How to Read and Diagnose the Fuel Gauge in a Car

How to Read and Diagnose the Fuel Gauge in a Car

Knowing how to read and diagnose the fuel gauge in a car is crucial. It will save you from running out of fuel in the middle of the highway.

The sad bit is when you don’t know how many miles it will take to get to the nearest refilling station.

The fuel gauge has two main parts: the fuel sender unit and the gauge.

The purpose of the fuel sender is to measure the amount of fuel your car has left in the tank and display the reading on the dashboard.

If any of them fail, the warning light may flash. Read on to find out how to read and diagnose the fuel gauge in your car.

How to Read and Diagnose the Fuel Gauge in a Car

The best way to diagnose the fuel gauge is to first understand how it works. The fuel sender unit is located at the side or top of the fuel tank.

More so, it comprises a form float connecting to a sliding wiper and a thin metal rod mounted to a resistor.

The wiper conducts electric current flow from the gauge to the resistor. When the float nears the top of the tank, the wiper slides to the ground side, which causes less resistance.

The float sinks, and the wiper moves to the other end as fuel levels drop, leading to low fuel needle gauge reading.

Often, the float motion doesn’t extend to the bottom. As a result, the fuel needle gauge will go below the empty level and stop, despite the tank having fuel.

The inaccuracy reading can also be caused by the fuel tank shape, which will make the float signal false fuel reading.

Related: Fuel gauge reads empty when full

Symptoms of a faulty fuel gauge

1. Fuel gauge stuck on full

The fuel gauge stuck on full symptom is common. It can occur when the gauge resistor malfunctions and transmits full voltage to the fuel gauge.

The fuel sending unit stays in constant motion, leading to the continuous movement of the wiper.

A defect in the wiring from the fuel gauge to the fuel sending unit can also cause a bad ground wire to the terminal. This sends a false signal to the instrument cluster.

Over time, the restrictive material strip wears, creating an open circuit. This demands you know the correct fuel levels of the car and the miles you cover to avoid running out of fuel unawares.

Read: Effects of overheating engine

2. Low reading when the tank is full

The float separating from the gauge arm is a significant cause of the fuel gauge reading empty.

A faulty resistor can also restrict signal, making the gauge read empty and halting the rest of the fuel sending unit.

It is wise to check the fuel pump wires since they may be corroded and stop voltage flow to the fuel gauge.

3. Fuel gauge stuck on empty

Fuel gauge stuck on empty is a typical sign you will notice when you refill your car, but the reading doesn’t shift. This problem results from a broken resistor. It could also be a separated float.

Like the fuel gauge stuck on full, you need to track the miles you drive and the car fuel consumption rate to avoid being stranded.

4. Fluctuating fuel gauge reading

Fluctuating fuel gauge reading can signify mechanical failure. It may also be due to a stuck fuel sending unit float giving an erroneous reading or shitting positions based on the vehicle’s movement.

For example, you could start driving while the gauge is half, and a few minutes later, it changes to full, then empty. It is wise you have it diagnosed to determine whether it needs a replacement.

Read: Home remedies for car radiator leaks

5. False low fuel light

A fuel gauge uses a series of electric connections. If they fail, so does the sending unit, which often signals false low fuel light.

But again, it is always wise to ensure the car has enough fuel. Achieving this will prevent the fuel pump from overheating or damage.

More so, continuously driving with low fuel makes the car suck debris from the bottom of the tank. This clogs the fuel tank strainer or the fuel injectors and can dwindle the engine’s longevity.

Read: Vacuum leak detection methods

Frequently Asked Questions about Diagnosing the Fuel Gauge

What happens if the circuit has problems?

The fuel sending unit and the fuel gauge ground get interruptions. They also fail to have a source and a fuel sender voltage, respectively.

How do you know if the fuel gauge has a problem?

You can conduct a voltage test on the sending unit to confirm the reading. The fuel gauge will be faulty or corroded if its reading differs from the fuel sending unit.

Can I drive with a faulty fuel gauge?

It is possible but risky to drive with a faulty fuel gauge. You can cause an accident or end up in a dangerous situation.

What are the causes of a bag fuel gauge?

Problems can result from a blown fuse, broken gauge, and damaged or disconnected wires. A faulty fuel gauge sending unit or a stuck fuel gauge sending lever can also cause lousy fuel gauge readings.

How to test the fuel sending unit?

You will need a Digital Multimeter (DMM) and Electrical Wiring (EWD). Ensure the tank is lower than half to prevent spilling, and confirm the plugs are clean, dry and the pins are straight.

The connector should also be seated. Next, turn the car to the ON position without starting the engine. Remove the pump module to manipulate the float arm and back-probe the connector voltage.

One of the pins should have 5V or 12V. The output voltage should change based on how you move the float arm. If the input voltage reading is incorrect, inspect the circuit.

You now know how to read and diagnose a fuel gauge in a car with the above information. The tricky bit is testing the instrument clusters, fuel gauge, and fuel sending unit.

More so, they can be confusing and produce false results. A wise move is you have a professional mechanic diagnose and fix the system to avoid mistakes.

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