Stick to the basics
One source tells you that air bags are the greatest safety innovation of all time. Another source says that they are media-hyped, technological overkill. Still another screams about innocent children being seriously hurt by them. Forget the politics and the philosophical discussions. The car has air bags, and it is your job to fix them.
Like most things automotive, passive restraint systems go by more than one name. They started out with the moniker “air bag,” which accurately describes their design function. A fancier way of saying the same thing is supplemental inflatable restraint (SIR), or supplemental restraint system (SRS). All 1998 model or newer cars sold in the United States have a version of a front air bag system. Usually a glance around the passenger compartment will reveal the manufacturer’s pet name for the system.
Repairing air bags is a little different from what you are used to. There is absolutely no room for artistic license. No shortcuts allowed, no re-engineering for ease of assembly, not even the option of making it better than new. When you are done with it, the passive restraint system must be exactly the way the engineers laid it out. If you deviate from the original, you open that can of worms that lawyers call “liability issues.”
The Hardware
The rules of engagementThou shalt not commit fraud. If you are paid to replace with new, replace with new.Thou shalt not endanger thy reputation. Never volunteer to cut corners on workmanship or parts quality; you can be sure somebody else will suggest it to you. If they insist, get it in writing. Thou shalt not seek technological break-throughs. Stick to the exact procedures spelled out by the manufacturer. If you see an easier, less expensive way to accomplish the same thing, apply for a patent and sell it to the OEM. Never try it out on a customer’s car.Thou shalt not walk naked through the legal thickets. Always cover your butt as best you can. Document everything you do to repair an air bag system. Document your sources for parts, keep copies of insurance estimates and take notes during telephone conversations. Always assume that the vehicle will be involved in another accident and be prepared to answer any questions that may come up.Thou shalt never forget that autobody repair is to be fun and profitable. It might not be the most creative part of your job, but repairing air bag systems is most likely a fundamental part of your shop’s business. Keeping the key aspects of air bag repair in mind will help you turn out quality jobs while taking into account the special conditions that surround them.The bags themselves are woven, usually of nylon, and folded into a package that is mounted in the steering column, dashboard, door, doorframe or seat. By design, the bag is porous. The gas that fills it dissipates through tiny holes in the bag. A front-mounted air bag inflates, protects the occupant and deflates in less than one second, often too fast for the occupants to be aware of the process. A front-mounted air bag is not supposed to be a soft, fluffy pillow; its job is to keep the occupant’s body away from the glass, dash and steering wheel during a crash that is occurring at road speed.
In order to protect the occupants, side air bags stay inflated slightly longer than the front impact bags. A door or seat frame-mounted side air bag is designed to push the occupant toward the center of the car, simultaneously cushioning contact with the door. The higher mounted side air curtain inflates to fill the space between the occupant’s head and the side glass, providing protection from both impact and broken glass.
The SRS inflation system is, in essence, rocket science. The small distance between the driver’s chest and the steering wheel requires almost instantaneous inflation of the bag. On the driver’s side, a minimal electrical current (about 750 milliamp) energizes an igniter (the squib) that starts the burning of sodium azide pellets, a process very similar to a solid-fuel rocket leaving the launch pad. This burning produces a large volume of hot nitrogen gas that is aimed directly into the air bag.
Typically, the passenger side air bag uses a different inflation system, which is usually a canister of compressed gas. The extra distance between the dashboard and the passenger requires a larger bag than the driver’s side and allows a slightly slower fill rate.
Because side-mounted air bags are optional equipment, manufacturers have the freedom to design their own systems. They can make their own choices about bag position, size, deployment criteria and method of filling the bag. Side-mounted restraint devices may use a solid fuel system (like the steering wheel-mounted air bag), a compressed gas system (like most of the passenger side, dash-mounted air bags), or a combination of the two.
Every front SRS system includes sensors that measure deceleration, a diagnostic module, a clockspring and the associated wiring harness. The system is designed to deploy the SRS if it detects a crash force equivalent to running straight into a brick wall at 10 mph to 15 mph. The diagnostic module monitors the system integrity, controls the dashboard warning light and stores diagnostic information. The diagnostic module self-checks the system every time the ignition is turned on, flashing the red (or yellow) warning while it goes through its tests. (The warning light should then go off and stay off.) Diagnostic modules also supply a backup source of energy to fire the air bags if battery power is interrupted. As an identifier, underdash SRS wiring is sheathed in attention-getting colors. Bright yellow is the norm, orange is allowed. All connectors in the air bag system are of a positive locking design.
The clockspring is located under the steering wheel and it connects the driver’s side air bag to the rest of the system. Because the steering wheel turns, anything electrical attached to the wheel (such as the horn switch) has to have some type of flexible attachment to the main wiring harness. Traditionally, the horn uses a spring-loaded contact against a plate on the back of the steering wheel, which is not a 100 percent trouble-free arrangement.
For dependability, air bags are hard-wired to the sensors and module through the clockspring, a sealed package containing a spiral of wire. There is enough slack in the wire for about five complete revolutions before the clockspring binds up. If properly installed, in the centered position, the steering wheel’s normal two turns left or right will not strain the wires.
The systems just described do not need a computer to activate the air bags. The diagnostic module conducts self-checks and stores diagnostic information. The sensors initiate the actual deployment of the air bags. These are considered to be dumb systems.
Smart systems add computer power to the diagnostic module, changing it to a control module with logic functions to determine exactly how the bags should be deployed. Smart systems are standard equipment in 45 out of 80 vehicles in NHTSA’s (National Highway Traffic Safety Administration) 2002 medium size (3000 lbs. to 3499 lbs. curb weight) passenger car list. The 2002 Taurus, Camry, Impala, Sebring, Accord and Altima all have what NHTSA calls “advanced air bag features.”
Integrating a control module into the SRS allowed the introduction of the two- and three-stage air bag. Severity of the crash is calculated by the control module, which then deploys stages one, two or three as required. The 2001 Windstar disables the passenger side air bag if it detects a person who weighs less than 45 lbs. in the seat. The 2001 Acura CL can disable the passenger bag (and notify the driver with a warning light) if a child moves out of the air bag’s “safe zone,” which is a common problem with children who fall asleep leaning against the door. Autoliv, a Swedish company and a supplier to Jaguar, uses an array of ultrasonic sensors to calculate a passenger’s height. This system is sophisticated enough to determine if the passenger is wearing a hat.
The federal government has ordered that by 2004, 35 percent of new cars must be equipped with “advanced” air bag systems, which take into account the height and weight of the seat’s occupant and react to changes in passenger position. As early as 1994, BMW had already adapted its smart system to adjust the deployment force according to seatbelt use. A few years further down the road, you can expect to see computer corrections added to compensate for temperature. Testing has shown that a cold environment can increase deployment time by 50 percent to 100 percent.
The Repairs
Disconnecting the battery will not clear codes from either a smart or a dumb SRS module. Only a scan tool with the specific OEM interface can clear codes, and some codes cannot be cleared at all. Some manufacturers require the replacement of the module every time the bags deploy. Hyundais set a non-erasable deployment code and illuminate the warning light if the front air bags deploy.
Air bags can go off with the battery disconnected. The module is designed to be a source of backup energy, capable of firing the bags for two to five minutes after the battery and ignition switch inputs have been removed. This is not a countdown function. Two to five minutes is the estimated time it will take for a capacitor inside the module to lose its charge. The proper approach is to consult the manual for the exact procedure, disable the system by following the procedure to the letter, go out for coffee, and then begin work on the system. Never attempt any type of repair near an air bag sensor while the system is live. Of course, this includes the side-impact sensors for cars with side-impact bags. If there is an air bag in the door, doorframe or on the side of the seat, there probably is a sensor right next to that grocery cart dent you are working on.
Two- and three-stage air bags can explode a second or third time if the initial deployment was only in the first or second stage. This means that a multi-stage bag hanging loose and looking harmless could be hiding the potential for injury. Approach all air bags the same way you would approach a land mine—it probably won’t go off, but you don’t want to be standing right there if it does.
The bags themselves are, obviously, a single-use item. Every manufacturer agrees on that. However, that is the only thing they agree on. Most manufacturers and insurance companies consider the clockspring to be a “must replace” item. Sensor replacement requirements are different for every car line. Some say “every sensor, every time,” while others tell you to test and replace as necessary. An important point to remember: A sensor, module, clockspring, etc., will probably come with new hardware and mounting brackets. The manufacturer includes these parts because they are single-use parts. Throw the old stuff away and use all of the new parts.
