How Gauges Work
by Bob Wallace
Electrical gauges are only of two types they measure volts or amps.
A volt meter has a high resistance and is put in parallel with the circuit at the point voltage is measured at.
A amp meter has a low resistance and is placed in series with the circuit.
Your high school physics were you learnt about ohms law and resistance in circuits is all you need to recalibrate gauges to different ranges.
To talk specifically about your two temp gauges.
Both of the sender units contain a thermo-couple. This is a junction between two dissimilar metals which when heated changes resistance and produces a small current.
When the junction is heated the current will always flow in the same direction for the same two metals - this in fact gives an increase in resistance in one direction and a reduction in the other!
So the oil temp gauge is a volt meter which measures the increase in voltage at the gauge due to a reduction in resistance the thermo-couple in the sender; hence the requirement for an external voltage.
But the head temp gauge is an ammeter measuring the current produced by the thermo-couple in the sender; hence the sender and gauge need no external power.
The choice of type of gauge (voltmeter or ammeter) is probably governed by the economics of making a thermo-couple with suitable characteristics for the required operating temperature range rather than the cost of the gauge.
Other gauges like an electrical vacuum/pressure
gauge will have some means of changing a variable resistance in the
sender unit and are of the voltmeter type so require an external
voltage.
