How does a fuel sender work

[davesboat]

"I,m in the process of fitting a new engine.I let my old fuel gauge go with the old engine instruments by mistake, it was a volvo penta one.
The sender unit in my fuel tank has 2 wires coming from it, a black which is connected to a lug on the tank and then is goes away up to the helm connected to the neutral block, the other wire is a green and yellow which I think is the sender wire.
I have bought a new fuel gauge( not a volvo one).
today I have been trying to wire up the new gauge with no luck,I can get the gauge to go up to 3/4 of a tank which is about the right amount of fuel in the boat, if i disconnect the sender lead the gauge still reads 3/4 of a tank, I dont understand why.
can anybody explain how the fuel sender works? I can not work out how it sends a signal with only 1 wire, surely there needs to be a circuit.
thanks"

[sandkicker]

"Fuel senders work as follows...
There is an arm connected to a float. The arm is conected to moveable contact on a resistor. As the arm moves the value of the resistor changes.
As the resistance changes the amount of current in the circuit changes. The circuit "starts" at the fuel gauge and a wire gos to the sender. As the float arm moves the resistance path to ground changes and the current that flows in the circuit changes. The meter actually reads current.

Two things to check..
1) the sender in the tank is connected to ground at the tank and back to battery. On aftermarket gauges, the metal mounting flange is the ground terminal, i.e., your missing second wire.

2) On aftermarket senders, the length of the float arm is often adustable. If improperly adjusted for your tank geometry, it will not read correctly."

[RicardoMarine]

""I can not work out how it sends a signal with only 1 wire, surely there needs to be a circuit."

This system does complete a circuit only when a negative lead is connected to both the sending unit and the gauge..... (should also be grounded to any metal fuel tanks)..... AND if the sender lead (in your case, the green wire) runs up and to the fuel gauge "S" terminal without interruption.

This completes the circuit for a gauge reading...... providing that the sender/gauge are matched and are working!

However, the actual gauge is powered and also grounded at the helm....... and it reads only that resistance to ground via the sending unit (of which is also grounded)!

As Robert says, the sender's job is to create this variable potentiometer type resistance. The gauge knows how to interpret this resistance and uses this for the needle reading.
NOTE: The gauge must match in both polarity and in ohms resistance with any sender in order to work properly.

."

[davesboat]

"Thanks guys, just got back from the boat and read your posts, I had a flash of inspiration while I was typing the problem on here, so I went back down to the boat and connected the neutral block in with the gauge and hey presto it worked.
I was trying to get the darn thing working with no batteries in the boat, I was just using a battery to power up the gauge with out connecting the boats neutral system up,
Soon as I connected the boats neutral system the gauge worked perfectly.
I disconected the sender lead to test and got the gauge to move.
Thank for the quick replies.
dave"

[RicardoMarine]

""Neutral"???? You may be thinking VAC! This is VDC!

Glad that you go it working! "

[davesboat]

"neutral, negative "-"
same difference for us in the uk.

glad its working too, its too easy to overlook the obvious sometimes."