See Figure 1
The Supplemental Restraint System (SRS) is designed to work in concert with the seat belts to further prevent personal injury during a head-on collision with another object. The SRS utilizes an air bag module, front impact sensors, a clockspring, and a diagnostic module.
With the battery cables connected, the SRS system is energized and monitoring the front impact sensors and the safing sensor for collision confirmation messages. When the vehicle strikes, or is struck by, another object (such as a tree, wall, another vehicle, etc.), the front impact sensors and safing sensor send impulses to the diagnostic module, which determines the force and direction of the impact. Based on this information the diagnostic module either deploys or does not deploy the air bag.
The only time that the SRS is completely disarmed with no chance of accidental deployment is when the battery cables have been detached from the battery and set aside, and at least 2 minutes have gone by to allow the system capacitor to discharge any residual energy.
Air Bag Module
The air bag module is the most visible part of the system. It contains the air bag cushion and its supporting components. The air bag module contains a housing to which the cushion and inflator are attached and sealed.
The inflator assembly is mounted to the back of the module housing. When supplied with the proper electrical signal, the inflator assembly produces a gas which discharges directly into the cushion. A protective cover is fitted to the front of the air bag module and forms a decorative cover in the center of the steering wheel. The air bag module is mounted directly to the steering wheel.Front Impact Sensors
The driver air bag system is a safety device designed to reduce the risk of fatality or serious injury, caused by a frontal impact of the vehicle.
The impact sensors provide verification of the direction and severity of the impact. Three impact sensors are used. One is called a safing sensor. It is located inside the diagnostic module which is mounted on the floor pan, just forward of the center console. The other two sensors are mounted on the upper crossmember of the radiator closure panel on the left and right side of the vehicle under the hood.
The impact sensors are threshold sensitive switches that complete an electrical circuit when an impact provides a sufficient G force to close the switch. The sensors are calibrated for the specific vehicle and react to the severity and direction of the impact.Clockspring
The clockspring is mounted on the steering column behind the steering wheel and is used to maintain a continuous electrical circuit between the wiring harness and the driver's air bag module. This assembly consists of a flat ribbon-like electrically conductive tape which winds and unwinds with the steering wheel rotation.Diagnostic Module
The Air Bag System Diagnostic Module (ASDM) contains the safing sensor and energy reserve capacitor. The ASDM monitors the system to determine the system readiness. The ASDM will store sufficient energy to deploy the air bag for only two minutes after the battery is detached. The ASDM contains on-board diagnostics and will illuminate the AIR BAG warning lamp in the cluster when a fault occurs.
When working on the SRS or any components which require the removal of the air bag, adhere to all of these precautions to minimize the risks of personal injury or component damage:
Handling A Live Air Bag Module
At no time should any source of electricity be permitted near the inflator on the back of the module. When carrying a live module, the trim cover should be pointed away from the body to minimize injury in the event of accidental deployment. In addition, if the module is placed on a bench or other surface, the plastic trim cover should be face up to minimize movement in case of accidental deployment.
When handling a steering column with an air bag module attached, never place the column on the floor or other surface with the steering wheel or module face down.Handling A Deployed Air Bag Module
The vehicle interior may contain a very small amount of sodium hydroxide powder, a by-product of air bag deployment. Since this powder can irritate the skin, eyes, nose or throat, be sure to wear safety glasses, rubber gloves and long sleeves during cleanup.
If you find that the cleanup is irritating your skin, run cool water over the affected area. Also, if you experience nasal or throat irritation, exit the vehicle for fresh air until the irritation ceases. If irritation continues, see a physician.
Begin the cleanup by putting tape over the two air bag exhaust vents so that no additional powder will find its way into the vehicle interior. Then remove the air bag and air bag module from the vehicle.
Use a vacuum cleaner to remove any residual powder from the vehicle interior. Work from the outside in so that you avoid kneeling or sitting in an uncleaned area.
Be sure to vacuum the heater and A/C outlets as well. In fact it's a good idea to run the blower on low and to vacuum up any powder expelled from the plenum. You may need to vacuum the interior of the car a second time to recover all of the powder.
Check with the local authorities before disposing of the deployed bag and module in your trash.
After an air bag has been deployed, the air bag module and clockspring must be replaced because they cannot be reused. Other air bag system components should be replaced with new ones if damaged.
DISARMING THE SYSTEM
To disarm the SRS, simply detach the negative battery cable from the battery. Isolate the battery cable by taping up any exposed metal areas of the cable. This will keep the cable from accidentally contacting the battery and causing accidental deployment of the air bag. Allow the system capacitor to discharge for at least 2 minutes, although 10 minutes is recommended to allow the dissipation of any residual energy.
ARMING THE SYSTEM
To arm the SRS, reconnect the negative battery cable. This will automatically enable the air bag system.