Air Bag (Supplemental Restraint System)

General Information

The Air Bag system or Supplemental Restraint System (SRS) is designed to provide additional protection for front seat occupants when used in conjunction with a seat belt. The system is an electronically controlled, mechanically operated system. The system contains two basic subsytems: the air bag module(s) (the actual air bag(s) themselves), and the electrical system. The system consists of:

The crash sensors
The safing sensor
The air bag module(s)
The diagnostic monitor
The instrument cluster indicator
The sliding contacts (clock spring assembly)

The system is operates as follows: The system remains out of sight until activated in an accident that is determined to be the equivalent of hitting a parked car of the same size and weight at 28 mph (40 km/h) with the vehicle receiving severe front end damage. This determination is made by crash and safing sensors mounted on the vehicle which when an sufficient impact occurs, close their contacts completing the electrical circuit and inflating the air bags. When not activated the system is monitored by the air bag diagnostic monitor and system readiness is indicated by the lamp located on the instrument cluster. Any fault detected by the diagnostic monitor will illuminate the lamp and store a Diagnostic Trouble Code (DTC).

Air Bag Module(s)

Exploded view of the air bag module mounting—driver's side

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The air bag module(s) are located in the steering wheel and the passenger side of the instrument panel, if equipped with a passenger side air bag. The modules are molded to fit into their designated areas, the driver's side is located in the center of the steering wheel. The passenger side module is molded to fit into the instrument panel and is located above the glove box, it is generally unnoticeable until deflated.

The module contain four parts: an inflator, a bag, a container, and the cover. The purpose of the inflator is to generate the gas needed to fill the air bag, it consists of a high strength steel casing containing a propellant that is activated by an igniter. The igniter is fired by the electrical signal received when the safing sensor closes. When the igniter is fired, the propellant discharge is ignited and fills the bag. As the bag fills, invisible "split seams" in the cover tear open allowing the bag to inflate and cushion the forward motion of the occupant and protect against serious injury.

Diagnostic Monitor

The air bag diagnostic monitor continually checks the SRS system to detect any trouble or fault

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The diagnostic monitor is located behind the center of the instrument panel, under the radio. The diagnostic monitor serves no purpose in the firing of the air bag(s). The main purpose of the diagnostic monitor is to monitor the operational status of the system through continual checks of the system's circuits. In the event of a fault detected, the diagnostic monitor will illuminate the air bag indicator and flash out a DTC. All codes are double digit and if a code is flashed, it can be read as follows: a flash-flash-flash, one-second pause, flash-flash, three-second pause, is a code 32. If numerous codes are stored, they will flash after the three second pause, if only one code is stored, the same flash sequence will begin again. In the event of a air bag indicator bulb failure, an audible tone will be emitted from the diagnostic monitor. this tone is a series of five sets of five beeps, this is code 55 indicating the indicator bulb is out and in addition the air bag system may require service.

The diagnostic monitor also contains the back-up power supply. In the event of a impact severing the battery or other electrical sources to the air bag system, the back up power supply contains enough power to deploy the air bag(s). The diagnostic monitor contains several small capacitors that store power. The back-up power supply must be discharged before air bag service can be performed.

Crash Sensors

Air bag firing circuit diagram

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The crash sensors are located in the front of the vehicle on either side of the front radiator support. In the event of a head-on impact (or at least a ³/₄ frontal impact, the sensing mass inside the sensor breaks away from the bias magnet. The sensor mass rolls along a cylinder toward two electrical contacts, if the deceleration is sufficient, the sensing mass will bridge the contacts and complete the primary deployment circuit to the air bag module.

The mounting and orientation of the sensors is vital to system operation. The sensors should be inspected by a qualified individual after any impact where the sensors could have been damaged. If any structural damage is evident, the area must be repaired to its original condition, otherwise the sensor orientation could be compromised. If the sensor(s) have received damage, they must be replaced.

Safing Sensor

The safing sensor is located behind the kick panel on the passenger side, below the instrument panel. The safing sensor operates identical to the crash sensor, except for the calibration. This is accomplished using a slightly weaker bias magnet. If the safing sensor contacts close simultaneously with the crash sensor, the air bag(s) will be deployed.

The safing sensor is essentially a safety for the air bag system. It protects the bag(s) from deploying due to an electrical short. The safing sensor is used to verify the force of a collision and complete the air bag(s) deployment. The sensor is located inside the passenger compartment so that a forceful enough impact could be detected, and a false signal from the crash sensor would not deploy the air bag(s) at an inappropriate time.

Air Bag Indicator

The air bag indicator is located on the instrument cluster and illuminates when activated by the diagnostic monitor. The indicator illuminates when the key is placed in the ignition and turned to RUN, this is the bulb prove-out. The indicator will stay illuminated after the vehicle is started while the diagnostic monitor checks the system for faults, if no faults are detected, the indicator will go out after about 6 seconds. If the indicator does not go out after 6 seconds, a fault has been detected in the system, a DTC will be flashed using the indicator.

Sliding Contact

The clock spring assembly is located in the steering column, behind the steering wheel. The function of the clock spring assembly is to keep the electrical connection intact while the steering wheel is rotated while the driver is turning the wheel to steer the vehicle.

System Service

CAUTION

Some vehicles are equipped with an air bag system. The system must be disabled before performing service on or around air bag system components, steering column, instrument panel components, wiring and sensors. Failure to follow safety and disabling procedures could result in accidental air bag deployment, possible personal injury and unnecessary SRS system repairs.

SERVICE PRECAUTIONS

Typically a warning label will be found on the vehicle regarding the air bag system

Always carry a live air bag module with the bag and trim cover facing away from your body

Always place an air bag module on a table or other flat surface with the bag and trim cover pointing up

Whenever working around, or on, the air bag supplemental restraint system, ALWAYS adhere to the following warnings and cautions.

Always wear safety glasses when servicing an air bag vehicle and when handling an air bag module.
Carry a live air bag module with the bag and trim cover facing away from your body, so that an accidental deployment of the air bag will have a small chance of personal injury.
Place an air bag module on a table or other flat surface with the bag and trim cover pointing up.
Wear gloves, a dust mask and safety glasses whenever handling a deployed air bag module. The air bag surface may contain traces of sodium hydroxide, a by-product of the gas that inflates the air bag and which can cause skin irritation.
Ensure to wash your hands with mild soap and water after handling a deployed air bag.
All air bag modules with discolored or damaged cover trim must be replaced, not repainted.
All component replacement and wiring service must be made with the negative and positive battery cables disconnected from the battery for a minimum of one minute prior to attempting service or replacement.
NEVER probe the air bag electrical terminals. Doing so could result in air bag deployment, which can cause serious physical injury.
If the vehicle is involved in a fender-bender which results in a damaged front bumper or grille, have the air bag sensors inspected by a qualified automotive technician to ensure that they were not damaged.
If at any time, the air bag light indicates that the computer has noted a problem, have your vehicle's SRS serviced immediately by a qualified automotive technician. A faulty SRS can cause severe physical injury or death.

DISARMING THE SYSTEM

CAUTION

The air bag system must be disarmed before performing service around air bag components or wiring. Failure to do so may cause accidental deployment of the air bag, resulting in unnecessary repairs and/or personal injury.

Position the vehicle with the front wheels in a straight ahead position.
Disconnect the negative battery cable.
Disconnect the positive battery cable.
Wait at least one minute for the air bag back-up power supply to drain before continuing.

ARMING THE SYSTEM

Connect the positive battery cable.
Connect the negative battery cable.
Stand outside the vehicle and carefully turn the ignition to the RUN position. Be sure that no part of your body is in front of the air bag module on the steering wheel, to prevent injury in case of an accidental air bag deployment.
Ensure the air bag indicator light turns off after approximately 6 seconds. If the light does not illuminate at all, does not turn off, or starts to flash, have the system tested by a qualified automotive technician. If the light does turn off after 6 seconds and does not flash, the SRS is working properly.