Fuel sending unit questions

[Basszilla4]

Hello everybody at iboats.com,

Quick question if you don't mind. Wondering what the difference between a two wire and three wire fuel sending unit are? I recently bought a set of Faria gauges for my boat. Comes with fuel gauge. Haven't received in mail yet. Looking on eBay, I see there are universal sending units available. Some with two wires, and some with three wires.

I was planning on getting gauge and seeing how many wires it needs to be installed. Then buying the appropriate sending unit. I had asked someone else who had told me that on a two wire unit, one is ground, and the other is ohm's and goes to the fuel gauge. Sounds logical. However, on the picture of the fuel gauge I bought, it appears there is spots for three wires, plus the one for the 12v light.

Maybe somebody could give me a quick lowdown on the differences. Any info would be greatly appreciated. Thank you.

Sincerely,
Errol (Basszilla4)

 

[Silvertip]

Re: Fuel sending unit questions

Fuel gauges (the actual gauge) for boats are generic and use a sender that provides a 33 - 240 ohm resistance to represent fuel level. There are actually four wires on the typical gauge. 1) +12V when the key is on, 2) ground, 3) internal light, and 4) the sender line from the sending unit in the fuel tank. There are several types of fuel sender but the most common is the two wire (and most inaccurate) because it uses a swing arm with a float on the end to move a variable resistor (33 - 240 ohms) as fuel level rises and falls. One wire on this sender goes to the SENDER terminal on the gauge. The other goes to ground. Three wire senders are a more sophisticated and use various technolgies to accomplish the same thing (33-240 ohms resistance) to the gauge. These senders require a battery voltage connection (+12V when the key is on). The gauges work the same with either sender but the three wires are far more accurate.

 

[Basszilla4]

Re: Fuel sending unit questions

Hey, thanks for the very useful info on that Silvertip. I am including a couple pictures of the ones I am looking at. The first one is the three wire, made my Moeller. It has the three wire with connector which I would have to cut off and splice wires, but for my tank that is 8 inches from mounting surface to bottom, this is 7 1/2 inch.

The other with the float arm is the universal. Adjustable from 4 inch to 24 inch. Only a two wire.

One thing I can't figure out is how the Moeller works. No float or anything like that. Both of these are roughly the same price. Leaning toward the three wire one especially after reading your reply.

Thanks again.

 

[Silvertip]

Re: Fuel sending unit questions

There are various technologies for the three wire units. The Moeler unit looks very much like it is a "capacitance" style sender. There are two tubes, one inside the other and the fuel level between the two tubes creates a difference in capacitance that is then converted into a resistance output for the gauge. If that's the type, those units can also detect the presence of water in the fuel. The reason the float style units are so inaccurate (usually) is that they determine fuel level -- not fuel quantity. Look at your fuel tank in two ways. First, most are long, shallow and wide so as fuel level drops the attitude of the boat affects how the fuel sits in the tank. Not only that, but it can slosh around easily causing the gauge to dance around. To better understand this, think of a dixie cup or cone shaped fuel tank. If you filled the cone with liquid, there is much more fuel in the top half than the bottom half. So while the gauge would say you had half a tank, that bottom half would disappear at an alarming rate vs the top half. So shape of the tank has much to do with accuracy. The more sophisticated senders still fall victim to that problem but the nice thing about them is that some can be calibrated for full and empty where you can do only one or the other with a float style sender. The big advantage to the better senders is you don't experience the needle-dance of the cheaper senders. Hope this helps.

 

[Basszilla4]

Re: Fuel sending unit questions

Hope this helps.

Yes it does help. Thank you for taking the time to lend some advise. Went and bought the Moeller three wire one. My tank is 12 gallons and a perfect rectangle. Shouldn't have the cone affect. If I need any help on installation, I will throw out another post. Thanks again.