In older vehicles, when you turn the key to the ON position or start your car, you watch the fuel gauge go to full, back to empty, and then to the position that highlights the current amount of fuel in your vehicle.
Newer vehicles require no such theatrics. Fuel gauges on newer cars are much more accurate and dependable. However, they still need maintenance and attention occasionally.
The fuel gauge is part of a cluster of other indicators that let you know how your car is running — from engine temperature and oil pressure to battery charge level and various warning lights. In many ways, it’s like an onboard computer screen for your vehicle.
One of the most common problems with fuel gauges is when they read incorrectly or not at all. When this happens, it’s important to get it repaired by a mechanic as soon as possible because driving for too long without knowing how much gas is left in your tank can lead to real problems (like getting stranded).
Most modern cars have fuel injection systems. This means that there is a sensor that sends a signal to the car’s computer indicating the amount of fuel left in the tank. In older vehicles, when you turn the key to the ON position or start your car, you watch the fuel gauge go to full, back to empty, and then to the position that highlights the current amount of fuel in your vehicle.
This is because there is a float in the gas tank that rises and falls with the level of fuel. The gauge needle moves with this float. If this float were to get stuck, it would give an inaccurate reading.
You would have no way of knowing how much fuel was in your vehicle without estimating your usage and driving a little further than you are comfortable with before filling up again.
The first thing you should do if you notice that your gauge is not working properly is fill up your tank and see if it will move into and stay at F (full). If it does but starts to drop once again, check your manual for information on checking fuses. There may be a fuse related to your gauges or perhaps just one that is related specifically to the fuel gauge.
If replacing a fuse doesn’t work, look into having your float checked out by
Older cars had the fuel gauge and fuel sending units connected directly to the battery. When you turned the key on, the gauge would go to full, then to empty, and then (hopefully) to the correct position. This is because there was a voltage drop in the circuit.
Nowadays, most cars have an “ignition feed” that runs from the ignition switch to the fuel gauge and sending unit. In these cases, if it still goes back and forth when you first turn the car on, it’s a different problem. The sending unit is going bad or needs replacing.
Bad Fuel Gauge
If your gas gauge stays at empty, but you have gas in your tank, then you have a problem with your fuel sensor or a clogged filter in your gauge. A clogged filter can be caused by sludge build up in your fuel tank.
If you have an older car with an analog display and you lose power to the fuel gauge, this could be caused by a loose connection or bad ground wire for the instrument cluster. You should also check for any blown fuses related to instrument cluster circuits.
If you’ve ever driven a car, you have probably seen the fuel gauge drop a small amount after filling up, and then go back to full. This is normal, and it happens because of something called hysteresis.
Hysteresis occurs when the magnetic field in the gauge’s electric motor keeps its armature from moving back to zero until there is enough motion to overcome this magnetism. The fuel gauge needle stays at the “E” until you drive the vehicle for a short period of time.
If your fuel gauge doesn’t move at all after filling up with gas, there is a good chance you have a bad sending unit in the gas tank that needs to be replaced. If your gauge doesn’t go to full after starting your car or turning on the key, it could be an electrical problem or even the fuel pump itself that’s causing the bad fuel gauge reading.
Most of our readers will have noticed that their fuel gauge doesn’t work right when they turn the key to the ON position. That’s because it’s part of a circuit that tests the fuel level sensor and related components.
If you do see the needle swing to full, it means there’s a problem with this test circuit and not with your actual fuel level.
This test is performed every time you turn your key to the ON position, so if you see this behavior every time, it’s an indication that there’s a problem with the fuel gauge itself or its associated wiring. However, if this only happens once in a while, it could be a sign that there are issues with one of these components:
- Fuel level sensor(s)
- The instrument cluster itself
- The associated wiring
If your fuel gauge is reading empty when you know there is fuel in the tank, the cause may be a faulty sender unit the part of the system that measures the level. The sender unit is a simple device that uses a float and lever system to measure how full your tank is. As it moves up or down, it makes contact with an electrical circuit that gives a reading on the gauge in your instrument cluster.
The issue can sometimes be caused by an electrical fault, but usually it’s simply because the sender unit is no longer working due to wear and tear. This can be fixed by replacing the sender unit with a new one, located at the top of your fuel tank.
If you have any doubts about doing this yourself, then don’t hesitate to get in touch with one of our local mechanics who will be happy to help you out.
When your fuel gauge reads incorrectly, it can be very frustrating. If you’re running low on gas and it says you have more than a quarter tank of gas, you’ll keep driving around unaware that you are about to run out of gas. If you’re full up on gas and the gauge says empty, then unnecessary panic sets in.
This could happen for two reasons:
The float is not set correctly. This is like the float in a toilet — it’s a piece of plastic attached to an arm that moves up or down depending upon the level of fuel in the tank. As the fuel level rises and falls, the arm does so as well. The arm is linked to some kind of potentiometer (which is just a fancy word for an electrical device with one variable wire). When the arm turns, it changes resistance on the variable wire, which changes how much power flows through it. If the float isn’t sitting right in the tank, then it won’t measure the correct amount of fuel.
The potentiometer has failed. It sounds like this isn’t your problem since you’re getting readings at all, but just so you know there is an electrical component that can fail and cause your fuel gauge to read incorrectly.