What are these extra wires on fuel sender unit?

exp2000

Petty Officer 2nd Class
Joined
Dec 10, 2009
Messages
148
Not sure where to connect these two wires on fuel sender unit. Can anybody help?

eb1ae32e.jpg
 

NYBo

Admiral
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Oct 23, 2008
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7,107
Re: What are these extra wires on fuel sender unit?

The two wires that are already connected are all that are needed, assuming the black goes to the gauge and the green/white goes to the battery negative. The colors are wrong, though; they may be reversed. The other two go to something else. Trace them back to see where they go.
 

Silvertip

Supreme Mariner
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Sep 22, 2003
Messages
28,758
Re: What are these extra wires on fuel sender unit?

The black wire connected under the nut is the "S" (send) wire and it should go to the "S" terminal on the fuel gauge. That should be a pink wire -- not black. The green wire on the left is secured to the tank which appears to be plastic and if it is, is serving as a tank "bonding" wire and would typically go the filler neck if the boat has one. It would be more appropriate if the tank were metal. The green wire with the black stripe (white plastic insulator) is the sending unit ground. It can go ground (battery negative). It does NOT have to go all the way up front to the gauge. Ground is ground wherever you find it. The red wire is apparently not connected to anything, nor should it be.

Here is a diagram of the fuel gauge system.

Gauge-SenderWiring.jpg
 

exp2000

Petty Officer 2nd Class
Joined
Dec 10, 2009
Messages
148
Re: What are these extra wires on fuel sender unit?

The black wire connected under the nut is the "S" (send) wire and it should go to the "S" terminal on the fuel gauge. That should be a pink wire -- not black. The green wire on the left is secured to the tank which appears to be plastic and if it is, is serving as a tank "bonding" wire and would typically go the filler neck if the boat has one. It would be more appropriate if the tank were metal. The green wire with the black stripe (white plastic insulator) is the sending unit ground. It can go ground (battery negative). It does NOT have to go all the way up front to the gauge. Ground is ground wherever you find it. The red wire is apparently not connected to anything, nor should it be.

Here is a diagram of the fuel gauge system.

Gauge-SenderWiring.jpg


I went back and looked at the photos I took as I was taking this boat apart. The red wire was on the center nut, green/white was on the left side, and I don't see black wire at all. Here is how it looked like. Now I don't know if that was correct to begin with; hovewer, gauge is showing that the tank is empty even though it is half full.
e9f186e2.jpg
 

Silvertip

Supreme Mariner
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Sep 22, 2003
Messages
28,758
Re: What are these extra wires on fuel sender unit?

You asked what the wires were for in your first post and you showed a picture with four wires. Now you say you don't see a black wire. You need to look at the gauge end of this circuit. Since there is a red wire and it is under the nut, there should be a red wire on the "S" terminal of the gauge. But of course it is also possible there are several splices between the tank and the gauge so who knows what you have. Next, is the wire with the white insulator actually grounded? Does the other green wire go to the fuel filler housing?

The fuel sender resistance is 240 ohms empty, 109 ohms half full, and 33 ohms when full (all are approximate). So disconnect the red wire and measure resistance from that sender terminal to ground (shell of the sender). If it measure 109 ohms (about) the sender is ok. If it measures infinity (open circuit) you have no ground or the sender is bad. If those check out ok, jumper the "S" terminal on the gauge to ground with the key on. The gauge should peg full. If not, make sure there is 12 volts and ground on the gauge. If there is, the gauge is bad. If not -- fix it. You can also reattach the sender wire and then disconnect that same wire at the gauge and repeat the resistance reading. If it measure infinity (open circuit) the wire between the gauge and sender is open or is not the correct wire.
 

NYBo

Admiral
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Oct 23, 2008
Messages
7,107
Re: What are these extra wires on fuel sender unit?

Your "before" picture clarifies things very well. Restore the connections to the way they were in that picture, and troubleshoot from there.
 

exp2000

Petty Officer 2nd Class
Joined
Dec 10, 2009
Messages
148
Re: What are these extra wires on fuel sender unit?

Thanks guys, I got a little sidetracked with a project. New unit has been ordered. I got the old one out, and it had been all rusted.
 

exp2000

Petty Officer 2nd Class
Joined
Dec 10, 2009
Messages
148
Re: What are these extra wires on fuel sender unit?

Put a new unit in and it shows nothing again. Can I directly connect wires from the battery to the unit, + to center nut and - to ground?
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: What are these extra wires on fuel sender unit?

No! Look at the diagram I posted. Do you see a + connection on the sender? NO - there is none. follow the diagram and the system will work. The center connection goes to the "S" terminal on the gauge. Connecting it directly to the battery will have you looking for another sender.
 

exp2000

Petty Officer 2nd Class
Joined
Dec 10, 2009
Messages
148
Re: What are these extra wires on fuel sender unit?

I have to be honest, I did not really look at that diagram before I asked. Shame on me. If I would have looked, I would not have asked such a stupid question.
 
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