Most oil pressure senders are a normally-closed switch that puts the
connector terminal to earth ground in the lack of rated pressure.  When the
pressure increases, internally opening the switch, the circuit loses it's
grounding and the light goes out.  If you touch the wire to ground, the
light should come on.
  You have apparently created a +12 V-powered wire that previously was a
negative (-) part of the circuit..  So, now, instead of a neg. wire seeking
earth to complete the bulb circuit, you have a hot wire that is contacting
ground through the oil pressure switch and frying the wire (or blowing a
fuse, which would be better!).  The wiring connector 'pin-outs' from
old-cluster to new-cluster are obviously not identical pin-for-pin.  This is
apparently causing your problem. Compare old wiring diagrams pin-for-pin to
the newer style version, and you will find the problem(s).  I don't
personally know the differences, but others here may be able to chime in to
assist you.

HTH,

Mike B.

----- Original Message -----
From: "Jonathan Poole" <jfpoolio@GMAIL.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Tuesday, April 01, 2008 4:16 PM
Subject: 91 instrument cluster, oil pressure led

> Howdy List,
>
> I retrofitted the instrument cluster from a 91 passenger van into my '83
> air
> cooled westy.  All is good with the retrofit,, except for the oil pressure
> led/lead.  I disconnected the ground for the buzzer and all, so no problem
> there but what has happened is that after wiring things up following the
> great guide at old volks home:
>
> http://www.oldvolkshome.com/late_van_cluster_install.pdf
>
> I fried my dual output oil pressure sender.  The reason that it was fried
> (or at least sauteed) is that the wire that runs from the instrument
> cluster
> to the sender is now hot or has voltage.  I disconnected the wire from the
> sender and the on the dash light doesn't come on at all (which is good for
> now).  I plan to replace the sender so that my oil pressure guage will
> work
> again but would also like to wire up the in dash oil pressure light as
> well.  Does anyone know if this wire should have voltage at the oil
> pressure
> sender or if there is a problem with my cluster or wiring?  Before this
> cluster change my oil pressure led and gauge both worked great.
>
> Thanks for any help,
>
> Jonathan Poole
>