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Air Bags: How They WorkPosted 12/10/2001
By Keith Reinhardt
Like our modern military, automotive air bags must maintain a constant state of readiness to protect citizens from harm. If one component of the system does not perform as designed, the end results can lead to both catastrophic physical harm to a vehicle's occupants and possible legal liability to those who have performed service to the vehicle's air bag system. For these reasons, it is critical that today's automotive technicians thoroughly understand the operation and approved repair processes associated with one of the most technological subsystems on the modern automobile, air bags.
How Air Bags Work
To have a frontal (driver or passenger) air bag deploy, certain criteria must be met. One of these criteria is that the frontal impact must be within a 60-degree window, occurring within 30 degrees from the vehicle's centerline. Another key element is that the crash forces are equivalent to a head-on collision with an immovable barrier at 10-15 miles per hour. Because a typical automobile accident only lasts about 0.125 (1/8th) of a second, air bags deploy within 15 to 20ms after the initial crash impact. To create a protective cushion between the occupant and the vehicle's interior, the air bag inflates at speeds up to 200 mph within approximately 30ms after impact. This allows the occupant to contact a fully inflated bag within approximately 45 to 50ms after the initial crash impact. Approximately 100ms after impact the bag deflates.
The entire deployment and deflation cycle takes place in less than one second.
Typical air bag systems consist of three components: crash sensor(s), a diagnostic module and air bag module(s).
Crash sensor designs include:
The diagnostic module performs several functions, including:
A misconception of earlier multi-point systems using electromechanical crash sensors is that the diagnostic module causes deployment of the air bag(s). On these systems, deployment of the air bag module(s) is directly activated by the crash sensors' contacts closing. In contrast, in a single-point system the accelerometer does not directly deploy the air bag(s), but only determines the threshold point for deployment. The diagnostic module's processor commands deployment. Be aware that some of these modules may contain a fuse and may also require a case ground for proper operation.
Air Bag Module
The air bag module is the most visible part in the system. It contains a base plate for mounting, an inflator module, an air bag, a cloth cushion with a lining coated with corn starch to keep an undeployed bag from sticking to itself, a trim cover and electrical connectors.
Most air bags are inflated using some type of pyrotechnic device. The most common inflation technique involves using current (approximately 750ma) to energize an igniter that then starts the burning of sodium azide pellets. The combustion of sodium azide creates nitrogen gas to fill the air bag. Most air bag modules offer a circuit resistance of approximately 1 to 3 ohms to the electrical circuit. Because the air bag module is ignited by means of current flow, an ohmmeter should NEVER be placed across the connector of an air bag module. For diagnostic purposes during air bag system service, a load tool, also known as an air bag simulator, must be used to provide a substitute resistance for the air bag module(s) (Figure 4). Note that Ford's load tool is the much smaller orange tool with the connector on the bottom.
Many vehicle manufacturers use a shorting bar clip on the air bag module that automatically shorts the circuit when the connector is disconnected (Figure 5). Some technicians have experienced a malfunction that occurs if the shorting bar does not release properly when the air bag module is reconnected. The shorting bar continues to short-circuit the wires to the air bag module. The diagnostic module will see this as a faulty air bag module due to a low resistance. The diagnostic module will then illuminate the Air Bag light and record a trouble code. A Service Precaution
Whenever you are performing service on or near the air bag system components or system wiring, to prevent accidental deployment, you must disable the system. Keep in mind, to deploy an air bag, electrical current must flow through the air bag module. For this reason, if the air bag module is disconnected and isolated from the electrical circuit, it cannot deploy. This is especially important to remember because if the ignition switch is left on, or the reserve power is still being supplied to the crash sensors, an inadvertent jolt or improper positioning of a crash sensor could cause a deployment.
In general, most air bag systems can be disabled by:
Common Diagnostic Issues
Unless the diagnostic module has the ability to flash codes, most air bag systems today require the use of a scan tool to access Diagnostic Trouble Codes. Because OEM-installed air bag systems differ in design and operation, even on like platform vehicles, you may find cases where the same service procedures do not apply to different vehicles of the same line and model year. Always refer to the appropriate manual.
Improvements in air bag operation are still under way. The so-called smart air bags will consider such factors as vehicle speed, seatbelt use, occupant weight and seat position. As a result, air bags will become safer for children and small adults. Other new innovations will include multi-stage air bags that will vary inflation rates, and side impact/head curtain air bags that are engineered to minimize risk to out-of-position occupants.
Remember, as automotive repair technicians, we have an ethical responsibility to maintain the air bag system's readiness to do its intended task of protecting vehicle occupants from injury. Putting all air bag system repairs into perspective, we are dealing with peoples' lives.
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