Today: An airbag answer that's "no charge."

Dear Car Talk

Dear Car Talk | Jan 01, 2010

Dear Tom and Ray:

I recently purchased a 2009 Chevy Traverse. My wife, who is a vertically challenged 5 feet 1 inch, likes to sit on a cushion to gain greater visibility. However, when she sits in the front passenger seat with a fabric material cushion, the warning light indicates that her seat's air bag is off. So, currently, in order to ride as safely as possible, she does not use the cushion. Is the air bag really turned off when she sits on the cushion? If it is, please explain to this non-automotive engineer how her sitting on a cushion is compromising the system

-- Calvin

RAY: If the light indicates "Passenger Air Bag Off," her air bag would not deploy in an accident.

TOM: Here's why. When air bags first came out, there were reports of some -- mostly very small -- people being injured by the force of the air bags.

RAY: So, in a technological fix, cars now come with smart air-bag systems. There are sensors in the front passenger seat that determine both the mass and the seating position of the passenger. And the computer then uses that information to determine whether the air bag should deploy, and, if so, with what force. Pretty good, huh?

TOM: Different cars use different sensing technologies. But in your particular car, a sensor in the front seat tests for "capacitance." What's that? It's the ability of an object to hold a charge. And since human bodies are 90 percent water, you can tell how big they are by how much of a charge they can hold.

RAY: The problem is, if she's sitting on a cushion, she's too far away from the sensor to conduct its test charge, so the computer concludes that there's no adult in the seat, and it turns off the air bag.

TOM: The sensor is designed to work at a small distance -- through stuff like winter coats and Bronko Nagurski Long Underwear -- but it can't work through a couple of inches of cushion.

RAY: So, what can you do? I see three options, Calvin. Option 1 is to forget about the cushion. That's obviously the safest approach. Without the cushion, we know the air bag will work, and she'll be sitting low in the seat, where the safety engineers expect the occupant to be during a crash. That makes the safety equipment most effective.

TOM: Option 2 would be to try a thinner cushion. You might find one that gets her close enough to the seat bottom to allow the sensor to detect that she's there. And the light will tell you if you've succeeded. If the "Passenger Air Bag Off" light stays off, she's fully protected by the air bag.

RAY: The third option is the bonbon-and-cheesecake diet. See if she can add a bigger, homegrown cushion of her own. That's the option I'd choose. Oh, wait -- I've already chosen that one.

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