There is basically two ways a gauge can be configured to work, one way has the current flowing through the sender unit the other does not, there is two distinctly different gauges for each method probalbly described best as two and three wire gauges.
there is a method available to use a two wire in three wire mode but lets not get too creative here.
firstly the two wire gauge as is obvious it has two wires one goes to the power supply (12 or 6 volt) and the other directly to the sender unit, the advantages here is simplicity the disadvantage is that all the current flows through both devices not always a good idea particularly with fuel gauges.
The three wire method uses the sender to "shunt" an internal resistor within the gauge itself, the advantage here is that nearly all the current flows through the gauge and not the sender unit leaving the sender to run cool,(within our applications the sender may be immersed when being used at its "hottest" thereby using the fuel to cool the resistor in the sender but leaving the sender cold when exposed).
mixing the two methods may result in excessive current being passed by the sender unit leaving the risk of igniting the fuel either in normal operation or of most concern when the sender fails (which it will) if it is not designed to be used in two wire mode.
see the next post for a pretty picture and words