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.

System Components

Typical air bag systems consist of three components: crash sensor(s), a diagnostic module and air bag module(s).

Crash Sensors

The purpose of a crash sensor is to detect a sudden deceleration of the vehicle and to close an electrical circuit if the sensor determines the severity of the impact warrants deployment of the air bag(s). Crash sensors generally fall into one of four design types, three electro-mechanical and one electronic.

Crash sensor designs include:

• A ball and magnet design that allows a ball to roll forward to complete an electrical circuit by touching two contacts. The size of the magnet calibrates the sensor (Figure 1).
• A spring band and roller design that allows the roller to move forward and close a contact when tension of a spring band is overcome. The spring band could be compared to a roll of carpet unwinding (Figure 2).
• A rotating weight design that allows a weight to move a rotor against spring tension to a point where contacts complete the circuit. The tension of the spring and size of the weight calibrate the sensor.
• An electronic accelerometer that is located inside the diagnostic module, which is mounted in the passenger compartment. This type of sensor is found on most late-model vehicles.
• Air bag systems may use one mounting location for crash sensor(s) inside the diagnostic module (single-point system), or up to five remotely mounted sensors (multi-point system).

  Single-point systems generally use an accelerometer located inside the diagnostic module, which is mounted in the passenger compartment. In addition, some signal-point systems include both an electro-mechanical sensor along with the accelerometer. Again, both sensors would be mounted internally in the diagnostic module (Figure 3).

  Multi-point systems use both the accelerometer (mounted in the diagnostic module) in combination with remotely mounted electro-mechanical sensor(s), or they employ only remotely mounted sensors. Remote sensor locations may include the radiator support, fender apron, cowl, or dash areas.

Diagnostic Module

The diagnostic module performs several functions, including:

• Continuously monitoring the air bag system electrical circuits.
• Controlling operation of the “Air Bag” or “Inflatable Restraint” indicator lamp in the instrument cluster to alert the driver of a detected fault.
• Recording, storing and providing diagnostic information for a technician.
• Providing energy reserves that supply backup power in case the vehicle's system power is low or lost during a crash.

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:

1. Removing the ignition key.
2. Disconnecting and taping the battery cables.
3. Removing the “Air Bag” or “SIR” fuse.
4. Waiting a specific time for the reserve power to deplete. (Note: some models require the backup power module to be disconnected.)
5. Disconnecting the connector to the air bag module(s).

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.

Future Innovations

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.

Keith Reinhardt is an assistant professor of automotive technology at Southern Illinois University Carbondale in Carbondale, Ill. He holds a master's degree in educational administration and is an ASE-certified master automotive technician/paint and refinishing. His e-mail address is kvette@siu.edu.

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