Fuel Sending Units

What is a fuel sending unit and where is it located?

The fuel sending unit is located at the back of the vehicle inside the fuel tank. It measures fuel level, allows fuel to flow to the engine and excess fuel to return back to the tank, and vents excess vapors from the fuel tank. On some vehicles, the sending unit includes the fuel pump and a fuel sock—a filter designed to prevent large material from entering the fuel pump. While fuel sending units may differ in design, their functional purpose remains the same.

Inside every vehicle there is a fuel gauge that provides the driver with a visual indication of how much fuel is left inside the fuel tank. This helps the driver monitor fuel level and know when the tank needs a fill to avoid running out of gas. The fuel gauge is just the indicator; behind it is the vehicle’s fuel delivery system, and a major component of that system is the fuel sending unit, which measures, then sends the fuel gauge on your dashboard information about the tank’s current fuel level.

Many different names are used when referring to a fuel sending unit, and they all refer to the same part. The most common are: fuel sender, fuel sender unit(s); fuel sensor, fuel level sending unit(s), fuel level sender(s), fuel level sender unit(s), or fuel level sensor(s), fuel pump sending unit(s), fuel pump sender(s), or fuel pump sender unit(s), fuel tank sending unit(s), fuel tank sender(s), fuel tank sender unit(s), or fuel tank level sensor(s), fuel gauge sending unit(s), fuel gauge sender(s), fuel gauge sender unit(s), or fuel gauge sensor(s). 

How a fuel sending unit works

How does a fuel sender unit work, exactly?  Well, inside of the sending unit there is a float, usually made out of foam, plastic, or even a thin metal. Connected to the float is a thin, metal rod which is mounted to a variable resistor on the opposite end. This variable resistor is an electrical device whose job is to resist the flow of electricity. The resistor itself consists of a strip of resistive material, and one side of this material is connected to the ground. A wiper, electrically connected to the fuel gauge, slides along this resistive material and this movement conducts current from the gauge to the variable resistor. Here is a diagram of an example fuel level sensor to help you get a better idea of where each of these different things are found and what they look like:

Now, if the wiper is close to the side of the strip that is grounded, this indicates there is less resistive material in the path of the current, and  the resistance is small; as a result of this lack of resistance, more current will flow from the fuel sender unit to the gauge. On the other hand, if the wiper is at the other end of the strip, there is more resistive material in the current's path, and the resistance is large; as a result greater resistance, less current will flow from the fuel tank sending unit back to the fuel gauge.

Okay, so you are probably wondering what this level of resistance and amount of current flow tells us (and the fuel gauge). Well, that depends on the application as it can vary, but, essentially, it is this difference between the wiper and the resistive material on the variable resistor that tells the fuel gauge the amount of fuel in the tank.

Let’s illustrate one particular application in which the fuel sending unit takes a reading and the level of resistance it encounters while doing this is very little. This signals that the float within the fuel level sensor is at or near the top of the vehicle’s fuel tank (essentially submerged in the fuel tank). This tells the fuel gauge there is a full supply of fuel in the vehicle’s tank, and the needle on your fuel gauge will point more towards the full side.

In contrast, using this same application as an example, if the resistance that the fuel gauge sending unit encounters while taking a reading is high, this signals that the float has dropped and is closer to the bottom of the fuel tank, telling the fuel gauge that the level of fuel in the tank is on the low side, and the needle on your fuel gauge will point towards the empty side. However, in another application this may be completely opposite; low resistance will mean the float is at the bottom of the diesel fuel or gas tank equaling low fuel while high resistance will mean the float is towards the top of the fuel tank equaling higher fuel level. So, again, it can vary depending on the application. Of course, no matter the application, there are varying levels of resistance between the two extremes as the float drops and rises, and indicated on your gauge as the needle goes from showing a full tank to a tank that is half full/half empty (depending on how you look at it), to a completely empty tank, and everything else in between.

How do I know my fuel sending unit needs to be replaced?

Fuel sending units are replaced if they are rusted, if the fuel pump has gone bad, or if the fuel gauge does not show correct fuel levels due to a bad float. Knowing what your vehicle’s fuel level is, extremely important; you don’t want to be running out of gas on that long road trip thinking your tank is full, or want to see the fuel gauge’s needle pointing to “E” after you just filled your fuel tank. So if your fuel gauge is giving you wrong information and making you guess at how much fuel you have left, then it’s important that you obtain a fuel sending unit replacement as soon as possible.

Can I replace the fuel sending unit myself?

Removal of the sensor requires basic hand tools, getting at the sender unit can be difficult since it requires removing the diesel fuel or gas tank for access. Having a copy of the service manual for your automobile and reading through the procedure will certainly help.

The fuel sending unit setup can vary from vehicle to vehicle, so the tools needed can range from a screwdriver to a set of ratchets and sockets to a drip pan to a gasket sealer.   The repair may require the disconnection of the fuel lines and fuel pump relay, and possibly the fuel tank. First, disconnect the fuel pump relay. Then disconnect the negative battery terminal. On some vehicles the fuel pump can be accessed by removing the back seat, but if you don't have the luxury of doing that, you'll want to drain the fuel tank of most of its gas. The next step requires a disconnection of the fuel filler hose and the fuel pump wiring harness. Then remove the fuel lines from the fuel sending unit. It's a good idea to support the fuel tank with a sturdy object—like block of wood and a jack. Then take off the fuel tank straps that hold the tank in place.

Once the tank has been removed, you may have to hammer the retaining ring with the help of a solid tool—like a chisel—to gain access to the fuel sending unit. For reinstallation, reinsert the unit into place. Then simply tighten up the retaining ring. An assistant can work wonders when it comes to lifting the gas tank back into place. You want to gently and slowly jack the tank up while your helper holds it and maintains its position. Then, to keep the tank in place, tighten the fuel tank straps. Then reconnect the fuel lines. Tighten the fuel filler hose clamp after the fuel filler hose is in place. Then connect the fuel pump relay. As a final step, connect the negative battery cable.