You don't even have to disconnect the battery. Excuse me for sticking my nose in here, but often other people will look at various answers when researching a problem for a car of a different model. Simply disconnecting the battery on some brands such as VWs, BMWs, Audis, and I've been told, some GM models, can leave you with an empty wallet and a real lot of frustration. The engineers for those brands designed in tricks to force you to go back to the dealership for such mundane tasks as simply replacing a defective battery. We've all heard the horror stories of cars that may not start, and if they do, they won't accelerate off of idle, and they won't come out of "park". They have to be dragged out of the shop, and skidded onto a flatbed truck for the trip to the dealership to have multiple computers unlocked. By the time everything is working properly again, the bill is usually over $950.00, . . . for simply disconnecting the battery.
What BMDOUBLE is referring to is what you'll find in a lot of service manuals, but for the type of work you described, this warning is strictly for liability and to avoid lawsuits. They even tell you to disconnect the battery if you're going to replace the wiper blades! They're protecting themselves from people who lack common sense.
The idea is in case the electrical wires are torn during a crash, there are storage capacitors inside the Air Bag Computer to keep it operating. Capacitors are like extremely small batteries that store a small charge, but it is plenty to fire an air bag. Once the battery is disconnected, the charge on those capacitors bleeds down to 0 volts in about two minutes, then the computer can't do anything.
You have never heard of an air bag deploying accidentally. We even remove them from the steering wheel and drop them on the seat, knowing they are safe to handle. There are some things, however, that you can do to deploy one. We consider that "common sense", but it really isn't common sense for competent do-it-yourselfers who haven't been trained on these systems.
A two-wire cable runs from the Air Bag Computer to the air bag assembly. The computer switches 12 volts onto one of those wires when it wants to pop the bag in a crash. (As a point of interest, we do that with a 9-volt transistor battery for demonstrations). There's a lot of safeguards built in to prevent the computer from deploying the bag when it isn't supposed to. What you can do if you aren't aware of it, is to hit those wires with static electricity. If you can see or feel a spark from your fingertip after sliding across the seat fabric, that is at least 3,000 volts, so that is way more than enough to ignite the pellet of rocket fuel in the bag. Logic would dictate that would be a possibility any time you unplug a connector in that two-wire cable going to the bag, but again, there's safeguards. In the half of the connector that is on the air bag side, there is a shorting bar built into it that shorts the two wires together when the mating half of the plug is removed. By shorting them together, even if they're hit with that 3,000 volt static discharge, it shows up on both wires equally. The difference in voltage between the two wires is 0 volts, so nothing happens. It is only if that shorting bar is damaged from someone digging in there with a pick that it becomes a safety concern.
There is a shorting bar in the bright yellow connector at the computer, and another bright yellow connector where the wires plug into the "clock spring". That is a wound-up ribbon cable in a plastic housing under the steering wheel. We used to see a brass nail head sliding across a brass ring for the horn circuit. That was fine for a horn, but air bag deployment is timed to within a few milliseconds, and that slip ring can't be trusted to not have an intermittent connection at just that instant it is needed. That's why they use the ribbon cable with its solid connections.
When an air bag deploys in a crash, the heat is going to melt the connector where the clock spring connects to the air bag. That is why we always replace the clock spring as well as the air bag and all the crash sensors. When an unscrupulous "mechanic" tries to reuse the old clock spring, the melted / deformed two-pin connector may go in and work okay, but it can deform or damage the shorting bar in the air bag's connector. That won't cause a problem until the air bag has to be unplugged for some other service, then the lack of the shorting bar leaves the air bag vulnerable to static discharge. Again, you would have to put some effort into getting the bag to deploy. That's why this is worth mentioning.
As long as I'm at it, I should mention that the best testing of the "squib", or "initiator wire" which is that two-wire cable with bright yellow connectors, is done by the computer itself. It will detect when those wires are shorted together, when there is a break in one of them, or when either one is shorted to the car's metal body or frame, and it will set the appropriate diagnostic fault code, turn the system off, and turn the red warning light on to tell you. At that point the computer will not try to deploy the bag in a crash. This wire is usually pretty short, so there isn't much that needs to be inspected to find a problem, but if someone were to go in there with an ohm meter, that could fire the air bag. Most digital volt / ohm meters are powered with a nine-volt transistor battery or four double "AA" cells providing 6 volts. Those can fire the air bag if one of the connectors is unplugged and the shorting bar is pushed out of the way with the meter's probe.
Again, if you just unplug a connector, the circuit will be immune to static discharge and it is safe to work around the air bag. Be sure to reconnect everything before you turn the ignition switch on. If you forget to reconnect a connector, that will be detected as soon as you turn the ignition switch on, during the system's six-second self-test sequence when you see the warning light turn on at that time. Even when you turn the ignition switch back off, then reconnect the connector, the diagnostic fault code will remain in memory for some time. On many car models the code will never self-erase even by disconnecting the battery, and the system will never turn on and watch for a crash. With those models, you have to use a scanner to manually erase the fault codes to reset the computer. That is not an expensive proposition if you have to visit a repair shop, but it's easier to just not turn the ignition switch on until everything is back together.
If you need to work with the air bag system, here's links to some guides that may be of interest:
https://www.2carpros.com/articles/air-bag-removal-steering-wheel
https://www.2carpros.com/articles/symptoms-of-a-bad-airbag-clock-spring
https://www.2carpros.com/articles/steering-wheel-clock-spring-removal
Thursday, January 9th, 2020 AT 3:43 PM
