This is a stupid question I have but can anybody tell me why the fuel gauge has a ground when it seems from every diagram I find that it is fed 12v via ign, and grounded after the sending unit acting as a variable resistor/potentiometer?? idk why I don't understand the reasoning for this, Anyone?
I wrote this a few years ago - just as part of a general discussion on the gauge/sender/chassis ground relationship - and to make sure I had the theory of operation for these gauges straight in my mind;
The components mentioned above - when connected by the pink and blue wires - form the "sensing leg" of the gas gauge. And as Mr. Bucket suggests, the sensing legs of all the gauges are fundamentally the same - only the method of grounding (via the frame rail for gas gauge and into the engine block for coolant temp/oil pressure) differs from one sender to the other.
But, if I'm reading it right, it appears that your question is: Are there actually two grounds required for each gauge? And if so, what is the purpose of the second ground point?
The answer is yes: each gauge has two coils - each of which requires both a 12 VDC supply as well as a connection to ground. Read on.
12 volts is supplied to the positive terminal of the sender (via the pink wire). It's source is the negative side of the gauge's measuring coil.
The less resistance in this total combined circuit, the greater the electron flow through the coil wires - and consequently the stronger the force is that it (the measuring coil) can exert on the gauge's magnet. The magnet is attached to the lower end of the indicating needle.
Now, to answer your question:
In addition to the sensing leg, each gauge has another - opposing - circuit called the "control leg". It is identical to the sensing leg in every way except that the negative side of its coil is lead straight to ground - at the common ground bus block. This as opposed to first going through a variable resistance and then to ground like the sensing leg. Also, within the actual gauge, it's coil is located on the opposite side of the magnet from the measuring coil's. This is a schematic of the gas gauge:
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So what you have is; two coils located on either side of an indicating needle - AKA the "ray". The ray is pivoted somewhere in the middle and has an integral magnet mounted at the bottom. This shows a coolant gauge and should help explain the mechanical action of the needle:
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These coils create their own magnetic fields - the strength of which is a function of the electron flow (i.e. current) passing through them. When these circuits are energized - by turning the ignition switch to the RUN position - the opposing magnetic fields begin to "fight it out". Each coil tries to pull the magnet - mounted on the needle - towards itself.
Due to the fact that it's negative leg is run straight to ground - resulting in nearly zero resistance, the current flow through the control coil is essentially constant (as is it's magnetic force).
On the other hand, due to the variable resistance generated by the sender (as a result of changes in the fuel tank level), the current flow/magnetic strength of the measuring coil can be higher or lower than that of the control coil.
The location of the two coils - to the magnet - is such that:
1. The control leg is always trying to peg the indicating ray to the FULL end.
2. The sensing leg is always trying to peg the ray to the EMPTY end.
So as extreme examples:
1. If the pink wire to the sender is broken/disconnected the control leg exerts the greater force and pegs the needle Full.
2. If the pink wire is allowed to contact ground (before passing through the sender), the sensing leg has a greater force and the ray pegs low.
As GM explains it (better and way more succinctly than I can):
VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.
When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil.
When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.
