Received an interesting question from a state fire instructor who is also an Assistant Chief in his local fire department. I think others will run up against the same issue so i wanted to share and get some dialogue going.

Here's the question...
"While conducting an extrication course, we set-up to detonate some airbags for the students. One of the vehicle we set up was a 2002 BMW 325ci. We dug into both the driverís and passengerís frontal airbags (to Hot Wire them for the demo).

We successfully fired the passenger dash airbag and disconnected our wiring to move to the steering wheel. Just after connecting our wiring to the steering wheel airbag the passengerís airbag (that had already deployed) fired again!

We continued forward and successfully fired the steering wheel airbag, disconnected the wiring and initiated a discussion with the students. While talking off to the side of the vehicle the steering wheel airbag fired a second time also!

In both instances the airbags deployed a second time about 90 to 120 seconds after the initial firing with no further input from us. Any thoughts? I have never experienced and airbag acting this way. Thank you in advance.

Here's my replyÖ
What you ran into is called either 'dual stage' airbags or 'dual threshold' airbags. That's the way most all of the frontal airbags are now in newer model vehicles. One airbag but you have a split airbag inflator charge; usually something like a 70% charge and a 30% charge. Both sit side by side in the same inflator housing. You'll probably see four thin wires coming out from the inflator instead of the usual two. If only one inflator fires in a real crash, then the airbag is softer and that is a good thing when the collision is not so severe and you need just enough protection that airbags should fire to protect you.

During a collision, the airbag 'brain' receives inputs from all the crash sensors, the speedometer, the brakes, even things like the seat track adjustments and the seat belts. From this data, it instantly calculates whether both charges should fire off (100% charge) or if maybe just one of the two charges in that airbag need to deploy. The larger one fires first in a real-world incident. The smaller charge is typically designed to heat and heat until it cooks off by itself. The second deployment will generally not be such a powerful charge as the initial one. During a crash, the larger inflator portion fires first if only one charge is needed.

What happened in your case was that the "thermal timer" for the second inflator charge activated due to the heat from the first charge deploying. The heat from the first deployment warms up the second, un-deployed charge, until it also self detonates. For one reason, this is because the automaker does not want their car leaving the scene with a live charge still intact. Welcome to the world of the new "Smart Airbag Systems"!

For anyone interested, I have a pdf file describing the procedure I use when Hot Wiring airbags before a University of Extrication hands-on class. If this is something you are interested in, email me at and I'll send it to you.