Limiting Coil and Operating Coil

Current from the battery passes through the limiting coil to the common connection of two coils at the lower terminal on the dash unit. The current is then offered a choice of two paths, one through the operating coil of the dash unit and the other over to the tank sending unit. When the fuel tank is low or empty, the sliding brush cuts out all resistance in the tank unit. Most of the current will then pass through the tank unit circuit because of the low resistance, and only a small portion will pass through the operating coil to the dash unit. As a result, this coil is not magnetized enough to move the dash unit pointer, which is then held at the "Empty" position by the limiting coil. When the tank is partly full or full, the float of the tank unit will rise to the surface of the fuel and move the sliding brush over the rheostat, putting resistance in the tank unit circuit. More current will then pass through the operating coil to give a magnetic pull on the pointer, which overcomes some of the pull of the limiting coil. When the tank is full, the tank unit circuit contains the maximum resistance to the flow of the current. The operating coil will then receive its maximum current and exert pull of the pointer to give a "Full" reading. As the tank empties, the operating coil loses some of its magnetic pull and the limiting coil will still have about the same pull so that the pointer is pulled toward the lower reading.