Air bag / ignition off ?

[Minty]

Question - do airbags work when the ignition is turned off?

[snuffle]

None that I know of.

[lost in espace]

If not why should you disconnect the battery for 10 minutes before fiddling with an airbag then?

[TheEnd]

lost in espace said:

If not why should you disconnect the battery for 10 minutes before fiddling with an airbag then?

People being overcautious.
The airbag needs a voltage spike to set them off. Unplugging a lead it's going to magic a 12v spike out of nowhere, but you will get a "can't be too safe" type of person that want's you to take the battery out of the car before going near it.
Have you even seen a brakelight flash when disconnecting a light cluster?

There is one good reason to disconnect the battery when fiddling with airbags, and that is to make sure that you can't switch the ignition on as the system will spot a missing airbag and keep the warning light on.
A lot of people do this when they remove a pretensioner seat, or a steering wheel, and switch the ignition on to release the steering lock or to roll the car.
You are then stuck with the light on the dash until it is reset electronically.

[TooLateForAName]

To stop any static or residual charge setting them off by accident when you fiddle with them. But they won't be set off by the proper system.

[Minty]

Hmm, so switching off whilst in a traffic jam for example is economical but risky??

Wonder if the Eco cars with automatic engine cut out get around this somehow ?

[TheEnd]

The eco car will still have the "ignition" on if it is doing that auto stop start stuff.

[PHmember]

Minty said:

Hmm, so switching off whilst in a traffic jam for example is economical but risky??

Wonder if the Eco cars with automatic engine cut out get around this somehow ?

Exactly how fast are you expecting the impact speed of the crash to be whilst sat in traffic with the engine off?

[Minty]

fair point, in the middle of a big jam it should be irrelevant.

so, 'ignition left on' should mean the airbag will be deployed in the worst case scenario....

[snuffle]

yes

[Rubin215]

Yes, they do.

The system has a residual charge that can still set it off.

After a vehicle collision, the Fire and Rescue Service will fit an airbag restraint over the steering wheel and take great care not to get between unshielded, undeployed airbags and anyone still in the vehicle.

We disconnect the battery as soon as feasible, and do not use radio apparatus within 10 metres of the vehicle.

We also strip away the interior plastics of the vehicle to expose curtain and side airbags, seatbelt pre-tensioners and other potential hazards before we do any cutting or spreading.

[anonymous-user]

The "Squib", which is the detonator for the main gas producing charge in the airbag assy, is "fired" using approx 30v, which is generated by the airbag control module (with a DC-DC convertor) and stored on a set of capacitors within that ecu. This does two things:

1) guarentees a "firing event" by pushing enough current to the squib to overcome any slightly dodgey high resistance wiring connectors etc (also squibs have a diliberately large current requirement to trigger to avoid false triggers from static (ESD) events)
2) guarentees a "firing event" even if the air bag modules powersupply has been interupted before the trigger event (think multiple roll over where curtain bags may only fire later in the crash etc)


These power storing capacitors have a "Load resistor" across them, which slowly discharges the voltage when the ignition if off. disconnecting the battery and waiting 10mins ensures that insufficient voltage is availible to trigger the squib, what ever the circumastances (when connectors within the loom are disconnected, you cannot guarentee what order the connector pins will seperate or mate in, hence it might be possible to accidentally produce a "trigger" signal.

[maniac0796]

The system tends to store a charge.

Apparantly, the correct procedure for taking them out is to put them in a cage. I just put mine at the back of the car outside whenever it's out. Then again, my airbag lights been on since i bought the car, so I doubt it works.

I have had a BMW one from the Napoli sat in my bedroom for 3 years now though Tempted to set it off soon, see how high it'll go.

[mcford]

As mentioned the system holds a charge. If you consider a collision on a car with the battery at the front, it'll be one of the first things damaged and possibly won't be able to supply electricity to the air bag system, hence the system storing enough electricity to deploy them.

[RobM77]

Slightly off topic, but is it true that an airbag can give you hearing damage?

[PHmember]

What?

[anonymous-user]

RobM77 said:

Slightly off topic, but is it true that an airbag can give you hearing damage?

A temporary "loss of hearing" is faily common post airbag firing, but i haven't seen any permanent hear loss reports?

[PHmember]

Pardon?

[Rubin215]

RobM77 said:

Slightly off topic, but is it true that an airbag can give you hearing damage?

Yes.

But if you are the driver of a car in a frontal collision, I would much prefer mild hearing loss to reconstructive facial surgery.

[RobM77]

Max_Torque said:

RobM77 said:

Slightly off topic, but is it true that an airbag can give you hearing damage?

A temporary "loss of hearing" is faily common post airbag firing, but i haven't seen any permanent hear loss reports?

I've heard plenty of stories to this effect, and being a keen musician it's always worried me. Obviously, I'd rather have hearing damage than massive injuries in a 60mph accident, but I do worry that if I had a bump in town where I might just be a bit sore for a few weeks that in such an accident I might damage my hearing.