fuel sender unit - fuel guage, somethings wrong

[Raycaster]

Fuel gauge is reading past FULL on 1/4 tank or less on a used legend XCalibur.

Google said "open circuit, probably replace fuel sender".

Fuel sender is the rod version with the float position giving full 30 Ohms or 240 Ohms empty.

I tested old fuel sender and it read 30 at the top and basically a inch from the bottom read 200 Ohms then nothing for the last 1/2 inch.

Bought a replacement and it reads EXACTLY the same, hmm. Guess that is normal? Maybe my ohmmeter just went out of range at the end...

The way I thought it should be is 30 Ohms at the top and 240 Ohms at the bottom.

The boat is in storage so I just yanked the old one out to compare with the new one.

 

So I'm down to 1) Gauge problem 2) wiring problem 3) a dud replacement.

Its not rocket science and will figure it quickly when I get the boat in the driveway but if anyone has some quick pointers before that I'd appreciate it.

[Fisherman]

Bad ground, corroded wires along the circuit.  The sending unit is a basic rheostat,  where the metal arm rubs along the wire winding around a longer post.  The shorter the distance between the metal arm contact and wire winding, the lower the ohms will read. 

There's a whole variety of different shapes but most use the same principle.

[BillM]

Ground would be a good bet.

[Terry]

2 minutes ago, BillM said:

Ground would be a good bet.

It usually is ground 

[Raycaster]

So I got the boat in the driveway making a few fixes...

I hooked up the original fuel sender again and started testing it with the new one I purchased.

Again the needle went to full with an empty tank. I removed from the tank and manual moved the ring to the top and bottom to see if the guage did anything.

Well... It actually moved a little more past full!

Not sure what the past owner did but it seems like it might be in reverse? I'm very tempted to reverse the cables on the Gas Guage but there are 4 wires to it. Ignition, ground, and wire from fuel sender. The 4th wire may be a light wire? Looking for a Legend Xcalibur 2007 wiring diagram before I screw something up.

[Raycaster]

Here’s a general description of the connections:

1. Power (12V) Wire: Red
2. Ground Wire: Black
3. Sender Wire: Pink or Yellow
4. Illumination Wire: Blue

Ok, getting closer.

[glen]

Is your ground wire grounded to the boat.  Is boat grounded on the trailer or in the water only.  

[Raycaster]

I have grounded the fuel sender straight to the battery for testing.

Today I will yank out the guage to test and get a 100% answer

[AKRISONER]

like so many problems I have with my boat...starting a new has been the solution. Its costly, but trying to fix the mess made in the past sometimes isnt worth the time or effort.

[smitty55]

I worked in the aviation industry as an avionics technician for over 35 years. Many of the instruments we did had a requirement to be tested for insulation resistance on a "Megger" tester which tests the insulation properties of wiring will under higher DC or AC voltages to see if the insulation breaks down. Some Instruments often had breakout boxes that would plug into the connector and every single circuit would be tested against every other one. This was also done to test the integrity of the wiring in rotors, stators, commutators and windings in transformers, motors and generators. There were also some tests that could be done using older analog ohm meters like a Simpson on the higher RX1000 scale that had higher voltage outputs that could detect insulation breakdown. I've even had motor stators test within tolerance but on the low end that barely passed speed tests, so I always replaced them. We also had a tester called a growler that you would place a rotor on and slowly rotate it to find shorted coils in the windings. So what I'm getting at here is that even though the windings on that rheostat could test out ok with a digital multimeter once power is applied there could be insulation breakdown that is causing your issues. Insulation can also break down at elevated temps as coils heat up. Here's a couple of articles on insulation resistance. Also keep in mind that any continuity test testing grounds should have less than 1 ohm readings, anything higher indicates poor ground.
https://www.instrumart.com/assets/Megger-Guide-to-Insulation-Testing.pdf

https://handtoolsforfun.com/how-to-check-insulation-resistance-with-a-multimeter/

[smitty55]

2 hours ago, AKRISONER said:

like so many problems I have with my boat...starting a new has been the solution. Its costly, but trying to fix the mess made in the past sometimes isnt worth the time or effort.

Agreed, if in doubt change it out.

 

[Raycaster]

Ok, hard wired everything.

Sender and ground good at console.

results: guage.