Mercedes Mullins
Positioning a fuel sending unit in a boat tank requires careful attention to detail. The sender float should move from front to back, and the positioning of the mounting holes is crucial. It is important to note that the holes are not symmetrically placed, and the sending unit must be clocked correctly. The compatibility of the sending unit with the fuel tank is essential, and the length of the sending unit needs to be measured accurately. Troubleshooting wiring issues and ensuring proper grounding are also key aspects of positioning a fuel sending unit in a boat tank.
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What You'll Learn
The float should move front to back
When installing a fuel sending unit in a boat tank, the float should move from front to back (bow to stern). This is because the mounting holes in the tank are not placed at equal distances, so the sender must be rotated to align with them. This will ensure that the float is in the correct position.
It is important to note that the orientation of the articulating arm of the float must miss the baffles and sidewalls of the tank. This means that not much consideration can be given to forward directions.
In some cases, the float may only move up and down on a rod, rather than front to back or side to side. This is the case for WEMA sending units, where the float is the only moving part. This design ensures that other objects in the tank do not interfere with the float's movement.
Additionally, it is important to seal the wiring connections with heat-shrink and elevate them to prevent them from laying in water, which can cause corrosion.
By following these guidelines, you can ensure that the float in your fuel sending unit is positioned correctly and functioning properly.
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Check the mounting holes and rotate the sender to align
When installing a fuel sending unit in a boat tank, it is important to ensure that the sender float is positioned correctly. The float should move front to back (bow to stern). If you are using an existing tank with pre-drilled holes, the position of the sending unit is dictated by the location of these holes.
To ensure the correct positioning of the fuel sending unit, follow these steps:
Check the mounting holes on the tank and the sending unit. The sending unit should have five holes, but they will not be evenly spaced. Line up the mounting holes on the sending unit with those on the tank. This may involve rotating the sender to achieve the correct alignment. Mark the screw holes in the tank for easier alignment, as the flange will cover them and make them harder to locate.
Once the holes are aligned, check the movement of the float arm. It should be able to move freely without obstruction. Hold the sender next to the tank to determine the direction of free movement before installing it. Duplicate this orientation when placing the unit into the tank. To avoid confusion, use a marker to indicate the direction of travel of the float arm once the screw holes are aligned.
After confirming the correct orientation and alignment of the mounting holes, proceed with securing the sending unit in place. Insert the screws and tighten them securely. By following these steps, you can ensure that your fuel sending unit is positioned accurately in your boat tank.
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The sender wire must be grounded
The sender wire should be grounded at the tank and back to the battery. The sender wire completes the circuit for a gauge reading. The gauge must match in both polarity and ohms resistance with the sender in order to work properly.
If the sender wire is not grounded, the gauge will not peg. To test this, ground the sender terminal of the gauge, and if the needle does not move to full, you have lost continuity to the sender wire or there is no ground to the tank.
The sender wire is one of three wires connected to the sender at the fuel tank. The other two wires are a bonding wire and a power wire. The bonding wire connects all metal in the boat together, and the power wire provides power to the gauge.
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The length of the float arm is often adjustable
The float arm should be adjusted to the depth of your fuel tank. For example, a 15.5" tank would require a 10.5" arm length. The float arm should also be positioned to miss the baffles and sidewalls of the tank. Ideally, the sending unit would be dead center at the top of the tank so that bow rise, falls, and turns have a minimal effect.
It is important to ensure that the wiring connections are sealed with heat-shrink and elevated so they will not lie in water. Corrosion on wires and terminals is a common problem with sending units.
To test if the sending unit is working, connect the wires and turn on the ignition. Push the arm to the UP/FULL position and observe the gauge reading. If the gauge reads correctly, the sending unit is working properly.
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The gauge must match in polarity and ohms resistance
When installing a fuel sending unit in a boat tank, it is important to ensure that the fuel gauge matches the sender unit in terms of polarity and ohms resistance. This is because the sender unit's job is to create a variable potentiometer-type resistance, which the gauge interprets to provide a needle reading.
The polarity of the gauge and the sender unit must match for the system to complete a circuit and function properly. The circuit starts at the fuel gauge, with a wire running to the sender unit. As the float arm moves, the resistance path to ground changes, altering the current in the circuit. This change in current is what the meter reads. Therefore, it is crucial that the polarity is consistent between the gauge and the sender unit to ensure accurate readings.
Additionally, the ohms resistance of the fuel gauge and sender unit must align. There are two standard ranges for fuel gauge and sender unit ohms resistance: 33-240 ohms and 0-180 ohms. The US standard sender and gauge have a higher reading at higher resistance, with 240 ohms indicating a full tank and 33 ohms indicating an empty tank. In contrast, the European "metric" standard operates in the opposite direction, with higher readings at lower resistance; a reading of 10 ohms indicates a full tank, while 180 ohms indicates an empty tank.
It is important to determine whether your boat's fuel system adheres to US or European standards to ensure the compatibility of the fuel gauge and sender unit. If you are unsure, it is recommended to consult the gauge manufacturer to ascertain the input range required for the sender unit.
By ensuring that the fuel gauge and sender unit match in both polarity and ohms resistance, you can guarantee accurate fuel level readings and avoid potential issues with your boat's fuel system.
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Frequently asked questions
To access the fuel sending unit in your boat tank, you will need to remove the seat bench. This is usually made of aluminium and can be removed by unscrewing along the outside of each seat and along the back lip by the livewells/storage lockers.
The sender float should move from front to back (bow to stern).
First, check that the sender wire is not shorted to the ground. If it is not, then the sender is either the wrong one for the gauge or it is defective. If you have a Moeller gauge, you need a Moeller sender. Ensure that the sender and gauge are matched and are working, and check that the sender is grounded to any metal fuel tanks.
Be careful to "clock" the sending unit correctly as the holes are not symmetric. You need to measure the sender length and confirm that it is correct for your tank.
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