The Supplemental Restraint System (SRS) is used on 1990 and later Pulsar and Sentra models. In the event of a substantial front end collision, the SRS is designed to provide additional protection for the driver, and on 1995 and later models the front seat passenger. The SRS assists the normal seat belt restraining system by deploying an air bag assembly, via the steering wheel or passenger side dash pad.
The SRS utilizes an air bag module, front impact sensors, a spiral cable, a warning lamp, and a control module. All SRS wiring and connectors are yellow coated to identify them; never probe or disconnect any SRS electrical circuit without first disabling the SRS system.
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 signals 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(s).
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-side 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.Spiral Cable
The spiral cable 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, G-sensor, CPU 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 disconnected. The ASDM contains on-board diagnostics and will illuminate the AIR BAG warning lamp in the cluster when a fault occurs.
All SRS electrical wiring harnesses and connectors are covered with YELLOW outer insulation. Do not use electrical test equipment on any circuit related to the SRS (air bag) sensors. When installing SRS components, always install with the arrow marks facing the front of the vehicle.
Several precautions must be observed when handling the inflator module to avoid accidental deployment and possible personal injury.
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 spiral cable 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 system turn the ignition switch to OFF
position. Then disconnect the both battery cables starting with the negative cable first and wait at least 10 minutes after the cables are disconnected. Be sure to insulate the battery terminal ends.
ARMING THE SYSTEM
To arm the SRS system turn the ignition switch to OFF position. Connect the both battery cables starting with the positive cable first.
The SRS or air bag system is equipped with a self-diagnostic operation. After turning the ignition key to the ON or START position, the AIR BAG warning lamp will illuminate for 7 seconds. After 7 seconds, the AIR BAG lamp will extinguish if no malfunction is detected. If the AIR BAG lamp does not extinguish after 7 seconds, check the SRS self diagnostic system for a malfunction.