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Understanding Airbags

Many shop personnel say ‘it’s not my job’ to know the basics about how and why airbags work. I disagree. Not only do we owe it to our customers, but we can use this knowledge to differentiate ourselves from the competition and get the job.

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Contributing Editor Keith Combs is the body shop manager at Bill Roberts Chevrolet, where he’s been employed for more than 25 years. He was named GM/ASE Master Collision Repair/Refinish Technician of the year in 2000 and was one of the first to achieve GM’s World Class Technician status in 2002.


Why didn’t my airbags go off? They should have gone off!”

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How many times have you heard similar concerns from your customers? You could just dismiss their questions and move on to repairing their vehicle. After all, the airbags are automatic and work when they’re supposed to, so the customer doesn’t really need to know how or why an airbag works. It would be a waste of your time to explain it.

Or would it?

I’m surprised just how many shop personnel have no clue how or why airbags work. Sure, some who care enough to take the time to learn, but many will just say, “Send it to the service department,” or “It’s not my job.”

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That may work for you, but I’m more inquisitive. Not only that, there’s a paycheck at the end of the repair that often comes from working on areas that we may already be into. I’ve seen instances where windshields are replaced, and then someone has to replace the dash and airbags. It’s so much nicer to do that while the windshield is out.
That’s just one minor example, but I’m sure you see my point.

We owe it to ourselves and our customers to at least have a clue about what goes on and why. This knowledge can also be what differentiates us from the competition and gets us the job. For example, on one occasion, I had a customer come to me for an estimate on her Mustang after getting an estimate at the local Ford dealership. She asked why her airbag didn’t deploy, and I gave her a straightforward explanation of the approximate parameters that need to be met to deploy an airbag. I then illustrated the apparent angle of impact that her car experienced and asked her if she came close to hitting her face on the windshield. She then realized her accident didn’t meet the conditions necessary to deploy the airbag and proceeded to tell me that the estimator at our competitor either could not or would not explain why her airbag system didn’t deploy.

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She was so impressed that I’d taken the time to answer her question that we were able to reap the rewards by repairing her vehicle.


If I had blown off her questions or had no clue how to answer them, she might not have chosen us. All she wanted was some reassurance that her airbag shouldn’t have deployed and that the technicians who’d be doing the repair knew what they were doing. I was able to give her that reassurance and, in return, she gave me her keys.

The Basics of an Airbag System
The more I research airbag tech-nology, the more I realize how limited my understanding is of the complexities of airbag systems. Would I like to know all the details of all airbag systems? You bet. But if I knew it all today, I’d have to study again tomorrow because the technology is everchanging. Instead, I guess I’ll have to be content trying to keep up with a general understanding of airbag systems and, more importantly, how to go about repairing them.

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That said, let’s take a look at the basics of an airbag system …

Airbag modules — This is the explosive part of the equation. In my limited experience, I understand there are basically two types of propellants used in airbag inflator modules: sodium azide and compressed gas.


1. Sodium azide propelled – The most common propellant or, should I say, fuel used for the longest time inside of these units has been sodium azide, also known as a type of rocket fuel. It’s a very stable, solid propellant that gives off a combination of nitrogen gas and sodium hydroxide (lye) when ignited by an electro explosive device. Although the quantities of caustic compounds are very small, you shouldn’t ignore them; as with any repair, always protect yourself from any possible danger.

FYI: The white powder you commonly find is cornstarch, which is used as a lubricant for the expansion of the airbag, made of a nylon fabric.

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Pre-1998, these units would commonly be single-stage with only one level of power. But today, you’ll sometimes find two-stage units that are designed to give the level of power needed for the conditions of the crash and the occupants in the vehicle. We really don’t need to know all the intricacies of all these airbag modules to repair these vehicles, but a simple understanding is essential to keep yourself safe. Be aware that even though the airbag has been deployed on these dual-stage modules, there’s a chance of another potentially dangerous explosion if the module somehow got a charge of electricity or possibly even excess heat.

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2. Compressed gas propelled – These days, we’re seeing hybrid inflators that use a compressed inert gas contained at a very high pressure (some are in the 3,000 psi range) capable of inflating approximately three times faster than its solid fuel counterpart.
In some cases, there will be a combination of more than one type of propellant in one airbag. Most often when you run across one of these, it’ll be involved in side airbag systems. These systems require faster deployment, since there’s usually less space between a side impact and the occupant.

Sensors — There’s a whole plethora of possible sensors in airbag systems. Although each system will have sensors, some systems are more complex than others due to the use of a combination of sensors.

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Sensors are actually switches of sorts designed to relay information of different types. You may have front-end impact sensors that relay the amount of G-force that’s being caused by the accident (by the way, those are the same forces that are trying to shove your face against the windshield and steering wheel). Some of the sensors may be located up front, or you may only have one inside of the sensing diagnostic module, depending on the design. Most of these are designed to sense an impact within about 30 degrees from a head-on crash on a frontal airbag system; on side airbags, they may incorporate side-impact sensors and possibly even rollover sensors.

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On top of the sensors I just mentioned, there are sometimes “passenger presence” sensors that work along with “seat position sensors,” which tell the system whether or not to arm itself or how strong of a charge should be deployed, depending on the weight of the passenger.


You’ll run into many different sensors on many different systems. Some of the sensors have different names than I’ve used here yet they do the same job, while other systems will have a completely different setup altogether. Just remember, these sensors or switches need to be in good condition so they can complete the circuit that they’re designed to complete – so look out for any damage. In the case of safety equipment such as this, I like to use the rule, “When in doubt, throw it out.”

Seatbelt tensioners — While all the action is happening with the airbags, some systems give you a tug from the seatbelt by igniting a pyrotechnic charge connected to the belt. This assists the airbags by holding you much tighter to the seat in an accident that will deploy them.

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Although not all systems have pretensioners, the ones that do often deploy seatbelt pretensioners during a rear-end collision to catch someone from reeling forward after the slingshot action received from a slam from behind. So if you have an airbag lamp illuminated after a rear hit, check for deployed pretensioners in the system.
While my above list of airbag basics isn’t exhaustive, it should at least give you a brief overview of some of the major items you find in a typical airbag system. You’ll also find knee bolsters, collapsible steering columns, SIR coils, and on and on, but if you understand the basic items above, you’ll know what to look for in a repair situation.


Repairing Airbag Systems
Repairing airbag systems requires specific procedures for specific vehicles, so I can’t actually show you step-by-step what to do in every case. What I can do is give you a quick overview of what you should do when you encounter an airbag system that has deployed.

One of the first things to do is get information specifically for the vehicle you’re working on. We first consult an airbag repair matrix that’s included with our estimating system. It gives you a list of items you have to replace when an airbag deploys, along with a list of items to inspect for damage. The airbag matrix also gives you disable procedures and wait time required when disarming an airbag system. Always disarm the system before working on an airbag circuit, and check the manufacturer’s service information to give you direction for the repair at hand. You’ll also occasionally need some type of diagnosis equipment to check the system as directed by the service information.

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Some manufacturers require dummy loads put into the electrical system and arming the system prior to installing the actual airbag modules. This checks the system without any possibility of deployment. Better safe than sorry. I’ve made such dummy connectors by taking the connector off the deployed airbag and adding a resistor, but you’ll need the service information to know the proper resistance needed for the system you’re working on.


Because it’s difficult to keep enough dummy connectors and the diagnostic equipment for all applications, let alone the service information, we have, on occasion, sent vehicles with airbag issues to other dealerships to complete the diagnostics.

I have, at times, started out by replacing all of the items listed in the airbag matrix and lucked out, not needing anything more to make that dreaded malfunction indicator go away. On the other hand, I’ve had occasions when, after installing all the items on our list, I still had a lamp to deal with. I’ve also seen vehicles that didn’t have the seatbelt pretensioners listed in the airbag matrix, requiring a scan of the vehicle to find the problem (in this particular case, all we had to do was replace the pretensioners to make the malfunction lamp go away).


Some of the systems require input to one of the modules from a diagnostic scan tool, while others just require all parts needed to be replaced. Whatever the case is, it’ll be spelled out in the service information for the vehicle you’re working on.

Keep Yourself Safe
Always protect yourself. It makes no sense to repair a system to protect someone else if you compromise your own health in the process. Although most of the residue from an airbag system is non-toxic, some of it is toxic. Protect yourself sufficiently.

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Also, any time you work with electrical circuits or components, know what you’re doing. If you’re using a multimeter or a test lamp, be sure you have one that has a compatible resistance level for the circuit at hand.


Another thing to keep in mind: With the growing popularity of hybrid vehicles, there’s the possibility of high-voltage circuits somewhere on these vehicles. Be sure you have the knowledge and capability to protect yourself and to keep from frying components in whatever circuit you’re involved in. Static can ruin electronic components so be sure you protect against static discharge.

I’m sure you’ve also heard the precautions about not laying an airbag unit face down and never painting an airbag (it can change the strength of the cover).

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And, if you’ve ever deployed a defective airbag for safety purposes, resist the obvious urge to see what or who you can propel. I just wish someone had taken the time to tell me that years ago.

Learning the How and Why
When I started researching this article, I was reminded of a story that a friend had told me about a disagreement he had with his wife. He made a comment that makes me smile each time I think about the look on his face when he said to me: “I’m looking for a car with just ONE airbag!”

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While I know he wouldn’t actually want his wife’s life to come to an untimely end, his comment illustrated the reputation that airbags have for saving lives – in this case, specifically his life and his life only.

The fact is, airbags do save lives, something I’m reminded of regularly. For example, one of our customers was involved in an accident where another car pulled out right in front of him while he was traveling at highway speed. The two occupants in the vehicle that pulled out were killed. Their vehicle had a disadvantage as far as the angle of impact and also didn’t have side airbags. Our customer’s vehicle was totaled, and not only did the frontal airbags deploy, but the side airbags also, probably because the sensors indicated a possible rollover situation. The customer was sore but lived to tell us about it. He insisted on buying another vehicle just like the one he had, which he credits with saving his life.

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Considering the important role airbags play in a collision and the countless lives they save, don’t we owe it to ourselves – and to our customers – to take the time to learn a little about how and why they work?

Contributing editor Keith Combs is the body shop manager at Bill Roberts Chevrolet, where he’s been employed for more than 25 years. He was named GM/ASE Master Collision Repair/Refinish Technician of the year in 2000 and was one of the first to achieve GM’s World Class Technician status in 2002.

Disarming Airbags

I’ve had requests to disconnect or otherwise disarm airbags. In cases like this, refer your customers to the National Highway Traffic Safety Administration’s (NHTSA) Web site at www.nhtsa.dot.gov (click on Vehicles and Equipment, then on Airbags). There, they can obtain information on how to get approval for adding an on/off switch or deactivating their airbag system if they’re considered at risk when faced with an airbag deployment. As much as airbags have improved collision conditions for most people, they’re not always better for all people in all circumstances.

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