See Figure 1
The Supplemental Inflatable Restraint (SIR) system helps supplement the protection offered by the driver's seat belt by deploying an air bag from the center of the steering wheel during certain front end collisions. The bag deploys when the vehicle is involved in a frontal crash of a sufficient force within 30 degrees of the centerline of the vehicle. Testing and any needed repairs should only be performed by a professional. Take cautions seriously when working near this system. Lack of care could result in possible air bag deployment, personal injury, or other needed repairs to the SIR system.
See Figure 2
The main portions of the SIR system are the deployment loop and the Diagnostic Energy Reserve Module (DERM). The main function of the deployment loop is to supply current through the inflator module in the steering wheel. This will allow deployment of the air bag in the event of a frontal crash.
The arming sensor, SIR coil assembly, inflator module, and discriminating sensors make up the deployment loop. The arming sensor switches power to the inflator module on the high side of the loop. Either of the sensors can supply ground to the inflator module on the low side of the loop. The module is only supplied sufficient current to deploy when the arming sensor and at least one of the two discriminating sensors are closed simultaneously.
Diagnostic Energy Reserve Module (DERM)
The DERM is designed to perform the following functions in the SIR system:
- Energy Reserve - The DERM maintains a 36 Volt Loop Reserve (36 VLR) energy supply to provide deployment energy when the cars voltage is low or lost in a frontal collision.
- Malfunction Detection - The DERM performs diagnostic monitoring of the SIR system electrical components.
- Malfunction Recording - The DERM provides the SIR system diagnostic trouble code information through a scan tool.
- Driver Notification - The DERM warns the driver of SIR malfunction by controlling the AIR BAG warning lamp.
- Frontal Crash Recording - The DERM records the SIR system status during a frontal crash.
The DERM is connected to the SIR wiring harness by a 24-way connector. The harness connector uses a shorting bar across certain terminals in the contact area. The shorting bar connects the AIR BAG warning lamp to ground when the DERM harness connector is unplugged. This will cause the AIR BAG lamp to come on steadily whenever the ignition is in the RUN or START position with the DERM disconnected.Air Bag Warning Lamp
Ignition voltage is applied to the AIR BAG warning lamp when the ignition switch is at the RUN or START positions. The DERM controls the lamp by providing ground with a lamp driver. The lamp is used in the SIR system to do the following:
- Verify the lamp and DERM operation by flashing seven times when the ignition switch is ON .
- Warn the driver of SIR electrical systems malfunctions which could potentially affect the operation of the SIR system. These malfunctions could cause a non-deployment of the air bag in a frontal crash, or deployment in cases less severe than intended.
The arming sensor is a protective switch that is located in the high side of the deployment loop. The sensor is calibrated to close at low level velocity changes, lower than the discriminating sensors. This will ensure that the inflator module is connected directly to the 36 VLR output of the DERM or ignition 1 voltage when either of the sensors closes.
The arming sensor consists of a sensing element, normally open switch contacts, two diagnostic resistors, and two diodes. These sensing elements close the switch contacts when the velocity of the vehicle changes at a rate that indicates a potential need for deployment.
The 51K ohm diagnostic resistor is connected in parallel with the switch contacts allowing a small amount of voltage to pass operation. This current flow results in voltage drops across each component within the deployment loop. The DERM monitors these drops to detect circuit or component malfunctions. The 2.49K ohm diagnostic resistor is connected in parallel with the diodes allowing the DERM to monitor the voltage applied to the high side of the deployment loop. These two diodes in the arming sensor provide an isolation between the 36 VLR output of the DERM and IGNITION 1 voltage.SIR Coil Assembly
See Figure 3
The SIR coil assembly consists of two current carrying coils, which are attached to the steering column. The coils allow the rotation of the steering wheel while maintaining continuous contact of the deployment loop to the inflator module.
See Figure 4
The inflator module is located on the steering wheel hub. The module consists of an inflatable bag and an inflator which is a canister of gas generating material and an initiating device. When the car is in a frontal crash of sufficient force. Current passing through the initiator ignites the material in the inflator module. The gas produced from this reaction quickly inflates the air bag.
DISARMING THE SYSTEM
See Figure 5
- To disable the SIR system, turn the steering wheel so that the vehicles wheels are pointing straight ahead.
- Turn the ignition key to the LOCK position and remove the key.
- Remove the AIR BAG fuse from the fuse block.
- Unsecure the left sound insulator by removing the screws and nuts.
- Remove the Connector Position Assurance (CPA) and the yellow two-way connector from the base of the steering column.
When the AIR BAG fuse is removed from the fuse block, and the key is in the ON position, the AIR BAG warning lamp will be ON. This is a normal condition and does not mean that the SIR system is malfunctioning.
ENABLING THE SYSTEM
- Engage the yellow two-way connector to the base of the steering column and the Connect Position Assurance (CPA).
- Install the left sound insulator with the mounting screws and nuts.
- Insert the AIR BAG fuse in the fuse block.
- Turn the ignition switch to the RUN position and verify that the AIR BAG warning lamp flashes 7 times and then turns off. If it does not turn off have your SIR system checked by an authorized service center.