Air bags have featured in the news in recent years due to 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 very close to the air bag or unbelted when it deployed. The deaths, in some situations, have been blamed on improper use of infant seats or not using seat belts. But yet others blame the deaths on government regulations that need 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 needs deployment speeds of up to 200 mph, which can create serious injury or even death to children and small adults who are not buckled up or are too close to the bag when it is deployed. In Europe, where rules let air bags that deploy with 30% less force, there have been no deaths associated with air bags.
The National Highway Traffic & Safety Administration (NHTSA) has reviewed their regulations and now lets auto makers to use less powerful air bags. NHTSA now lets auto makers to add switches for deactivating the air bags, and even allows consumers to have a switch mounted (by a professional) to deactivate their air bags under certain circumstances. Auto makers have equally 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.
In recent times there have been many recalls for airbag related issues
Irrespective of what type of air bag system a car 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.
THE BIG BANG THEORY
One story all air bag systems share in common are crash sensors. In theory, an air bag is only meant to deploy in a frontal collision that is severe enough to trigger the control electronics, or in some situations a mechanical firing pin.
Most vehicle companies say a car has to experience an impact that generates at least 7 G’s of force before it will trigger the air bag. For most car, this would be the equivalent of hitting a solid barrier at 12 to 15 mph, or hitting another car at a speed of about 25 mph. In reality, the air bag may deploy at higher or lower speeds.
Air bags also are not mean to go off in rollovers or side or rear collisions, but they sometimes do. Weird things can occur 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 happening when in fact it really is not. Of course, in such situations the air bag does nothing to protect the occupant since it is only designed to protect during frontal collisions.
WHAT IS AN AIR BAG CRASH SENSORS
An air bag is only as great as its control system. On most cars, the bag is triggered electrically. Found 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 car use only a single electronic crash sensor found inside the passenger compartment. The same arrangement is now being used in many newer domestic cars.
There are many 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, kept in place by a small magnet. When the sensor gets 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 the impact.
Another often used 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 cars have “spring and mass” crash sensors in which a spring loaded weight is deflected by the impact to close a switch. Most newer cars now have solid state crash sensors that contain either a piezoelectric crystal or a “micromachined accelerometer” chip that creates an electronic signal when jolted.
Since many crash sensors will automatically reset themselves after a hard jolt, some vehicle makers say their crash sensors do not have to be changed 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 issue, of course, is knowing for sure whether or not a crash sensor has been damaged or has failed to reset itself.
To avoid 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 often 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 let the bag to deploy unless it also experiences a certain rate of deceleration (often 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 assist 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
THE MECHANICAL AIR BAG SYSTEMS
A few older cars (Jaguar & Toyota) are equipped with a self-contained “mechanical” air bag system, manufactured by Breed Automotive (a similar system is also offered 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 necessity for a power supply as well as any control electronics or external crash sensors, which makes it much easier (and cheaper) to change after an accident.
THE CRASH SENSOR CHECKS
The air bag control module self-checks the crash sensors each 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 found, the air bag warning light will come on and often 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 issue. Air bag service information can also be found on the vehicle manufacturer’s website. Loss of circuit continuity anywhere in the air bag system, or loss of power to the air bag module are frequent causes of trouble codes.
Because crash sensors are sealed units, you cannot always know their true condition by outward appearances. Any sensor that is obviously sustained physical damage due to collision or other damage should be changed. But what about ones that look okay? Most electromechanical crash sensors are configured to be electrically open in their rest condition. So one quick check you can handle is to check for continuity with an ohmmeter. If the sensor contacts are closed, it has not reset and should be changed.
CAUTION! Do not attempt to check or change 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).
Also, never 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 car that has been in an accident to see if the sensor is electrically open, however, does not necessarily imply the sensor is okay since the sensor may have sustained internal damage from the force of the collision that may keep it from working properly. Because of this, other vehicle makers say crash sensors should always be changed if an air bag has been deployed in an accident. These include BMW, General Motors, Isuzu, Mercedes, Mitsubishi, Porsche, Saab, Volkswagen and Volvo.
THE AIR BAG SENSOR REPLACEMENT
Many 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 ensure 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 mounted in exactly the same location and the same position as the original. The sensor must also be firmly linked so it won’t break loose in a future collision. Altering the mounting position of a sensor may make it to trigger the air bag accidentally or not at all.
WHAT ARE AIR BAG MODULE REPLACEMENT?
If your car has been involved in an accident, it may be necessary to change the air bag module in addition to the airbags and/or crash sensors. The module on many late model cars 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 changed when your vehicle is repaired. Car makers also want the module changed to minimize their liability in case your car is involved in another accident.
If you are attempting to change your own air bag module, disconnect the battery cable, wait 15 minutes, then locate the module (often 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 mounted and linked to its wiring harness.