A simple resistive device, the temperature sender on a motor vehicle provides a resistance proportional to the temperature of its probe.
The sender is screwed into the engine block or cylinder head at a relevant point. It has a probe which goes into the waterway, measuring the temperature of the coolant passing it.
Typically only one wire is connected, this being a positive feed from the temperature gauge. The wire is then earthed via the resistance to the engine block into which the sender is screwed.
Probe +-+ |
(Earthed on block)
Problems and Remedy
Although it is a reliable device, problems do occur. Typically a problem will result in the temperature gauge reading either fully hot fully or fully cold - when the sender is earthed (short circuit) or disconnected (open circuit). Calibration problems may also occur, resulting in a believable, but incorrect reading - although they are more usually due to failure of the voltage regulator.
A temperature sender is a cheap unit to buy (a few dollars), and assuming all the connecting wires are sound and the voltage regulator is accurate, replacement is much more sensible than attempting repair.