Fuel gauge always says full. Which variable resistor/potentiometer?

[Onegazza]

I have a 0-190 ohm fuel sender in my tank and a corresponding 0-190 ohm fuel gauge however, it only ever says full (and if I'm lucky, 3/4 full means I'm almost empty). Cable length is around 3.5m of 1mm (1yr old).

What size potentiometer/variable resistor do i need in order to get this reading correctly and does it need to be in series or parallel? I previously tried a 1kohm with no luck.

 

[Scott Danforth]

you need to get the float arm to properly move

also, if your sender is at the back of the tank and you have a shallow tank, it will most likely read full if your bow is always pointed upwards. does your boat sit level?

 

[Onegazza]

you need to get the float arm to properly move

also, if your sender is at the back of the tank and you have a shallow tank, it will most likely read full if your bow is always pointed upwards. does your boat sit level?

Sorry, this wasn't my question. Even if the fuel sender is removed from the tank and moved up and down manually, my gauge still reads full.

 

[Scott Danforth]

Sorry, this wasn't my question. Even if the fuel sender is removed from the tank and moved up and down manually, my gauge still reads full.

then you have either a bad gauge or a bad sender or bad wiring

 

[Onegazza]

then you have either a bad gauge or a bad sender or bad wiring

Are you sure on that? It's pretty common to need a potentiometer on a fluids gauge.

 

[dingbat]

Are you sure on that? It's pretty common to need a potentiometer on a fluids gauge.

The sending unit is a calibrated potentiometer.

Have you measured sender output to verify its 0-190 ohm output?

 

[Scott Danforth]

Are you sure on that? It's pretty common to need a potentiometer on a fluids gauge.

never ever ever had to use a potentiometer on a gauge ever.

and I must have designed at a minimum, 800 different fuel tank installations from marine to off-highway to on-highway

So I am pretty sure

 

[mike_i]

sounds like a bad gauge

 

[Silvertip]

Look very carefully at the fuel sender. If you got overly aggressive at some point tightening the electrical connection nut, you may have spun the entire assemby to where it shorted against the main support rod which is ground. Shorting the sender terminal creates a full indication.

 

[Onegazza]

So, after a thorough investigation today, as the wet, grey British winter draws in, I found two problems:

1. The wires on the fuel sender were noted incorrectly. It was noted black as ground and blue as signal in the manual however, removing the fuel sender from the tank and connecting directly to the gauge, it started functioning correctly with the cables swapped round

2. I noticed the horn had stopped working. Putting a multimeter on it, the voltage across is was dropping to 0V. Wiring directly to the fuse box positive and ground terminals confirmed the same - the cables from the battery had reached end of life. Because ultimately the resistance increases and the fuel sender is wired to ground, this would have caused a continual full reading anyway because the resistance was high.

As I had all the kit with me, and with it tipping down with rain, I put the covers over, turned on the torch and got to it and replaced with new cables in the loom from the battery to the fuse box, disposing of all the old ones. Replaced with proper tinned marine wire and solder heatshrink connectors

Hey presto, horn burst back into life also and fuel gauge all working correctly.

...turned out the tank was full anyway so, siphoned out 40 litres to stop it overflowing as I removed the fuel sender and watched the gauge increase back up as I filled the tank back up again afterwards.

 

[Scott Danforth]

glad you got to the bottom of the problem