atomarc is dead on with his process. Rusty, it sounds like you have a broken or loose wire somewhere. Sometimes the ends touch sometimes they don't causing the swing. The mechanism in the sender could also be the culprit causing the same effect. As atomarc says, when the side of the gauge going to the sender touches ground the reading is empty...when its floating in the breeze (open) it reads full. The sender unit is essentially a variable resistor with a slider. As the gas level drops, the slider moves closer to ground. On a '39 the oscillations are dramatic because it wasn't until later that the gauges were "dampened" to reduce the bobbing of the needle as the gas sloshes in the tank.
