Resetting service warning lights

The are also possibly backup batteries in some parts of some cars. But that's not to say they will feed the airbag trigger mechanism. Which I'd guess will need a fair bit of current.

Quite.

But if you disconnect the battery at the start, chances are it will be fine by the time you've actually got to the airbag to unplug it.

A minute after disconnecting the battery seems far more likely to me.

And it's going to take more than a minute to put down the tools used to disconnect the battery and pick up those needed to remove the airbag. ;-)

Can you explain why you'd need an airbag to work some 30 minutes or more after a car has totally lost power in an accident, etc?

Any form of electrical storage costs money. And the bigger that store, the more it costs. Car makers are well known for not spending a penny where a farthing will do.

It?s hard to conceive of any reason why you would want airbags powered for so long, and easy to think of reasons why this would be a bad idea. Seems far more likely to me that the capacitor will be designed to discharge via a resistor over a much shorter period (less than a minute I?ll wager).

Tim

Are you going to disconnect the only thing that will supply current after a few seconds, the battery?

Do you have a clue about how much energy is needed to detonate the explosives in an airbag?

Correct. I was talking about "honest" john.

Want to post a link to one that says 30 minutes?

I would sincerely hope the designers have enough sense to put high value bleeder resistors across any significant capacitive elements to drain any residual charge within 2 minutes.

Its faulty, the light is/has been on, so what you just said doesn't apply or even make sense. That is the whole point, you do not have a clue as to what will happen when the system has/is indicating a fault. It may fail to go off when it should, it may go off at some random point, it may even work as expected. Clearing the warning doesn't make it safe unless it also passes the tests, ie. its unsafe and therefore illegal under UK law. Its just as illegal as havining spiky bits of metal sticking out of a car.

It's not that simple, Dave. Modern electronic modules and CPUs are made to be so energy-saving it results in as much high-impedance circuitry as possible, so even relatively small capacitances can take an age to discharge unless the designers have provided bleeder resistors.

True but that isn't a service warning light, its a fault indicator that requires service.

dennis@home explained on 28/04/2018 :

It is technically possible to enable them to still fire days later, it is also possible to have them still able to fire at 29 minutes later, but not at 30 minutes later.

Why don't you tell us, since you're such an expert?

The manual for mine (not a Volvo) states to leave it for 5 minutes after disconnecting the battery. And they've probably added a couple of extra minutes to get to this figure just to be on the safe side.

Given that its acceptable to disable an airbag (say when you have an infant in a read facing seat on the passenger seat), then I would be surprised if driving with a non functional airbag is actually an offence.

I would expect any safety critical design to fail safe...

For example I had a dodgy sensor on one car with ABS. If you hit a pothole with the rear offside wheel it would cause the ABS warning light to come on. At that point the ABS system as a whole was then inhibited, and the breaking system reverted to a normal non ABS one. A design decision, that elected to possible disable the breaks entirely on detection of a fault, would hopefully not get through a design review!

"Brakes".

You're welcome.