It's one of these:
http://www.radioshack.com/product/index.jsp?productId=2036270

pointed from ground to +12v.

The only explanation I had, was that if you hooked up your battery
backwards (oops!) and ran power through the cluster backwards, all the
current would pass through this diode and blow a fuse, and not damage any
of the gauge circuits.  But I'm not sure if that really would be a design
goal.

On Thu, Jun 7, 2012 at 1:30 PM, David Beierl <dbeierl@attglobal.net> wrote:

> At 12:50 PM 6/7/2012, pickle vanagon wrote:
>
>> We made some good progress on this project last night, but there something
>> that is puzzling me about the foil.  In the bank of resistors on the back
>> of the foil, the third one down is a diode which goes from ground to
>> positive 12v.  What is the purpose of this kind of arrangement??
>>
>
> If it's a Zener (symbol would have little wings slanting up and down from
> the ends of the line) it could be an overvoltage limiter.  Even a plain
> diode (reverse biased of course) could be to suppress ringing/spikes when
> the panel is shut down.  If it's forward biased (arrow pointing toward
> ground) then you haven't traced it correctly.
>
> VW had a tech note at some point about adding a diode when replacing the
> panel voltage regulator, but I don't remember what precisely it connected
> to.
>
> Got a number on it?
>
> Yours,
> David
>