Radio code savers and airbags- IMPORTANT

Brett Dikeman brett at
Fri Nov 1 02:42:58 EST 2002

At 1:00 PM -0500 10/31/02, Huw Powell wrote:

>Putting a diode in the doohickey you are making will prevent this
>reversed current flow, which, as you point out, would be very bad for
>the 9 volt battery.  1 amp diodes are pretty cheap at RS as well.

Page 68.30A of the 100/200 Bentley covers this- and Audi recommends a
diode too.  They also mention Snap On makes a prebuilt version.

>Another minor warning, be careful to avoid turning anywthing on in the
>car while this is set up, if you pull that 9v down far enough you'll
>lose the radio anyway.  Not sure if the 9v battery has enough oomph to
>run the interior light(s), door chime, etc.

A slightly bigger warning here- on the same page, is a boxed note,
which I quote:

(bold)WARNING - ALWAYS(/bold) separate airbag power supply connector
before using a computer memory saver.  Failure to do so may result in
accidental activation of the airbag."

The diagram shows a single-pin cylindrical connector, and identifies
it as being "located behind the center console."  Further
instructions identify it as being on the driver's side, and easily
accessed- remove the side panel in the driver's footwell(ie, the
carpeted panel right where your right knee would rest.) I've removed
that panel before- I think there's barely two screws involved.  I
strongly suggest reviewing this section of the Bentley lest one
unplug the wrong connector.

Rarely does the Bentley offer such warnings without merit.  In fact,
a quick search of the web turns up the answer; google to the rescue.
I think I searched for "computer memory saver tool"(in quotes to
specify the phrase) along with the word 'airbag'.)  I found a
discussion on the BMW list whereby an engineer, who worked on such
systems, reveals the reason - and it makes sense.

A good parallel here would be old Apple II's- if you flicked the
power supply on+off, or tripped on the power cord...sometimes it
would survive, but most of the time- you'd get a really nasty screen
of flashing characters; sometimes the system would hiccup and start
executing another part of the program and (very quickly) crash.

Apparently, the risk is that early airbag controllers were not very
foolproof and could accidentally, during/after a "brownout", get
confused- and possibly end up  executing "set off the airbag" code.
Lack of battery power isn't a problem, by the way- the airbag system
has its own energy supply, located, along with the sensor, in the
center console...roughly under the ashtray, if I read the diagrams

To be honest, I have no idea why this would be different from
situations such as a dead/dying battery or charging system
problems-but I personally am not one to argue with the Bentley when
explosives are involved :-)

"They that give up essential liberty to obtain temporary
safety deserve neither liberty nor safety." - Ben Franklin

More information about the 200q20v mailing list