An unwanted air bag deployment

Jun 1, 2009 by

The owner of a Volkswagen Passat decided that he would buy a cool new GPS navigation unit to spruce up his ride. He went to the local professional grade aftermarket specialty stereo dealer, selected an appropriate model, and then paid for the Navigation Head Unit and the installation.

 

After the installation, he turned in a parking lot, and BANG!!! the driver’s side air bag deployed. He was slightly injured. The allegation was that the stereo store had negligently installed the GPS Nav Unit, which caused the air bag to deploy. GEI was assigned to answer the question, “Why did the air bag deploy?”

The first items we examined were the bumpers and chassis. There was no impact damage to either the front bumper or the underside of the frame. The deployment was caused by an errant electrical signal, not by any vehicle impact.

Next, we extensively examined the wiring under the hood, under the dash, and behind the dashboard.  Overall, the interior wiring was in horrible condition.  Many wires behind the dashboard were cut and roughly spliced, tapped into with aftermarket quick connectors, or crudely taped together.  Several pieces of wire were missing their insulation in various lengths and section along the wires.

At first glance, we suspected that the issues with the wiring behind the dashboard were the cause of the errant electrical signal.Upon further detailed inspection, we determined that none of the wires that had been cut or exposed had anything to do with the driver’s air bag deployment.

The center console was in equally bad shape. Someone had installed a mobile phone on it and the level of fit and finish matched the wiring under the dash, which was to say, very unprofessional. Additionally, there was evidence that someone had installed and then removed an aftermarket alarm system. We examined that wiring as well and determined that none of the center console wiring could have produced the unwanted air bag deployment either.

We then turned our attention to the steering wheel area.  The steering wheel was disassembled and the air bag container assembly was examined. No evidence was found of any failure with the air bag assembly itself.  We then focused on the wiring assembly behind the air bag that was contained in the air bag clock spring behind the steering wheel.  It’s called that because it looks like a clock spring that winds and unwinds.This is how an unbroken electrical connection to the air bag is maintained, while still permitting the steering wheel to turn back and forth.  It contains an outer ring that rotates around an inner ring.  The two rings are connected by a flat ribbon cable. The ribbon cable has four wires inside, two for the horn circuit and two for the air bag deployment circuit. As the inner ring of the clock spring rotates inside the outer, the cable is wound tighter and looser, thus allowing the steering wheel to rotate and maintain a constant electrical connection with the steering wheel mounted horn button and the air bag.

At this point, we removed the clock spring assembly for closer examination.  When we opened the clock spring casing to expose the ribbon cable, we found that the cable had a non-original, non factory, very unusual fold in it, located very near to the inner connector. The cable’s fold brought the horn’s positive wire into very close proximity to the air bag deployment wire.  When the driver turned the steering wheel in just the right way, at just the right time, the two wires touched.  When the driver accidentally beeped the horn, it provided power to the air bag deployment circuit and the energized air bag then deployed.

The opposition contended that the stereo installation employee had opened the clock spring and damaged the cable. We proved at trial, to the jury’s satisfaction, that this did not occur. First, the startling difference between the professional level installation of the Nav Head Unit, and the junior-high level cell phone and alarm installations, made it easy to distinguish who did what. In a high volume, professional quality shop, as was the case in this instance, technicians do not waste time removing anything they do not have to. With a typical flat rate installation charge by the company, there is strong pressure on technicians to finish the current job and move on to the next job.

There were no Navigation Unit circuits anywhere on the steering column. This meant there was absolutely no reason for the Navigation Unit installer to waste the considerable time it took to remove the steering wheel, remove the clock spring assembly, open it, remove the ribbon cable (for an unknown reason), stuff the cable back in (incorrectly), replace the clock spring assembly, and then reinstall the steering wheel. And if, for the wildest of reasons, he had removed the ribbon, he certainly would have obtained the proper tools to correctly repack the ribbon, as was befitting a professional who would be cognizant of the dangers associated with handling delicate wires of an air bag detonation circuit.

So, post trial, what do we think happened? Our best guess is a previous owner tried to install a steering-wheel-controlled anti-theft alarm system by himself. He got it all disassembled, couldn’t make it work, and abandoned the project, removing all the parts he had previously installed. What he didn’t know was that he had created a time bomb. The right amount of steering wheel turns in the right direction, coupled with an accidental beep on the horn at just the right time, equaled a very loud BANG !!!

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