Quick answer:  There are electrodes on the side of the tank, connected to a
small "black box" on the side of the secret compartment next to the water
tank.  A green wire comes from this box to the LED panel.

The panel supplies current to this wire through a large (1 megohm)
resistor, and measures the resulting voltage on the line.  If you have
access to a high-impedance voltmeter, measure from the green lead to ground
(panel turned on)  while draining the tank from full to empty.  As it
drains, you should see first < 3v, then >3 <5, then >5 <7, then >7
(corresponding to Gn, Y, R, off).  If you have values like this, then your
panel needs fixing.  If not, then it may or may not be the panel.

More later.
david

At 02:27 PM 2/21/2000 , Tobin Copley wrote:
>Folks:
>
>Quick Q: the water level indicator light in my 82 westy seems to be stuck
>one level above "empty".  I'd assumed we were just low, but I filled it up
>with water yesterday, and the level indicated did not change.  I'm assuming
>there is some sort of sender unit in the tank someplace, but Bentley does
>not discuss this.

David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"