01-02-2014, 02:07 PM #1
Let's Just Unplug the "Brain"…Can't We?
Received a question from an experienced vehicle rescue instructor in NC that is worth sharing. Here is his question and below that, my reply. How would you answer this same question if it came up in a discussion at your station?
"Hey Ron.... I need some help on a subject. I had a department call me the other day. They had an instructor tell them that there was a wiring harness for the airbag system under the dash that you could unplug and it would disarm all airbags according to that instructor. I told them that if you had major damage to the dash, how could you find it? Also, I said it would not disarm the cylinder fired airbags. Could you give me your input?"
My reply is;
"I've had similar comments or questions from students in the past. Your response was about what I tell them.
Yes, there is a main wiring harness and connector that comes into one side of the airbag control unit; what we commonly refer to as the airbag 'Brain'. It runs signals into the Brain from the crash sensors and then is also the wiring harness that takes the deployment signal from the Brain to all the individual airbags whether they are stored gas or the chemical type. It is also the main 12v power supply to the Brain.
The airbag brain is not always under the dash although this is a one place where it can be found. The most common location is along the center 'transmission tunnel' or hump between the two front seats. If you have access to the Crash Recovery System software, look up a few models. You can see both the under-dash and the between-seat locations on the Crash Recovery as they use a yellow box diagram to indicate the Brain.
I have attached a screen shot below of an Audi A3 convertible. You can see the airbag Brain location deep under the instrument panel and dash of this particular vehicle.
If you have people trapped, in reality, I explain to students that it is not practical to dig through and under the center console or deep under the dash to try to disconnect this main cable connector to the Brain. Crews should be shutting down 12-volt power and respecting the airbag inflation zones. Reputable vehicle rescue instructors SHOULD NOT be telling students that this is practical or even a recommended practice by any means. It is NOT I assure you.
Even with 12-volt power shut down as should be down at extrication incidents, each individual airbag is still vulnerable to deployment due to static electric discharge. They should be treated with respect at all times.
One interesting side note is that there is one style of airbag that responders might consider "unplugging". Disconnecting the power wires running to the stored gas inflator of a roof airbag unit might be possible with some designs. Stored gas inflators for these airbags might be able to be exposed by stripping away the trim. With that out of the way, a responder might see the wiring going to the inflator. There is a big, yellow safety plug within a few inches of the inflator that can possibly be disconnected.
This is the only airbag design where I would ever even consider having a responder unplug or disconnect any airbag wiring. I do not mention this possibility in my classes and only explain this aspect if I am specifically asked about it by a responder who has seen the exposed connector, wiring, and stored gas inflator and is curious."
01-02-2014, 04:15 PM #2
Thank you for the share. I agree its best to DC power supply and respect the zones. One thing I teach probies during extrication training is to know what your cutting. These vehicle manufacturers are hiding sensors, gas cylinders, pretensionsers, and airbags everywhere now. IMO its best to rip off the plastic covering inside of posts and or rockers that you intend to cut or spread.
I do have one question though. What is the average time capacitors still hold charge after battery DC. I have heard many numbers.
01-02-2014, 04:24 PM #3
As a vehicle rescue instructor, you would be wise to make sure all your members understand that airbag systems in vehicles use a capacitor to store energy; even after the battery has been shutdown. It used to be in the "old days" of airbag designs (the 1990's), that capacitor 'drain time' could be easily ten minutes; even up to 20 in some old vehicles. GM had one early car with 7 capacitors and an excessive drain time of 20 minutes. That is not the case any longer.
When asked about the 'typical drain time' for an airbag system on a late model vehicle, I explain that it can be as short as one second or as long as ten minutes. The average time is 2 to 5 minutes however.
Regardless, make sure you explain to the members that they still need to respect the undeployed bags whether the power is down or not.
Remember also when talking about capacitors, mention to your personnel the fact the hybrids and EVs also have high-voltage capacitors. They too have a 'drain time'. In this case, when you're talking about DC voltages like 300v+, the drain time is generally advertised by the manufacturers as ten minutes.
01-02-2014, 04:58 PM #4
Thank you very much, I appreciate your input. I will print this out and include this in next shifts training. Like you said though, I am big on zones, and never assuming that DC the battery will disarm airbags. http://www.youtube.com/watch?v=b1Qj75pbl8o This video is a good reminder of just that.
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