Air bags have been in the news in recent years because of deaths that have resulted from air bag deployments in relatively minor low speed crashes. The victims have been small children or infants in the passenger seat, or small female adults drivers who were too close to the air bag or unbelted when it deployed. The deaths, in some cases, have been blamed on improper use of infant seats or not using seat belts. But others blame the deaths on government regulations that require auto makers to use air bags that deploy with sufficient force to protect an unbelted 160 lb. male adult in a 30-mph crash. This requires deployment speeds of up to 200 mph, which can cause serious injury or even death to children and small adults who are not buckled up or are too close to the bag when it deploys. In Europe, where rules allow air bags that deploy with 30% less force, there have been no deaths attributed to air bags.
The National Highway Traffic & Safety Administration (NHTSA) has reviewed their regulations and now allows auto makers to use less powerful air bags. NHTSA now allows auto makers to add switches for deactivating the air bags, and even allows consumers to have a switch installed (by a professional) to deactivate their air bags under certain circumstances. Auto makers have also developed smarter "adaptive" air bag systems that can vary their deployment speed and force depending on crash circumstances. Such systems modify air bag deployment force based on occupant size, seating position and impact speed.
For more information, see NHTSA's website information on Air Bags & air bag on-off switches
In recent years there have been many recalls for airbag related problems. For more information on this subject see Airbag Recalls.
Regardless of what type of air bag system a vehicle has, though, all motorists are urged to use their seat belts, to sit as far back as possible from the steering wheel, and to put infants and children 12 and under in the back seat.
Air bags have to deploy within milliseconds when a crash is detected.
BIG BANG THEORY
One thing all air bag systems share in common are crash sensors. In theory, an air bag is only supposed to deploy in a frontal collision that is severe enough to trigger the control electronics, or in some cases a mechanical firing pin.
Most car companies say a vehicle has to experience an impact that generates at least 7 G's of force before it will trigger the air bag. For most vehicles, this would be the equivalent of hitting a solid barrier at 12 to 15 mph, or hitting another vehicle at a speed of about 25 mph. In reality, the air bag may deploy at higher or lower speeds.
Air bags also are not supposed to go off in rollovers or side or rear collisions, but they sometimes do. Weird things can happen during an accident, and impact forces can be transmitted in such a way that the air bag sensors are tricked into thinking a frontal collision is occurring when in fact it really is not. Of course, in such situations the air bag does nothing to protect the occupant because it is only designed to protect during frontal collisions.
Here's a YouTube video clip of air bags being deployed intentionally by rescue workers:
AIR BAG CRASH SENSORS
An air bag is only as good as its control system. On most vehicles, the bag is triggered electrically. Located in the front of most domestic vehicles are one to three "crash" sensors. These are positioned well forward in the crush zones so they will react almost instantly to the sudden deceleration that results from a frontal impact (anything up to about 30 degrees either side of center). Many European vehicles use only a single electronic crash sensor located inside the passenger compartment. The same setup is now being used in many newer domestic vehicles.
There are several different types of crash sensors. A commonly used sensor is the electromechanical "gas dampened ball and tube" design. The sensor is nothing more than a small tube with a switch at one end and a gold plated steel ball at the other, held in place by a small magnet. When the sensor receives a hard enough jolt to knock the ball loose from the magnet, the ball rolls down the tube, hits the switch and closes the circuit. The tube is slanted upward so the ball should return to its original position after an impact.
Another common crash sensor is the "Rolamite" design by TRW. Inside is a small metal roller that rolls forward under sudden deceleration and trips a switch.
Some vehicles have "spring and mass" crash sensors in which a spring loaded weight is deflected by the impact to close a switch. Most newer vehicles now have solid state crash sensors that contain either a piezoelectric crystal or a "micromachined accelerometer" chip that produces an electronic signal when jolted.
Because many crash sensors will automatically reset themselves after a hard jolt, some car makers say their crash sensors do not have to be replaced after an accident as long as the sensors have not suffered damage. These include Acura, Audi, Chrysler, Ford, Infinity, Lexus, Mazda, Nissan and Toyota. The problem, of course, is knowing for sure whether or not a crash sensor has been damaged or has failed to reset itself.
To prevent false deployments that might result from bumping into objects or a slow speed fender bender, most air bag systems also have one or two "safety" or "arming" sensors which are usually located inside the passenger compartment (under the dash or seat, in the airbag control module, or in the steering wheel air bag module). The safety sensor will not allow the bag to deploy unless it also experiences a certain rate of deceleration (usually less than that of the crash sensors).
If both the crash sensors and safety sensor(s) are triggered by a collision, then and only then does the electronic control module say okay and ignite the air bag inflator. It does this by applying voltage to the "squib" inside the sealed inflator. The squib sets off the sodium azide pellets inside the inflator, which produce harmless nitrogen gas as they explode. The gas rushes through a filter that traps any unburned particles of propellant and helps cool the gas, and forces the bag to burst from its housing and unfurl in 30 to 55 milliseconds (less time than it takes to blink your eyes).
Airbag control module
MECHANICAL AIR BAG SYSTEMS
A few older vehicles (Jaguar & Toyota) are equipped with a self-contained "mechanical" air bag system, manufactured by Breed Automotive (a similar system is also available from Breed as an aftermarket add-on driver side air bag system). The mechanical air bag system does not have electromechanical or electronic crash sensors. It uses a mechanical trigger to ignite the propellant that inflates the air bag. It works something like a land mine. Inside the air bag assembly is a single mechanical impact sensor that trips a firing pin when a severe enough jolt is experienced. The firing pin ignites a primer which sets off the sodium azide pellets to inflate the bag. The mechanical system eliminates the need for a power supply as well as any control electronics or external crash sensors, which makes it much easier (and cheaper) to replace after an accident.
CRASH SENSOR CHECKS
The air bag control module self-checks the crash sensors every time the engine is started, so unless the air bag warning light is on the sensors are assumed to be okay. If a fault is detected, the air bag warning light will come on and usually deactivate the air bag system. Using a scan tool, you can pull the trouble code from the system and refer to the appropriate diagnostic chart in a service manual to troubleshoot the problem. Air bag service information can also be found on the vehicle manufacturer's website (Click Here for a list of websites and access fees). Loss of circuit continuity anywhere in the air bag system, or loss of power to the air bag module are common causes of trouble codes.
Because crash sensors are sealed units, you cannot always determine their true condition by outward appearances. Any sensor that is obviously sustained physical damage as a result of a collision or other damage should be replaced. But what about ones that look okay? Most electromechanical crash sensors are designed to be electrically open in their rest condition. So one quick check you can perform is to check for continuity with an ohmmeter. If the sensor contacts are closed, it has not reset and should be replaced.
CAUTION! Do not attempt to check or replace any crash sensor unless the air bag module has first been deactivated (or deployed as a result of an accident). This can be done by unplugging the air bag connector at the base of the steering column and waiting at least 10 minutes or longer depending on the application (always refer to a service manual for the proper deactivation and removal procedure).
The following YouTube video clip shows some doofus sitting on an air bag when it goes off. This is extremely dangerous as air bags deploy with explosive force:
Also, do not use a self-powered test light or jumper wires on the wiring of a live air bag system. Apply voltage to the wrong circuit and you could accidentally trigger the bag!
Testing a electromechanical crash sensor in a vehicle that has been in an accident to see if the sensor is electrically open, however, does not necessarily mean the sensor is okay because the sensor may have sustained internal damage from the force of the collision that may prevent it from working properly. Because of this, other car makers say crash sensors should always be replaced if an air bag has been deployed in an accident. These include BMW, General Motors, Isuzu, Mercedes, Mitsubishi, Porsche, Saab, Volkswagen and Volvo.
AIR BAG SENSOR REPLACEMENT
Several very important points must be kept in mind about replacing crash sensors. One is to make sure the replacement is the correct one for the application. Crash sensors are calibrated for specific vehicle applications, so compare OEM part numbers to make sure your parts supplier has given you the correct replacement crash sensor.
The mounting of the crash sensor is also critical. A replacement sensor must be installed in exactly the same location and the same position as the original. The sensor must also be firmly attached so it won't break loose in a future collision. Altering the mounting location or position of a sensor may cause it to trigger the air bag accidentally or not at all.
AIR BAG MODULE REPLACEMENT?
If your vehicle has been involved in an accident, it may be necessary to replace the air bag module in addition to the airbags and/or crash sensors. The module on many late model vehicles records hard codes and crash data at the moment of impact. This information cannot be cleared with a scan tool, so the dealer will tell you the module also has to be replaced when your vehicle is repaired. Car makers also want the module replaced to minimize their liability in case your vehicle is involved in another accident.
If you are attempting to replace your own air bag module, disconnect the battery cable, wait 15 minutes, then locate the module (usually somewhere under the dash) and disconnect it from the wiring harness. Do not reconnect the battery cable until after the new air bag module has been installed and connected to its wiring harness.