[Ronbo]

The sending unit is merly a resistive element that changes value based on tempature.

With this in mind disconnect the lead to the unit and briefly short it to ground. This should cause the gauge to swing. You can also test this at the gauge's sender terminal itself with a jumper wire to ground (disconnect existing wire first).

I've seen sending units with both coefficiants (either raise or lower resistance with temp increase) so I can't tell you which to look for.

You'll want to verify +12v to the gauge's IGN terminal as well as GND connection.

Oops, looks like Patrick beat me to it...