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Old 27 April 2008, 12:00   #1
nik
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fuel gauge problem

Hello, My fuel gauge is misreading. It is showing a full tank when its not full. I am a bit stumped as to where the fault lies. I have no idea whose make of sender is in the tank, but here are some pictures.
I have put an ohm meter on the sender and it reads from about 25 ohms to 240 ohms depending on the position of the slider. When it is plugged into the boat and the slider is moved from one extreme to the other, the fuel gauge moves from completely full to nearly completely full suggesting that the sender is not giving enough resistance. But I cant fathom why. If there were too much resistance, I would be looking for a bad connection, but its not.
I have no idea how this type of sender works, but it looks unmaintainable.

Any ideas? Thanks, Nick.
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Old 27 April 2008, 12:36   #2
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Quote:
Originally Posted by nik View Post
Hello, My fuel gauge is misreading. It is showing a full tank when its not full. I am a bit stumped as to where the fault lies. I have no idea whose make of sender is in the tank, but here are some pictures.
I have put an ohm meter on the sender and it reads from about 25 ohms to 240 ohms depending on the position of the slider. When it is plugged into the boat and the slider is moved from one extreme to the other, the fuel gauge moves from completely full to nearly completely full suggesting that the sender is not giving enough resistance. But I cant fathom why. If there were too much resistance, I would be looking for a bad connection, but its not.
I have no idea how this type of sender works, but it looks unmaintainable.

Any ideas? Thanks, Nick.
to diagnose the problem properly you really need to know what the system was designed to do. IIRC there are a few different standards for senders, and one of them is something like 30-250 ohms for empty-full scale and the other one works the other way round (high res when full, low when empty) and is roughly 200-10 ohms.

it sounds like you may have the former (or its a coincidence) in which case perhaps its the gauge which is faulty rather than the sender. it could even be just a setting on the gauge needing changed to tell it which sender type you have.
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Old 27 April 2008, 13:14   #3
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This gauge works, no current = tank at 0, full current = tank full. so either the sender has lost some resistance, or the gauge has become a lot more sensetive.
As I said, I have never worked on this system before. But is it possible that to get the gauge to match the sender, a shunt resistor could be placed across the gauge terminals, and due to vibration, metal fatigue has broken a wire of the resistor thus making it misread?
Purely speculation on my part. The gauge is hard to get at so I cant see the terminals.

Nick.
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Old 27 April 2008, 13:40   #4
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I guess it's this.

http://www.tek-tanks.com/senders_gauges/fuel.html
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Old 27 April 2008, 13:41   #5
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This gauge works, no current = tank at 0, full current = tank full.
nik - not sure you can translate the fact that the needle moves when you power up the gauge into "the gauge works". It could still be faulty.

So to summarise I think what you are saying is:

Sender in empty position 25 Ohms
Sender in full position 240 Ohms
Sender in empty pos. gauge powered on - reads almost full
Sender in full pos. gauge powered on - reads full
gauge power off - reads zero

Where does it read when powered up with sender disconnected?
Where does it read when powered up with sender shorted out?

If the gauge is working these should be FULL and EMPTY respectively. Even if these are correct then I would still say it doesn't garuntee that the range is correct.
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Old 27 April 2008, 14:36   #6
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[QUOTE=Polwart;246580] nik - not sure you can translate the fact that the needle moves when you power up the gauge into "the gauge works". It could still be faulty.

Thanks Trolli, that looks like the one. And it looks like the gauge is matched to the sender.

The gauge has a full range of movement. When the sender is disconnected, or the power is turned off, the gauge reads zero. when the power is on and the sender is connected, the gauge reads full. And moving the slider on the sender from top to bottom results in a very small movement in the gauge needle in the full region.

Nick.
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Old 27 April 2008, 14:43   #7
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[QUOTE=nik;246587]
Quote:
Originally Posted by Polwart View Post
nik - not sure you can translate the fact that the needle moves when you power up the gauge into "the gauge works". It could still be faulty.

Thanks Trolli, that looks like the one. And it looks like the gauge is matched to the sender.

The gauge has a full range of movement. When the sender is disconnected, or the power is turned off, the gauge reads zero. when the power is on and the sender is connected, the gauge reads full. And moving the slider on the sender from top to bottom results in a very small movement in the gauge needle in the full region.

Nick.
Had identical symtoms on my truck. The sender had on open circuit in the "reostat" element that the slider contacts. New sender fixed the problem.
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Old 27 April 2008, 15:28   #8
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I have no idea whose make of sender is in the tank
itts maid bi sum nobburs corld wema

sum nobburs corld wema

joyn de bloo wyre fromm de sennder too de bloo wyre onn de gayge an den taik dem too erth. (dats planit erth too yew). joyn de blakk wyre froom de sennder too de blakk wyre onn de gayge. doo nott taik deez wyres too erth. taik dem too fkin marrs ifn yew wont butt downt taik dem too erth. taik de redd wyre fromm de gayge to de 12v +. an robirts yorr farthers bruvver. dats ifn yew noe hoo yorr farther iz ov corse

garF
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Old 27 April 2008, 15:33   #9
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Had identical symtoms on my truck. The sender had on open circuit in the "reostat" element that the slider contacts. New sender fixed the problem.
I dont think this is the same. I am familiar with the rheostat type sender, but I dont think this is one of them.

http://www.tek-tanks.com/senders_gauges/fuel.html

"Tek-Tanks electrical fuel senders and gauges are fitted as original equipment by most major UK boat builders. Made from an aluminium head assembly with a stainless steel tube, they are reliable and easy to install. A float, holding twin magnets, rises and falls with the fuel level and triggers a series of electronic switches housed and sealed within the tube."

I think I will have to speak to them tomorrow.

Nick.

Thanks Garfie, could be the same one as tek tanks.
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