Why does my fuel gauge read backwards?

If you've replaced a faulty boat fuel gauge with a new one, wired it correctly, but find the readings are "backwards"—you MOST LIKELY got the wrong type of fuel gauge!

There are two types of Fuel senders: 240-33 Ohms and 0-190 Ohms

The 240-33 Ohms type will present a resistance of 240 Ohms between the signal and ground wire when EMPTY (float at the bottom) and 33 Ohms when FULL (float at the top).

The 0-190 Ohms type will present a resistance of 0 Ohms between the signal and ground wire when EMPTY (float at the bottom) and 190 Ohms when FULL (float at the top).

So, before you buy, see if your old (broken) Marine Fuel Level sender is sending/presenting 'some' kind resistance and try and figure out what type you need.

If you (or a friend, an electrician would be ideal) has a multi-meter, set to the resistance scale and (with the wires from the sender disconnected) measure across the signal and ground wires and see what the ohm reading is, and if you know the tank is full, 3/4, 1/2, 1/4, empty do you read:
full = ~240 Ohms
3/4 = ~188 Ohms
1/2 = ~136 Ohms
1/4 = ~84 Ohms
empty = ~33 Ohms

OR do you read:
full = ~0 Ohms
3/4 = ~48 Ohms
1/2 = ~95 Ohms
1/4 = ~143 Ohms
empty = ~190 Ohms

NOTE: the '~' means approximate

In the above measurements if you can remove the old Gas sender unit from the gas tank and then move the float all the way down (empty) and up (full ) and in the middle (1/2 tank) to try and get readings would be best.

Yes, if your marine fuel tank sending Unit is COMPLETELY broken this probably won't work, but a lot of times the sender starts 'breaking' by giving flakey readings during part of the range (variable resistor wearing out) and you may get readings at some positions.

Another 'trick' to try, if you have access to an electronics parts store (like the old Radio Shack) would be to get a few resistors. Get a 243 Ohm, a191 Ohm and a 33 Ohm (all 1/2 Watt, these are 'common' resistance values).

Then hook these up in PLACE/INSTEAD of the sender (ACROSS the two wires that WERE connected to the sender) one at a time, using a set of jumper clips (you can get these at the same store).

Try the 33 Ohm first, if when you turn the key on (to accessory or the setting where you would normally see the fuel level) you see a Full Tank indicated (or very close) then you need a 230-33 Style (you can turn off, switch to the 241 Ohm, turn back on and you should read Empty).

If you saw close to empty with the 33 Ohm resistor, turn off and switch to the 191 Ohm resistor and turn back on, now you should read close to Full (if you don't then something else may be wrong, like your gauge could also be bad). If you do read close to Full with the 191 Ohm resistor then you need the 0-190 Style.