To test the gauge, use a variable resistor you can get from RS or other
electronic supply store, or fixed resistors with these values:

Full - 37.5 ohms
Empty - 290.0 ohms

do this directly at the terminals behind the clock assembly, IIRC to avoid
intoducing error from the wiring between the sender and gauge.  Use a known
good 12v supply and separate ground to chassis.

Then test the wiring between using a voltmeter for continuity, especially
the ground connection.  Clean up all of the contacts between the two
points.  The ground point for the sender is on the crossmember above the
sender and is a real bear to get to and loosen unless you drop your tank.
 You can also check the sender for those two resistance readings to see if
it is in spec.

--
Alan Felder
Austin TX
82 Diesel Westy