To check the amount of fuel left in the gas tank we generally look into the fuel gauge mounted on the vehicle’s dashboard. The needle on the gauge moves from F to E as we use the gasoline. But we have to understand that many times it doesn’t show the exact fuel level of the tank because it is controlled by a sensor or Sending Unit present in the gas tank. Sometimes, the needle of the gauge stays on full and doesn’t move for a while though the vehicle puts up a few miles after filling the gas. This is because the fuel consumption has to cross a certain level in the tank for the sending unit to respond to the drop in fuel level and send an indication to the gauge. Often times it happens even with the empty tank that the gauge reaches the empty mark even when there is still plenty of gas present in the fuel tank. Before we understand the fuel gauge problems, let’s first look into its working mechanism.

Fuel Gauge Mechanism

A fuel sensor or sending unit consists of a foam float attached to a metal rod that moves up and down with the level of fuel and a resistor attached to the rod that controls the amount of current sent to the fuel gauge. The current passing through the resistor is controlled by a small mechanism in the resistor. It contains a wiper which slides over the strip of resistive material which is connected to the ground on one side. The entire fuel sensor assembly is typically attached to the fuel pump inside the gas tank. While the fuel pump is responsible for pumping the fuel from the tank to carburetor or fuel injectors, fuel sending unit sends an electrical signal from the fuel level sensor to the gauge based on the level of gas in the tank.

When the gas is full and the float is at the maximum level, the wiper of the resistor goes close to the grounded side of the strip resistance. This cause drops in resistance across the resistor and allows maximum current to flow through it. Thus the maximum signal is passed to the fuel gauge and the needle of the gauge indicates the maximum gas level at F. As the gas level decreases in the tank, the float sinks, and the wiper moves towards the other end of the grounded side. This makes the resistance to increase and reduces the amount of current flow to the fuel gauge. Thus the needle shows a decreasing reading in the gauge. When the gas is low, the float goes to the bottom, resistance reaches a maximum and very little current is sent to the gauge. Thus the needle is at the minimum reading i.e at “Empty” or E with a warning light on indicating low fuel level.

What Issues Can A Fuel Gauge Have?

If the fuel gauge is not calibrated correctly it might show inaccurate readings other than the actual gas levels. On the other hand, a bad sending unit or a faulty wiring between the sending unit and the gauge can also tend to create problems with the fuel gauge. The needle will behave erratically and it either reads the same or always shows full or empty irrespective of the gas levels. For instance, the reading may show half full even though the tank is empty or it may get stuck at empty through the tank is filled up with gas. It is risky for a driver to rely on faulty fuel gauge or sending units to know the gas levels. Particularly, it is more problematic with situations like the tank is nearly empty and the gauge is not showing the right readings. Any issue with a fuel gauge or sending unit should be checked and fixed immediately to avoid troubles while driving.