Electrical gauges have been common on vehicle engines for many decades, used to monitor engine vital signs. The two gauges that are probably the most important to us are oil pressure and water temperature. These tell us two crucial symptoms of engine behavior. Over time, these units are subjected to temperature extremes, dust, and vibration. It’s no surprise that these components eventually fail, offering us a gauge that is intermittent or showing a zero or full-scale reading only.
There are three pieces to traditional non-digitized gauge systems that can fail: the sending unit, the gauge itself, or the wire and connections associated with them. An oil sender uses pressure applied on a diaphragm to move a sliding resistor based on oil pressure. A water sensor is placed in the coolant flow and its electrical resistance changes with temperature. Fuel level gauges behave similarly, using a resistance from the sender to indicate the proper level using the gauge.
In all cases, voltage is applied to the sender and the sender resistance allows more or less electrical current to flow to the gauge based on the parameter being monitored. Minimum current causes the gauge to read at one end of the scale, and maximum current moves the needle to the opposite extreme.