The air bag or Supplemental Restraint System (SRS) is a safety device that is designed to be used in conjunction with the seat belt. Its purpose is to help protect the driver in a frontal impact exceeding a certain set limit. The system consists of the air bag module, three impact sensors, a clockspring and a dedicated air bag control module.
The air bag is a fabric bag or balloon with an explosive inflator unit attached. The system employs impact sensors and a "safing" sensor, as well as an inflator circuit and control module.
When the control unit receives the sensor signals, power is supplied to the inflator circuit, either from the battery or backup system. A small heater causes a chemical reaction in the igniter; the non-toxic gas from the chemical mixture expands very rapidly (in milliseconds), filling the bag and forcing it through the cover pad. Since all this is happening very rapidly, the expanding bag should reach the occupant before he/she reaches the steering wheel/dashboard during a frontal collision. The chemical reaction is complete by the time the air bag is fully inflated; as the occupant hits the bag, the gas is allowed to escape slowly through vents in the back of the bag.
Air Bag Module
The driver's air bag module is mounted directly to the steering wheel beneath a protective cover. Under the air bag module protective cover, the air bag cushion and its supporting components are contained. The air bag module contains a housing to which the cushion and inflator are attached and sealed. The air bag module is non-repairable. If it is dropped or damaged, it must be replaced.
The inflator assembly is mounted to the back of the module. The inflator seals the hole in the air bag cushion so it can discharge the gas it produces directly into the cushion when supplied with the proper electrical signal. Upon deployment, the protective cover will split horizontally.
The passenger's side air bag door on the instrument panel above the glove box is the most visible part of the passenger side air bag system. This system includes an extruded aluminum housing within which the cushion and inflator are mounted.
The module cannot be repaired. Following a deployment, the air bag module and instrument panel assembly must be replaced.
A passenger side air bag on/off switch is fitted to all models except the quad cab.Impact Sensors
The three impact sensors used in the SRS system verify the direction and severity of an impact. One of the sensors is called the safing sensor. It is located in the Air Bag Control Module (ACM), which is mounted to a bracket under the instrument panel, on top of the floor pan transmission tunnel. The other two are impact sensors and are mounted on the left and right inner fender extension panels behind the grille. The sensors are calibrated for the particular vehicle that they serve.
The impact sensors are threshold-sensitive switches that complete an electrical circuit when an impact provides a sufficient deceleration force to close the switch. The safing sensor is an accelerometer that senses the rate of deceleration. The microprocessor in the ACM monitors the sensor signals. A pre-programmed decision algorithm in the microprocessor determines when the deceleration rate indicates an impact that is severe enough to require air bag system protection.
The two impact sensors are available for service replacement. The safing sensor is only serviced as part of the ACM.Clockspring
The clockspring is mounted on the steering column behind the steering wheel. Its purpose is to maintain a continuous electrical circuit between the wiring harness and the driver's side air bag module. This assembly consists of a flat, ribbon-like electrically conductive tape that winds and unwinds with the steering wheel rotation.
The air bag control module (ACM) is mounted on the transmission tunnel beneath the dashboard.
The ACM contains the safing sensor, and a microprocessor that monitors the air bag system to determine readiness. It also monitors the impact sensors to determine when the proper conditions exist to provide the electrical signal that deploys the air bag. The ACM contains On-Board Diagnostics (OBD), and will light the air bag warning lamp on the instrument panel if a (monitored) air bag system fault occurs. If the light does not come on, does not go out or comes on when driving, the system must be diagnosed and repaired by your dealer or reputable shop. The system is NOT repairable at home.
The ACM also contains an energy-storage capacitor. The capacitor stores enough electrical energy to deploy the air bag for up to two minutes following a battery disconnect or failure. The purpose of the capacitor is to provide air bag system protection in a severe secondary impact if the initial impact somehow damaged or disconnected the battery, but did not deploy the air bag.
This unit is not serviceable and must be replaced if damaged or defective.
DISARMING THE SYSTEM
- First read the system precautions.
- Disconnect and isolate the negative battery cable(s).
- If the air bag module is NOT deployed, wait at least two minutes for the system capacitor to discharge.
ARMING The SYSTEM
Assuming that the system components (air bag control module, sensors, air bag, etc.) are installed correctly and are in good working order, the system is armed whenever the battery's positive and negative battery cables are connected.
HANDLING A LIVE 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 (such as when removing the steering wheel), 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.
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.Clean-Up Procedure
Begin the clean up 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 a unclean 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 vehicle a second time to recover all of the powder.
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 must also be replaced if damaged.