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Fuel level sending unit signal

What kind of electrical singnal does the fuel level sending unit send to the fuel gauge? (trying to figure out where the system is bad that is preventing the gauge needle from moving.) 1980 Pininfarina Spider

It usually is just a DC voltage. The gauge supplies regulated power to the sender unit which has a float tied mechanically to a variable resistor. As the fuel level changes so does the voltage going back to the gauge.

Your gas gauge is really an ammeter. It has a 12 volt wire connected to one post. The other post is connected to the gauge sender which is a variable resistor connected to a float inside the tank. The sender is grounded and the current measured by the gauge is controlled by the position of the float which changes the resistance (and gauge reading) as the float moves up and down…Most gauges are “dampened” so they don’t move around quickly as the gasoline sloshes around in the tank…