Kudos to Jim G for his detailed description of the mechanics of airbag deployment in S4 Spiders, but I believe the primary matter of concern in system functionality of vintage airbags goes beyond an electrical check of deployment circuitry and lies in the patency of the propellant. From a Popular Science article, here discussing the Takata fiasco:
“Automakers and federal regulators have found multiple causes for the inflators turning deadly. Among them: bad oversight on the manufacturing floor; the design of the car itself; and years of exposure to high heat and humidity in certain regions (a number of the deaths have occurred in Texas and Florida). Such heat and humidity can break down the propellant wafers, so that the ammonium nitrate propellant—a chemical than can be used in making bombs—burns too hot and too quickly, creating excess pressure in the metal housing that then explodes with deadly force
Here’s the article in its entirety:
I suspect ammonium nitrate was also the propellant of choice in S4’s and those cars living in high heat and humidity environments may be more susceptible to chemical degradation, resulting in anomalous deployment.
Everyone has no doubt seen video of airbags going off (as shown in the linked article) but last weekend, I volunteered at Reno SCCA’s Street Survival program for teen driver training and as always, a highlight is controlled deployment of removed airbags on the course. Two were set off and the surprisingly loud BANG
jolted all of the assembled multitude. A third airbag was deployed with a pylon atop and launched the hapless object 100’ skyward.
In conclusion, I am concerned a nearly 30 year old airbag, especially one that has endured an unfavorable environment as described above, could be counted on to perform as originally designed and may in fact be a source of additional injury in a crash. I don’t know what crash statistics are for pre-airbag equipped Spiders, but would be inclined to disable a S4 SRS and count on properly used seatbelts to be my source of protection.
Now, can we talk about the reliability of elderly seat belt webbing?