Re: Battery replacement: 1995 A6

[DeWitt Harrison <de@aztek-eng.com>]

David,

Try this old indian trick - no PC flack, pour favore - to
prevent memory loss in automobile electronics during
_engine off_ maintenance procedures. Buy any old unregulated
12 VDC, 300mA (or there about) wall adapter, a 50V, 3A silicon
rectifier diode and an automobile cigar lighter, a.k.a. "accessory"
plug at Radio Shack.

Cut off the power plug that came on the "wall wart" and
replace it with the 12V car accessory plug. Wire + to
center, - to the side contact.  Wire the diode
in series with the positive lead, cathode marking band toward
the center terminal of the accessory plug. Connect the
negative lead directly to the side terminal of the plug. (The
diode may not even be necessary. It is there to prevent the
possibility of current from the battery flowing backward
into the little auxiliary supply - for example because you
wired it backwards - and to bring the open circuit
output voltage from the AC wall adapter, which tends to run
14VDC, closer to the 12.5 battery potential (engine off).
The wall wart will not be harmed if it supplies current to
the battery since that current flow is limited and loading
the output will quickly and harmlessly drop the output
voltage to match the battery voltage.

Plug the accessory plug into the car's cigar lighter and plug
the wall wart into AC power.  Now (and not before) you can
disconnect the battery while continuing to supply voltage to
the items which require power in order to retain their puny store
of data. This could include the radio programming, etc.
(It's a crime, really. It is trivial to design circuits which retain
data with the power removed. Maybe there is some bizarre
marketing requirement for this stupidity.) DO NOT switch
on any electrical equipment while in this mode since 300 mA
or whatever isn't going to go far. Switching on an electrical
consumer will essentially short out the little auxiliary supply
and bugger the exercise. Cheers,

DeWitt Harrison
Boulder, CO
88 5kcstq


David Kroth wrote:
> 
> The A6 is new to me and I'm considering replacing the 3.5
> year old battery as insurance before the cold weather really
> hits.  In reviewing the Bentely manual there is a warning
> on page W 27-1 (Volume 3, I think) that says:
> 
>    For automatic transmission vehicles, the transmission
>    control module (TCM) "basic setting" must be reestablished
>    using the VAG 1551 scan tool (ST), whenever the battery
>    has been disconnected or replaced during repair.
> 
> What's up with that?  Do I really have to find someone with
> a VAG 1551 every time I disconnect the battery?  Is there
> some other way to "reestablish the basic setting" of the
> TCM without the 1551?
> 
> This could be a real thorn in the foot.