[Tony Smith]

Polarity makes no difference.

The gauge works by current flowing though a wire winding around a bi-metal strip. As the winding heats the strip, it bends, deflecting the needle on the gauge. The current flow, and the frequency of the circuit opening is dependant on the temperature of the sender on the Cyl Head.

For the gauge to correctly indicate, it needs the correct voltage and amperage flowing through the wire to cause the strip to heat at a certain rate, but polarity doesn't change the heating of the bi-metal strip.

The gauge operates with MORE current showing a COLDER indication, and LESS current showing a HOTTER indication. A full 6v across the gauge should move the needle fully to cold.

Tom, I'd be putting a Multimeter on the back of the gauge with the Ignition on + engine cold, and then again as the engine is running at operating temp (and showing off the scale).

To me, it seems like you are putting less than 6v through the gauge. This could be caused by the 12v reducer not giving you a full 6v at the gauge, or defective wiring with too high a resistance, or a poor ground (either at the Instruments, or engine to hull).

There is also the chance that the gauge or sender are defective, but as they seem to be showing some of the right functions, I would say they are working OK if given the correct diet of electrons.