Fresh water level sensor on a 1999 Pollensa?

My fresh water level reads zero. It doesn't have the usual three prongs, it must have a variable resistance float, I think. There are only two wires coming from the top of the tank and they read open circuit.
Has anyone removed one, did you find a fault, is it repairable? If not can anyone suggest a replacement? I used to remove float level sensors from car fuel tanks in the '70's and I guess this is like that. I will have to remove the tank. It would be nice to think it is a broken wire inside the tank that I can fix.
Next job will be to fit a 100 litre tank, I can't see why they weren't fitted with 100 litres when they were new.

Thanks Ian, yes I am in the middle of that. I filled the tank, at which point the analogue meter should shoot over to the 'full' side of the dial. It still said empty. I got underneath expecting to see four or five wires but there were only two coming from the top of the tank, which means it must be a variable resistor inside. I cut the wires and shorted them together, went back inside and pressed the 'water level' button and the needle shot across to the full position. So I am pretty sure it is a variable resistor with an open circuit somewhere.
As for the 100l tank I know that will weigh 100kg when full - but I would rather drag that around the countryside than pay a site fee just so i can refill it. It will on average, if we fill it and use it till its empty, be 50kg. If my maths serves me right !
What does a 110 AH battery weigh? I haven't weighed it but I guess 20kg. And a spare wheel? Dunno, but I have got one.
I may well go to SV Tech and get it up plated to 3500kg. I still don't know why A/S ever put campers out at 3300kg, and I think still do with Nuevos?

I think they weren't fitted with 100 ltr tanks from new for the same reason as they currently fit showroom glitter instead of 'the proper job'. As it were. In other words you really need at least 120W of solar power, preferably 200W so they fit 60W. You need 100litres of fresh water so you get 60 litres. Doesn't matter so long as you get shiny alloy wheels which, once they have done their job in the showroom, go all grotty.
"Look sir, it has a solar panel"
"ooh, that's nice"
"Look madam, it has a fresh water tank"
"well of course it does. I don't care or know what size tank I will need, but that means I can go a whole day without refilling it."
"It has got air conditioning in the living area"
"Ooh so we can go to France in the Summer"
(Titter titter I'm not going to tell them they will need to be in a camp site with mains electricity to make it work. And it weighs 32 kilograms, but they don't know how heavy that is anyway. And it's on the roof, swaying the body around when you go round a corner )
"Ah and look at the shiny wheels"
"Very pretty."
"and it has a heating system you can control with your phone, (in case you have forgotten what a 'heating on' switch looks like) so you can sit in bed and turn it on and off"
"Ooh how lovely. Where do I sign?"

. . . . . .

Anyway back to the point. I removed the tank to discover that it has two stainless steel probes both the whole depth of the tank. So the meter is obviously reading the varying resistance between the probes. I measured it. As you would expect it is infinity when the tank is empty, and as I was filling it I could see the resistance dropping. When full the resistance between the probes was 24k. So I need to remove the pcb and find out why it isn't registering properly. If I short the two probe wires the meter goes to full deflection, so some of the electronics is working.
I want to fit a 100l tank, but before I spend £150 on one, I want the meter level to work.

That Topargee you mention Bargee seems to imply that you have to fill the tank (knowing what the capacity is of course) and set the Topargee to zero and then monitor it.

There should be another sort. An ultrasonic distance sensor. But of course it would be more expensive. Donkeys years ago I did a short contract for a company in Cheltenham which manufactured sensors that water companies would install under a bridge and measure the distance to the river / canal below. It sent out an ultrasonic sound wave, started a clock, picked up the received reflection, stopped the clock and so as we know the speed of sound, it measured the distance. Thus they could tell the height of the water. I was really chuffed at that job: it was to repair about 800 units they had made which failed the final test. They expected a failure rate in manufacture of between 1 and 2 %. I wondered why they thought that I, a telecomms engineer, could repair them. By lunchtime on the first day I had discovered that there was nothing wrong with the manufactured items, it was the test bed that someone had designed wrongly. I fixed it and lo and behold the units passed the test one by one. At about 4pm I thought oh dear, I have done myself out of a job, but at least my contract is for a week so I'll get a week's pay. So I went to the boss and said sheepishly, there's nothing wrong with the manufactured items, it's the test bed that was faulty. The boss offered me a full time job.
That would have been about 1973. If I had had my wits about me I would have seen that the device was in fact a distance measuring device and it could be fitted to the back of a car to warn the driver how close they were to something they were reversing up against. I could be a millionaire by now!

The Bargee wrote: I tell the Admiral how much water she has used for each shower.

 How brave! I still have the mental scars....  and unnecessarily high water bills...

Cymro

Haaa Haaa Heee Haaa Haaa. I found the problem! No new gauges needed.
Those of us who form the majority on here will recall valve radios, which warmed up faster than a touch screen panel on a modern vehicle. And worked more reliably.
You remember the volume control, which went 'scratch, scratch' when you rotated it? That is because the wiper on the potentiometer inside was dirty, causing a bad connection between the wiper and the resistive layer inside the pot.
This is what happened with my water level meter. The 'set' potentiometer had gone dirty after 25 years. When I rotated it, (with the water tank full) the level needle shot across to 'full', and when I wiped the potentiometer to and fro, the needle jumped about all over the place which clearly showed a dirty connection inside the pot. Doing this several times cleans the connection between wiper and wiped surface inside. So I wiped it to and fro a few times and now all is restored, it works properly. Hooray.
So having spent the best part of two days removing the tank, then removing the probes and replacing it all, all I needed to do was to screw the 'set' control backwards and forwards a few times 
Now that is what I call a repair. No computers involved, no £95 bill for plugging in a OBD2 diagnostic machine. Just ordinary volts travelling across a resistive layer and on to a variable potentiometer wiper.
Off to CAK tanks in Kenilworth tomorrow to get a 100 litre tank . . . . . .If they have one in stock of course. I know they are advertising the correct size on ebay so a quick phone call to confirm before I go. . . . . .

I started to say that we have just been away for over a month trailing round France and Germany, I left the thing alone, periodically pressed the 'water level' button and it only ever showed 1/4 full, however much water was in it. So it didn't fix itself.
Anyway now it is fixed I am fitting a larger water tank of 100 litres. Here's a photo of the inside of the new tank - who would have thought you could take a photo of the inside of a black tank with your arm taking up most of the access hatch opening? The original probes don't reach the bottom because the new tank is bigger so I will be using Cunning and Guile to extend the probes to the bottom.