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      ….has anyone experienced erratic fuel gauge readings? Sometimes I get what I would consider normal, accurate readings of what I would expect the fuel level to be, at other times the gauge immediately jumps to full-scale ‘full’, which I know is wrong. I’ve checked all the obvious things, connections at the gauge itself and connections on the sender unit.

      Answers on a postcard, please!

      Ian Culley/B32 Another Fantasy


        Ian. Hi, has it always been like this? Or is it a new fault? Does the guage level differ when the battery is low comparied with when the engine is running? IE reads higher when the voltage is higher?

        If so, I believe that there is a voltage regulator in this curcuit normally its inside the guage. Its a long time since I did my basic electronics course 🙂 but, I believe that it will be a resistor and transistor (2 wire and 3 wire components) on a small board. Or posibly a little chip or IC inside the case.

        These guages work on resistance within the sender so it could also be the sender is shorting or jumping on the mechanical scale.

        I would suggest that practically its not something that can be “fixed” unless your proficient. But you may spot a dry joint or broken wire inside if you take a look. I would look on ebay for a new tank sender and guage from a custom car guage supplier. They are cheap and easily fitted and calibrated.

        Hope this of use.

        Best regards.




          Thanks for your comprehensive reply! The fault is a new one (last six months or so on a 13 year old boat) and seems to be completely independent of voltage: engine on, engine off, shore power + charger on or off makes no difference so I suspect it’s an intermittent contact somewhere. I can live with it because I keep accurate records of engine hours run which I can correlate with estimated fuel consumption, so I’ve got a pretty good idea when I need to refuel.

          Thanks again for your help

          Ian C


            It sounds like the sender, with low fuel in the tank they are reasonably easy to get out, six or eight screws from memory. It may be worth taking it out and taking a look. There is a float on the end of a wire arm that can be a bit of a fiddle to get out through the opening but in went in that that way…………. Mark the tank and top of the sender before you take it out so that it goes back in the same way around. The business end is at the top where the pivot point is. Check everything is tight and making contact electrically.

            You can use a multi meter on the ohms scale to check the change in resistance. Start at the highest scale and work down. Connect the meter across the two terminals and move the float up and down, the resistance should go up and down on the meter. If there is a dead spot where there is no resistance the sender is faulty.
            You can get a cheap Multi meter from Maplins for about £7.

            A multi meter is a really good tool to have on a boat. Not sure of your knowledge so I will assume it’s a little apologies if it’s more…….
            If you have not used one before the instructions are pretty good. You can measure resistance (ohms) Voltage (volts) power(amps) continuity (wire connected from point to point)

            Using ohms law, if you have any two of the above values you can work out the 3rd value. IE. If you know the voltage and the resistance you can work out the current (amps) and this will enable you to work out power in watts. You can even get an Iphone app that does the maths!!!

            Basically Ohms law says I =V/R ……..
            I is amps or current. V is volts R is resistance
            If you know any two you can calculate the third.

            If you know the voltage and the amperage you can work out the power in watts.

            Again basically;
            Amps x Volts = Watts
            Watts / Volts = Amps
            Volts x Amps = Watts
            Again, apologies if you know this, but hopefully other readers will have a “light bulb” moment and buy a multi meter. 

            Hope this post helps you and others.

            best regards.



              …..thanks Ant.

              I have a multimeter on Another Fantasy and know about the I=V/R stuff from a-level physics (albeit years ago!!) so I guess I can get by using fuel used = hours run x consumption rate per hour.

              All the best

              Ian C


              Hi Ian

              Just had this happen to me on way from Cherbourg to St. Vaast.

              Turned out to be the sender unit. Take it out and you’ll see a pin and rack afair (electrical connection bit). Give it a clean with some white spirit and see if that helps.

              Otherwise, if it’s suddenly going towards full, it’s getting a stray full voltage from somewhere, so check cables and connectors.




                Hi Bob, does the sender unit just “screw in”?


                  Hi CaSam

                  Not sure about the B39 but on my 2002 vintage B32 the sender is mounted by I think it was either five or six set screws around the periphery of the unit. It’s easy enough to remove but a bit fiddly to replace, you need to make sure it’s properly bedded down on to the tank itself. Mine is still playing up so I need to do a ‘Bob’ and remove and clean it to see if that helps. I’ll let you know…..

                  Ian C/Another Fantasy


                  @CaSam wrote:

                  Hi Bob, does the sender unit just “screw in”?

                  As IanC says above.


                    Problem fixed at last! While passing the time of day in a windy Salcombe I decided to have one more look at the sender. I noticed that although I’d cleaned the male (sender) part of both spade connectors on the sender unit the female connectors, particularly the negative one, were corroded inside. A good dose of WD 40 and poking around inside the spades with a thin piece of cloth and hey presto! system back to normal and it’s been OK ever since.

                    Ian Culley
                    Another Fantasy

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