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How Much Fuel? Faulty Fuel Gauge Sending Units

Gas tank

Some vehicles, especially as they get older, will register a full tank of fuel and then either “stick” there and show no consumption over time or will suddenly drop to empty (or near empty) once a couple of gallons have been burned off. A few will just show a vaugely correct reading of the amount of fuel available.

When this happens, drivers have two choices. Either the fuel gauge readings can be fixed or the trip meter can be used to guesstimate how much fuel is left and when the next fillup should happen. Mechanics, of course, will want to go with the first option and just fix the problem. It is almost always going to be the sending unit.

The fuel gauge’s sending unit is located in the fuel tank and is usually part of the fuel pump assembly on modern vehicles, but can be its own unit entirely as well. It will consist of a read-in device (usually a float on a lever) a translator, and an electrical circuit. Often a separate circuit also checks for very low fuel and signals the empty light on the dashboard. This is why it’s not unusual for the gauge to not be working but the empty light to correctly illuminate when the tank is near empty.

Before purchasing a replacement sending unit, though, do some basic circuit tests to make sure that the sending unit’s circuits are not at fault for the bad readouts. Run continuity checks on the entire circuit, not just the parts that are easy to access. Replacing a sending unit only to find out that the circuit is still bad is not a fun way to waste a Saturday.

Once the circuit has been ruled out and it’s definitely the sending unit at fault, remove and replace it. That usually requires that the fuel tank be drained and lowered from the vehicle. The sending unit is either part of the fuel pump assembly or is separate from it and using the same access hole or on its own. This will depend on the vehicle model. Carefully disconnect the wire clips from the sending unit’s circuit, remove the unit, and replace with a new one. It’s a good idea to use a bucket of fuel or liquid to test the new sending unit before installation. Just submerge it at various levels or move the float around to see that it changes the gauge readout. When satisfied, replace the unit and re-mount the fuel tank.

Procedure for dropping and re-installing a fuel tank are relatively straightforward. Find them here.

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