Outside of electrical supply and ground issue's, this is the main cause of fuel sending unit failures. You can see the wire wrapped around an insulated curved piece where a contact connected to the float arm slides causing the resistance to the gauge to vary showing different fuel levels. Note the broken wire by the red arrow. This causes the gauge to only work if the contact is above the break in the wire because everything below the break is disconnected from the rest of the wire and therefor will not make a reading on the gauge. This break in the wire caused my gauge to only read the fuel level if the level was above 1/2 tank. Otherwise the gauge showed empty. Just thought some might be interested if they have never seen this before.