The vibrations of the instrument cluster end up by developing cracks in copper routes, most often around heavy parts that stress the PCB more due to bigger inertia. The German manufacturer of instrument cluster, VDO, was very 'careful' to use as little copper as possible on the PCB. cracked copper routes around mounting pins of the fuel gauge (see photo below).cold solder joints of the yellow connector on the PCB of instrument cluster.bad wiring between fuel sender and instrument cluster.bad contact between the copper slider and the rheostat inside fuel sender.So now you need to diagnose why is the current flow from the fuel sender interrupted. I hope that now we're clear, I almost passed out here When the current flow through coil L2 is interrupted, the fixed magnetic field "wins" and the needle stays at an intermediary position (around 1/2 tank in your case). The needle moves according to this magnetic "tug of war" inside the can of the gauge.
The resulting magnetic field acts on a rotating magnetic armature (a small cylinder inside the coils) that has the needle of the gauge attached to its axle. The idea is to have a fixed magnetic field created by L1 and a countering variable magnetic field produced by another current coming through the fuel sender (which is a variable resistor or a rheostat). By 'opposing' I mean wound in opposite directions so they produce opposing magnetic fields when a current passes through them.
Inside the gauge there are two opposing windings or coils (L1 and L2). I've made this drawing to keep things simple.
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