Technical Fuel level sending unit

[80spider]

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, carb.

 

[sludgeguts]

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, carb.

They don't, at least ones i have played about with in the dim and distant past didn't.
The 'sender' is basically a variable resistor. power goes through the fuel guage and along the wire to the 'sender' in the tank. the amount of fuel in the tank governs where the contact on the variable resistor is, so with a full tank, the sliding contact will be at the lowest point on the resistor - therefore allowing highest current to pass to ground.
To test the fuel guage, turn ignition on and take the power lead off the 'sender' and ground it - no resistance should make the guage show full tank. If you suspect the 'sender' try cleaning up the grounding first.
the whole thihng is relatively simple... 12v to ignition sw, through fuel guage, through the 'sender' and to ground.