There are no external front or side crash sensors used so a wiring problem can not occur. On older models with front crash sensors, the Air Bag Computer monitored the state of each pair of wires to each of the two front sensors. If any one was grounded or open, it was detected, the system was shut down, and a diagnostic fault code was set in memory. If the two wires to one sensor were shorted together, the system was also shut down. The sensor also shorted those wires together in a crash, but the computer knows that isn't a logical state at 0 mph. If the short occurred intermittently at higher road speed, it needed either sensor AND the safing sensor inside the computer to trip at the same time to deploy the air bags. That was very unlikely to happen.
On the '99 model, there are no external sensors so there's no wiring problems to worry about. Everything is done through an accelerometer inside the computer. There are only three ways for the air bag to deploy. If the computer has been removed and reinstalled incorrectly and is tipped in any direction, it will require a different speed to trip the sensor. The computer has a sticker on top warning of this possibility. If it is tipped forward, it will trip at a much lower speed.
The second way for it to deploy is the vehicle actually hit something. It is typically set to not activate below around 15 mph.
The third, much less common way for it to deploy is through static electricity. For this to happen, there is only one possibility. The clock spring has to be coming apart. That is a wound-up ribbon cable in a plastic housing under the steering wheel. All of the other connectors in the "squib", (the firing wires going to the air bag), have gold-plated terminals with shorting bars. When any connector is unplugged, those shorting bars engage the two terminals and short them together leaving them unsusceptible to static electricity. The system will be shut down. When the clock spring comes apart, the open circuit will be detected and the system will be shut down, but static electricity can trigger the driver's air bag if it reaches those wires. That is real unlikely to happen. It is so unlikely that we took no special precautions other than common sense handling procedures when replacing those clock springs. There is no clock spring for the passenger side air bag. They are hard-wired so static electricity is not a concern.
If there were any type of wiring problem, the system would have shut down and the warning light on the dash would have been on. Under normal operation, the light will turn on for seven seconds when the ignition switch is turned on. The computer performs a series of self-tests during that time. When the light goes off, the tests passed successfully and the system is armed. If the light was off prior to deployment, the system was working properly. There are so many safeguards built into these systems to prevent unintended deployment. If the bags popped for no reason, you would be hearing about numerous lawsuits. If you think the system activated both bags for no reason, the computer can be sent in to have the crash data retrieved. Vehicle speed, direction of travel, rate of deceleration, and whether or not the brakes were applied are all recorded. That can prove or disprove a person was speeding, and in this day of anti-lock brakes and no skid marks, it can prove or disprove the driver had the brakes applied just prior to a crash.
Wednesday, August 10th, 2011 AT 7:20 PM