About I-CAR - Current Events/News: e-newsletters: Advantage Online: August 30, 2004

August 30, 2004 -When working on a collision-damaged vehicle there is often a risk factor involving undeployed airbags. Simply disconnecting the battery does not eliminate that risk. Late model vehicles can have airbags in several locations. What can collision repair technicians do to protect themselves?

Inflating With Sodium Azide Or Compressed Gas

Figure 1 - Sodium azide is a solid pellet type fuel housed within an airbag assembly.

Figure 2 - Compressed gas inflators have exhaust ports for the escaping gas to inflate the airbag.

When front driver airbags were first developed, the main inflator propellant used was sodium azide. With these types of inflators, an electrical signal causes the pellets to begin burning, creating nitrogen gas, which deploys and inflates the airbag (see Figure 1). Sodium azide was also the propellant for the first front passenger airbags.

Today, many airbags are deployed by compressed gas cylinders. A compressed gas inflator operates by sending an electrical signal from the restraint system control module to a small disk (performing a function similar to a soda bottle cap) on the gas inflator. The disk burns and the escaping compressed gas inflates the airbag (see Figure 2). Compressed gas cylinders may be stored in a compressed state upwards of 435 kPa (3,000 psi). Puncturing the cylinder will cause a high burst of high-pressure gas that could result in flying metal, plastic, or glass. This can occur even if the airbag system is disabled.

One of the reasons for choosing compressed gas as a propellant is the space restriction where these airbags are located. The compressed gas inflators are the shape of a long cylinder. With this design, the inflator can be positioned inside the instrument panel, seat back, door panel, or along any rail or pillar of the vehicle. Another concern with sodium azide is heat. When sodium azide burns, it generates a great amount of heat. If located in the soft trim areas of a vehicle, fires may be caused by airbag deployment. Compressed gas allows airbag deployment temperatures to be kept lower.

Airbag Locations

Front airbags are located in the steering wheel for the driver and the passenger-side instrument panel for the passenger. Side airbags are located either in the doors, seat backs, or pillars. Side curtain airbags are dropped down from the headliner along the roof rail. Knee airbags deploy from the knee bolster.

Airbag Removal And Storage

If it is necessary to remove an undeployed airbag, it should be deactivated. To deactivate and remove an airbag module and inflator, the 12-volt electrical system of the vehicle must be de-energized by following the vehicle maker’s specific procedures. Not all passive restraint systems de-energize the same. Depending on the discharge rate of the capacitors used within the system, they may discharge as fast as 30 seconds or it make take a number of minutes before airbag removal can safely occur.

Figure 3 - To limit the chance of injury during accidental deployment; keep airbags facing away from your body.

Figure 4 - When removing a seat that contains an undeployed airbag, ground the seat to vehicle ground.

Figure 5 - Carry an undeployed side curtain so that if activated, it would deploy out or downward.

Deactivating an airbag assembly or even disconnecting the airbag connectors does not eliminate the risk of deployment. Since only a small amount of current is required to activate an airbag circuit, static electricity could still deploy an airbag. The best way to avoid static electricity buildup is to handle the airbag like a computer module. Ground yourself with an anti-static discharge strap, attaching the strap to vehicle ground. Once the airbag is out of the vehicle, remove the wrist strap and carry the airbag so that it would deploy away from your body (see Figure 3).

When removing and installing a seat, the technician or the seat should be grounded to prevent static electricity from deploying the airbag (see Figure 4). Do not hold the seat with your hands over the undeployed side airbag, and carry the seat so that the undeployed airbag is away from your body.

Hold an undeployed side curtain airbag so that if deployed, it would fold out or downward, not toward you (see Figure 5).

The undeployed airbag, or seat, should be stored in a safe location where it is not subjected to conditions that could cause an airbag module to deploy, such as stray voltage or static electricity.

Single-Stage Or Dual-Stage

The driver and passenger airbag have the potential to be a dual-stage design. This means that even though the airbag is deployed, there may still be a live charge inside the airbag module that could be deployed.

For technicians, two ways to identify if an airbag is a two-stage design is to refer to the vehicle maker service manual or when the airbag is removed, look for two wire connectors attached to the airbag module. All dual-stage airbags have two connectors, and single stage airbags have one connector going to the airbag module assembly.

Conclusion

With resources and knowledge, collision repair technicians can safely perform tasks on vehicles equipped with compressed gas airbag inflators and other passive restraint devices.

To learn more about occupant restraints and protection devices in today's vehicles such as pop-up roll bars, seat belt pre-tensioners, dual-stage airbags, and Occupant Classification Systems (OCS), look for the new I-CAR program Advanced Restraint Systems (RES02) coming to your area soon.

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