Air Bags and the Collision Industry

More than just a bag of hot air?Air bags. What can you say? They've been praised and ridiculed, saved lives and cost lives, been promoted and protested, caused lawsuits because vehicles had them and caused them because they didn't have them-and on and on.As a collision repair technician, how does all this affect you? Well…it doesn't. Regardless of everything else, you still have to work on and around the things day in and day out. So what's best for you to do? Learn what you are and aren't supposed to do and find out how to work safely around them.In the collision repair industry, air bags come in two basic varieties: those that are already deployed and wrapped in wrinkled sheet metal and those that aren't deployed. We'll cover the things techs need to know about each kind a little later, but first, here are a few words about how air bags work.Air Bags at WorkThe majority of air bag systems have at least two deceleration sensors, one or two air bags, a wiring coil for the driver's side, some shorting bars and an electronic module.When the sensors detect a predetermined rate of deceleration-usually from hitting another vehicle-one of the sensors applies a voltage to the air bag and the other supplies a ground. This closed circuit sends current through a heating element, which ignites a substance called sodium azide. This burns very quickly and produces a large amount of nitrogen gas that inflates the air bag. This happens in twenty-thousandths of a second. Immediately after the air bag deploys, all of the nitrogen escapes through holes in the back of the bag and allows it to deflate.Now, a word about sodium azide, which is the same substance used in the solid rocket boosters for the space shuttle. In essence, it is rocket fuel. That may be a little unnerving to some people, but it is necessary to use that sort of substance to make the system operate fast enough. (We'll address some frequently asked questions about this substance a little later.) There are alternative systems, some using pressurized gas, that may eventually replace sodium azide. But working around the substance poses no risk to repair technicians who apply proper safety precautions.Another characteristic of air bag systems that technicians need to be aware of involves the control modules. In an accident, the voltage side sensor and the ground side sensors could close and provide completed circuits to both inflator modules (air bags). This type of system does not require the control module to be connected in order for the air bags to deploy. It is used in this application only for codes and to provide auxiliary voltage in case the battery is damaged in the initial impact. Be aware that disconnecting the control module on this type of system does not prevent accidental deployment.But this application poses its own potential problems. Because the module contains both sensors, striking the module with a hammer or dropping it a few inches can cause the air bags to deploy. Avoid the headaches (pun intended):Disconnect the control module any time you need to move it or work around it. But remember, as we saw in the first type, disconnecting the module does not eliminate all possibility of unintended deployment.Disabling the Air Bag SystemSo what is the proper way to disable an air bag system? Some service publications will tell you to disconnect the system's electronic control module (because they have voltage storage) and the battery, then wait a certain number of minutes before working around the system. But you can also go the direct route. Removing the available power sources is a good idea, but the best precaution is to find the air bag inflator module connector and separate the mating halves. This connector is usually located near the bottom of the steering column or the glove box.Notice that when you do this, a small metal wire now contacts both of the terminals going to the air bag inflator. This is called a shorting bar, and it helps to ensure that the bag isn't accidentally deployed as a result of electrostatic discharge (ESD) from a technician's hands. Inflator connectors have a design that moves the shorting bars out of the way when the two halves are mated. During normal operation, it requires at least 7V to activate an air bag at the necessary speed. However, a lower voltage-even static voltage- can cause a bag to deploy if given time.An additional word of caution might be useful here: Just because a vehicle has two blown air bags does not necessarily mean there isn't a need to disconnect anything. Check to see if the vehicle is equipped with side air bags and, if so, disconnect those before proceeding. Side bags have their own modules and sensors and work independently from the front bags. Banging on a door may well bring an unanticipated surprise, as well as interrupting your otherwise regular heartbeat.TestingSo what is the proper way to test an air bag? The first thing to keep in mind is to never apply a test light to an air bag circuit. It doesn't matter if the light is powered or not. I personally witnessed a case where a technician stated that a test light could not deploy an air bag and then proceeded to do just that. (But that's OK. They're only a few hundred bucks a piece.) If you need to perform electrical tests on an air bag system, use only a high-impedance digital volt-ohmmeter (DVOM).The only test involving the inflators themselves is a resistance check (usually about 2 ohms) of the igniter or squib.After that, test the rest of the electrical wires and components just as you would any other electrical system, except with the bags themselves disconnected. The wiring should show continuity, but the sensors will have resistive values (never zero) for their internal resistors, and coils typically run in the 1/2 ohm to 1 ohm range. All of these values should be given in available service material.Scan tools are also useful in providing diagnostic code information for troubleshooting air bag units. Many control modules will store information not only on codes but also on problems that may be occurring intermittently, the recent status of the air bag light, and whether or not the on/off passenger bag switch was activated. Different manufacturers have varying amounts of scan tool data available, so it behooves the technician to look through the data for assistance in diagnostics, rather than relying solely on codes.In addition to scan tools, some applications have "load tools" or "dummies" available that can be inserted in place of either air bag or to replace a bag and coil combination. Load tools have the same resistance as the air bag or bag and coil combination, and simulate the air bag to allow scan tool testing of the system. These test units can be relatively inexpensive and can greatly assist in system diagnostics.Repair/ReplacementWhen it comes to repairing a post-crash air bag system, the first thing a technician needs to consider is the manufacturer of the vehicle. Some OEMs require every component of the system to be replaced if a vehicle is in an accident severe enough to deploy the air bags. This doesn't always sit well with insurers, but that is the manufacturer's recommendation. Other makers require that the deployed air bags and any sensor in the area of the impact be replaced. Some control modules are only good for one crash, regardless of where they are located, and they must be replaced. Basically, it's a real hodge-podge of requirements.Because it is common for wrecked vehicles to have broken glass, some of them collect water before they are repaired. If water seeps inside any sensor or control module, replace the component. Many of today's control modules are located under the seats and have a tendency to fill with water and either fail or cause premature deployment.If your job requires a wire repair, refer to your service material for the necessary procedure. Some vehicles have very specific wire replacement requirements to maintain terminal integrity, some require harnesses to be replaced as a unit (no repair at all), and some permit the repair of nearly all wires. If a wire repair is to be made, always use a sealing type connector to prevent moisture intrusion. Small amounts of corrosion in an air bag system could be the difference between having the system operate correctly or not at all.QuestionsAny time some new technology becomes available, there are always a lot of questions, rumors, misunderstandings and the like that get in the way of facts. Even though air bags have been common for 10 years or so (actually, the first ones were available in the '70s), there are some questions that regularly pop up that need to be addressed here.What about components from salvage yards? Can those be used? Auto manufacturers and insurance companies in the United States say no. The salvage parts industry, on the other hand, sees this as a future profit center. Efforts are underway to develop a certification program for salvage air bags that would inspect them for damage. But because no insurers in the United States currently write for salvage air bags, installing even a certified bag would be considered fraud where a new, OEM part is listed on the estimate (See "Non-OEM Parts Rate Same As, Better Than OEMs During CIC," ABRN News, November 2000).Are there any other items I should check during air bag repair? Yes. Anytime a vehicle is involved in an accident severe enough to deploy the air bags, it is necessary to inspect the seatbelts and possibly the knee bolsters. The general requirement is that every occupied seatbelt needs to be inspected for signs of damage caused by heat and strain. Seatbelts are made of nylon and will stretch when exposed to the forces of a severe accident. This stretching makes them weaker, and they may possibly fail if the vehicle is involved in another accident. Some manufacturers recommend that all occupied belts be replaced regardless of the inspection results, especially if they are more than 10 years old. The other remaining check is on the knee bolsters, the padded areas at the base of the dash that are supposed to prevent or minimize damage to the knees in the event of an accident. If they are removable, examine them for signs of denting and replace them if damaged.Why are there special requirements for some windshields? On certain vehicles, the passenger-side air bag is designed to deflect off the windshield before it comes in contact with the passenger. As a result, a special urethane adhesive is required for installing the new glass to prevent the windshield from popping out if the air bag should deploy again. Ensure that the proper adhesive is used because old-style black window ribbon will not hold the glass securely enough. Vehicles with this type of arrangement will almost always break the glass upon deployment.Should the driver's side coil be replaced with the inflator module? This is another one of those good ideas but one that is not absolutely required. Because the coil is lo cated directly below the inflator module, it is exposed to all of the heat (more than 400