Just as a general discussion on the gauge/sender/chassis relationship - and to make sure I have the theory of operation straight in my mind;
The components mentioned above - when connected by the pink and blue wires - form the "sensing leg" of the gas gauge located in the instrument panel.
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.
In addition to the sensing leg, there is another - opposing - circuit called the "control leg". It is identical to the sensing leg in every way except that the negative side of it's coil is lead straight to ground (rather than through a variable resistance and then to ground like the sensing leg). Also, it's coil is located on the opposite side of the magnet from the measuring coil's.
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.
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.
Below are some images showing the top of the sending unit and how the pink and blue wires are connected to it.
Hot wire as it connects to sender head - there is a brass stud on the sender head and the connector is just a push fit onto it:
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Grounding lead at sender end:
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The other end of the grounding lead - where it bolts to the frame rail:
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Note: The feed wire is
not one continuous run from the instrument panel to the sender head. For reasons explained below, it is in two sections.
The first section starts at the firewall connector, goes down and runs back towards the rear of the truck (within the rear lighting harness loom). At a certain point - near the rear wall of the cab - it pops out of the loom and ends there. It is terminated using the female half of a pin/socket type connector - the same connector that is on the sender head.
At that point an extension is added on - the length of which depends on whether the truck is equipped with dual or single tanks. In the case of a single tank mounted on the LH side, the extension is only about 3 feet long and it goes directly to the sender.
If the truck is equipped with dual tanks, the extension is longer and is lead over to the 6-way switching valve on a frame rail cross member. Inside the switching valve control head, the pink wire is split into 2 different leads. It exits the switching valve as a pink/black wire (to the auxiliary tank) and a pink/white wire (to the production tank).
I only mention this extension connector because if you are having gas gauge issues, this could be a possible point of failure in the sensing leg circuit. It is pretty well hidden and can only be seen with the bed (or LH tank) removed. This is it's location:
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