Fuel gauge wiring and explanation

[GlennC]

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

[GlennC]

Here you will see a typical circuit of a three wire system as used on a 45 jeep, possibly a good example to use considering the era.
any way within the gauge you will see two coils RH and LH.
the LH is connected to the power supply and then to the RH continuing on to ground, this is the path that MOST of the power(current) travels, without a sender connected you will expect to see full scale deflection (fsd),empty or full would depend on the manufacturer but you would see fsd and not somewhere in between.
You will see the sender is connected to the junction of LH and RH coils and in effect is connected across the RH "in parallel" or "shunt" for the purpose of current flow both coils are in fact resistors.
in use when the RH resistance is decreased then greater current flows and vice versa, this changing resistance (R) is achieved by the sender unit,this is achieved with minimal current flow through the sender it self.
a greater understanding of resistors in parallel would make this even clearer but is not overly necessary
the way that the pointer is moved is by virtue of the magnetic field that is created "around" the two coils when current flows through them (best kept that simple).


Glenn