connector located at the base of the steering column.
- Connect the negative battery cable.
ENABLING THE SYSTEM
- Disconnect the negative battery cable.
- Turn the ignition switch to LOCK , then remove the key.
- Engage the yellow SIR connector and CPA located at the base of the steering column.
- Install the steering column filler 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.