Fuel reading inaccurate

A.F.C. 8511

Petty Officer 3rd Class
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Dec 7, 2017
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85
The fuel gauge reads full after being filled (go figure), but I do not believe it is accurate. I have a 22 gallon moeller fuel tank, my fuel gauge reads half full, but we put 18 gallons in at pump. This same scenario at various fuel levels. Is it gauge or sending unit. The tank came new in mail. Im wandering if sender/float was damaged?
 

HotTommy

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Mar 15, 2013
Messages
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Most fuel gauges are simply ohmmeters that read the resistance in the voltage coming through the sender in the tank. The sender changes the resistance as the fuel level moves a float attached to it. The resistance the sender provides varies by model and manufacturer. Likewise gauges vary by model. If the sender in your new tank is not intended for the same resistance range as your gauge, you will get erroneous readings. You need to confirm the ohm range for for the sender and gauge to determine if that is the problem. .... If they are properly matched it could be a faulty gauge, faulty sender or poor connection in the wiring connecting them and to the ground. If you decide to start troubleshooting them, come back and we'll have some more info.

BTW, my new Mercury 115 has a gauge that displays the fuel used. I reset it to zero at every fill up and subtract the amount used from the tank capacity to know how much is left. It is very accurate.
 

alldodge

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Mar 8, 2009
Messages
40,581
Remove the sender and check the float for full an free movement. If all good, reconnect the wires and move the float up/down and see if you get full scale
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
Agree with Tommy. All sending units don't have floats. If your new tank has a sending unit that does not, it's output must be matched to the gauge you are using. Might be worth a call to Moeller regarding how to proceed from this point.
 

clemsonfor

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Joined
Sep 19, 2005
Messages
1,011
Mine on my pntoon os a little hoaky. It seems to lead you to beleive there is more fuel that there is. I would thjnk they took into account the shape of the gank when calibrating it but it does not apper so. I have a triangle shape tank in the motor pod. So when its full there is more fuel at the top, so a droo of 3 verticle invhes of fuel in the tank when its full represents more gas used that when at the bottom of the tank on the skinny side of the triangle, yet the gauge seems to correlate to the fuel level of the tank when i look from the back through the back flat back side (its opaque) or i should say when looking at it from behind the boat. But like i said there is more fuel in the top half of the tank than the bottom half...at least when viewed from the back of the tank...but i get what i said contradicts it self cause i said there is more in ine half than the other half..haha. but the upper portion contains more fuel than the lower portion, there thats better. And like i said seems like my gauge just reads the verticle level of fuel in the tank without regaurd to actual quanity contained in the tank. But i have learned about where and how much fuel is left jn the tank from looking at the level and the gauge.
 

A.F.C. 8511

Petty Officer 3rd Class
Joined
Dec 7, 2017
Messages
85
Most fuel gauges are simply ohmmeters that read the resistance in the voltage coming through the sender in the tank. The sender changes the resistance as the fuel level moves a float attached to it. The resistance the sender provides varies by model and manufacturer. Likewise gauges vary by model. If the sender in your new tank is not intended for the same resistance range as your gauge, you will get erroneous readings. You need to confirm the ohm range for for the sender and gauge to determine if that is the problem. .... If they are properly matched it could be a faulty gauge, faulty sender or poor connection in the wiring connecting them and to the ground. If you decide to start troubleshooting them, come back and we'll have some more info.

BTW, my new Mercury 115 has a gauge that displays the fuel used. I reset it to zero at every fill up and subtract the amount used from the tank capacity to know how much is left. It is very accurate.

Hot Tommy were is that gauge on your Merc 115?
 

HotTommy

Lieutenant Junior Grade
Joined
Mar 15, 2013
Messages
1,025
The gauge I referred to is called a Mercury Smartcraft Systems Monitor SC1000. Newer ones may have a different display, but the one I have has a small (2.5" x 2.5") display like this:
s-l640.jpg

I've seen them for sale between $200-300 complete with the cable that plugs into a port on the engine. On this display you press the MODE button to change the display between RPM, fuel used, gallons per hour, voltage, water temp and engine hours. I mounted mine on the console.
 

HotTommy

Lieutenant Junior Grade
Joined
Mar 15, 2013
Messages
1,025
The cable connects to a plug inside the cowling. I suspect the location varies some by specific engine. Google Mercury SC1000 install for some pictures and examples.
 
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