Radio code savers and airbags- IMPORTANT

John Corbs jcorbs at
Fri Nov 1 07:37:24 EST 2002

I recently had to take the wheel off to replace the left instrument switch
cluster and followed these instructions.

Used the 9v battery solution without a diode.  Fortunately my friend whose
garage I was using pointed out the risk of turning other things on.  As it
was, the 9v got very hot.

I located what I believe was the wire for the airbag power supply and
disconnected it as instructed.  HOWEVER mine did not look anything like the
one in the diagram.  The good news was that it appeared to be the only
option that was accessible from that side, so crossed my fingers and had no

----- Original Message -----
From: "Brett Dikeman" <brett at>
To: <human747 at>; "Lines Peter" <Peterl at>
Cc: <gkot at>; <quattro at>; <200q20v at>
Sent: Friday, November 01, 2002 2:42 AM
Subject: Radio code savers and airbags- IMPORTANT

> 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
> correctly.
> 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 :-)
> Brett
> --
> ----
> "They that give up essential liberty to obtain temporary
> safety deserve neither liberty nor safety." - Ben Franklin
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