Presume it has a separate light bulb with it's own connectors?
If so, one will be the feed from the switch, the other the negative
Not aware of the model you have, but if I were to guess I'd go 1 for feed and 2 for earth
That said, a lot of Smiths gauges need to be wired through a regulator - that's where my knowledge ends!
A quick google through this up:
Inside the gauge is a bimetallic element (a flat strip of two metals bonded together). This strip will bend when its temperature changes because of the different coefficients of expansion the two metals have. This strip is wrapped with a resistance heating wire. One end of the metal strip is anchored, the other is connected loosely to the gauge needle. One end of the resistance wire goes to the 10V source, the other is connected to the sending unit.
The sending unit (temperature or fuel) is a variable resistor. 10V is supplied to one gauge terminal and the electric current flows through the resistance wire, out the gauge, down the wire to the sender and finally to earth. As the sending unit's resistance value changes it "throttles" the current and therefore the heating of the resistance wire inside the gauge. The more current that flows, the more heat generated, the more expansion there is of the bimetallic strip and therefore... more displacement of the gauge needle (hot and/or full).
The problem is that current and voltage are proportional. If you increase the voltage supplied to the system but the resistance values remain the same, the current has to increase. So if you supply 12V you are supplying 20% more voltage (and current) than the gauge was designed for. Thus, it will read high and physically the gauge itself will run hotter than intended which will shorten the life of the gauge and sending unit.
HOWEVER, you cannot talk about supplying a gauge 12V... that only happens when the engine is not running. As soon as you start the engine and the alternator starts producing electricity and the car's voltage system is no longer at 12V, it is at 14V or more. So, you will not be running your temperature gauge with 12V (or 20% too much voltage), you will be running around 14V, or 40% too much voltage and current. Yes the gauge will work but it will not be accurate and it won't last long.
Not very helpful of me - but I'd buy a TIM gauge
