Stupid fuel gauge question

nola mike

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My gauge is on empty, I have a full tank. Going to tackle it on tuesday, but when I measured resistance at the sender between the 2 posts just in passing a few weeks ago (wiring colors appear screwed up) I got 240 ohms. Looking that up now it appears that that should be the reading on an empty tank. Going to clean the posts and recheck, but assuming that I get the same results, that indicates a bad sender/stuck float, correct? I don't remember if I disconnected wires for the reading, though I don't think that should make a difference...
 

Scott Danforth

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Pull the sender and check the sender
 

gm280

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nola mike, could easily be a float arm stuck. So yes pull the unit out and see as Scott previously stated.
 

nola mike

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That's what I was thinking before I went on an electrical/gauge witch hunt. I'll pull and post the results...
 

Silvertip

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One common problem with senders is that people tend to tighten the nut securing the pink wire to the threaded terminal so tight that the stud itself begins to turn. That can spin the connection on the under side of the mounting flange and create problems. Sender resistance should be checked with the pink wire removed otherwise you are measuring the sender to ground along with whatever resistance the gauge is presenting in parallel with the sender..
 

nola mike

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Silvertip, yeah when I disconnected the wires I got no continuity. Pulled the sender (had a stripped bolt that I had to grind out) and the float arm was stuck, and there was no continuity between the terminals. So got a new sender. Bench tested, and it checked out before install. I calibrated it according to instructions, but it seems to read very high. I added 17.5 gallons gas (27 gal tank)-it read 3/4 at that time. Now I've gone at least 35 miles and it's still reading full. First step is going to be to check resistance and see if I have a gauge or calibration issue. Any other troubleshooting ideas?
 

gm280

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nola mike, unless you have a reed type fuel sensor, the reading you get is only a suggestion and you will have to learn how the reading equates to actual fuel in the tank. Even my vehicles read off. Technology hasn't changed much in true fuel readings it seems.
 

Silvertip

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Calibrate the sender for EMPTY since that's what you are trying to avoid. You know when it is full because you filled it.
 

nola mike

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Any suggestions on how to do that? I measured the resistance after at least 10 gallons and I was still at 33 ohms. Today added 15 gallons when the gauge was 3/4-and I didn't fill all the way. Im going to shorten the arm on the sender. Other option was to add a resistor.
 

nola mike

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I'm thinking at least part of that is that the sender is in the rear of the tank, and even at rest the bow might be angled up
 

Fed

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At 33 Ohms the gauge should have read full.
There's 2 systems, American & Euro and they must be matched as they work in reverse to each other.
Test the gauge to see what type you have & to confirm it's working properly.
gauge.jpg

Then test the sender to see that it's the correct one.
sender.jpg
Notice with the Euro one the Ohm readings are reversed, this means if you have the Euro system the gauge test has to be reversed.

All bets are off if you have a mismatch.

What sender did you buy?
 

nola mike

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Sender/gauge combo is correct, gauge works as it should. Sender is also working. I think with the sender in the rear, + any tilt in the boat, the gas stays full to the top for a while. A shorter arm would make it more sensitive to changes in the tank level I think
 

gm280

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Sender/gauge combo is correct, gauge works as it should. Sender is also working. I think with the sender in the rear, + any tilt in the boat, the gas stays full to the top for a while. A shorter arm would make it more sensitive to changes in the tank level I think

nm, just remember, anything you do to the float arm to make it more sensitive will also make it more susceptible to the gauge fluctuations when under way.
 

nola mike

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interestingly enough, I still had the old sender. Arm length was 4.75" despite a 6" tank depth. So the old arm would have read empty with 1/6 tank remaining--IF the tank is level at rest (still not sure if that's the case). Shortened the new arm to match. We'll see.
 

Fed

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I've been through this with my own gauge & tank mike and once you get to 'know' your tank you will feel very confident with gauge.
I can go for 10 or 15 miles without the gauge dropping from full but when it gets to below 1/8 of a tank while stationary it's time to panic.
Take a jerry can if you decide to test its lower limits.
 

Silvertip

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You calibrate by bending the arm or adjusting float position or both. You adjust for Empty resistance reading with float at or near the bottom of the tank (your choice). You always check fuel gauge on the water because that's the attitude the boat is in. It does not sit that way on a trailer.
 
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