I hate contradicting mods sorry but a temp gauage is in actual fact a volt meter and if it fails it will not work at all, you put its full voltage across it, I think its 9 volts as there is a stabiliser live one side and other to earth it will show full reading (9 volts if it was scaled that way), you put a resistor between guage and earth and it will give an inbetween reading depending on the value of the resistor. A temp sender unit is in fact a variable resistor that varies with temperature, hence different voltage readings on guauge as engine heats and cools, then change voltage scale to temperature scale and hey presto. The sender unit sometimes shorts to earth which is why you need to disconnect it to find out, if the guage stays at full then the wire has to be touching earth, sorry.
