Carrying an explosive device in your car that is capable of killing you doesn’t sound like a good idea, but most of us do just that. I’m talking about airbags, not hand grenades. OK, airbags save a heck of a lot more lives than they take, but nevertheless are potentially dangerous.
They are often thought of as inflatable balloons that will prevent injury in the event of a crash. No doubt they do a good job and, with seatbelts, do lessen the severity of injuries that would be incurred in most types of collision, but they are neither balloons nor foolproof. Like any mechanical or electro-mechanical system, things can go wrong. There are on-board diagnostics relating to airbags, but their usefulness varies from car to car. Unfortunately, the proof of the pudding is in the eating, or in this case, the crashing.
The airbag inflates when a direction-sensitive impact sensor tells the airbag ECU that a collision has occurred in its operational area. The bag in fact deflates immediately it has filled, thus providing a shock-absorbing effect.
On some vehicles there could be as many as 14 bags strategically placed. Direction-sensitive sensors are required because a bag in the steering wheel, for example, should only inflate in the dead-ahead position or up to 30 degrees either side of it. Impacts outside these limits should cause the side bags to inflate. Obviously, multiple bag systems will have sensors all over the place to protect from all various impact directions.
Early impact sensors, bearing in mind that airbags have been around for about 30 years, relied on a ball shooting along a channel when an impact occurred.
The ball would then overcome its holding spring’s pressure and make an electrical contact to activate the bag. Later versions operated electronically, but each required an impact of at least 25km/h to activate.
When the ECU receives the sensor signal a firing pin triggers a detonator and ignites a compound in pellet form that produces carbon dioxide and nitrogen gases under high pressure to inflate the bag. The whole sequence takes only milliseconds.
However, if you sit too close to the steering wheel and a bag inflates, it would probably feel worse than a right-hander from Mike Tyson. Ideally sit no closer than about 25cm to the wheel and don’t hang trinkets, charms, plastic doggies and other potentially dangerous things between you and the bag. People have been seriously injured or even killed by airbags.
After a bump in which the bag fails to inflate an airbag may become unstable, so be aware. A steering-wheel bag that breaks away from its necessarily fragile holder and hits you at about 300km/h will not improve your day. Many rescue workers and technicians have found to their cost that after an accident in which the bag failed to deploy, it could be activated by no more than turning the steering wheel.
Working on airbags without special knowledge and equipment is not recommended. But, if you do decide to check something, first disconnect the battery and allow 15 minutes for the capacitors to drain.