Hunter 306 Tank Sending Units

[Arbee]

Over the past couple of years our waste tank sending unit, and fresh water to a lesser degree, have gradually become less reliable, to the point where our waste tank now shows permanently between 30% - 50% full (e.g. it is currently empty and still showing 30%). Then, after the most recent diesel fill, that sending unit now shows (and alarms) that the diesel tank is always empty. So I thought it time to investigate and fix them all.

After consulting the manual and the Hunter Owners Store, I was expecting to find a top mounted sending unit with serial code on each of the three tanks. What I found is a top mounted diesel sending unit with no serial code, and two external sensors on the side of the waste and fresh water tanks (photos attached). Our boat is a 2004 model, so perhaps they changed the sending units towards the end of the run.

Before I contact the online store directly, I thought I'd check in here and see if the brains trust can shed any light or similar experiences with these sending units. Not sure if they are a "must replace" or can be cleaned or otherwise repaired.

Thanks for any advice and guidance.
Edit: since posting I've just discovered the water/waste tank units listed as "Water Sensor Moda" on the Hunter Store. So only the diesel tank unit is now a mystery.

Robert
Hunter 306 "Charisma"
Westernport Marina, Melbourne, Victoria, Australia

 

[dlochner]

The diesel sending unit has a standard SAE 5 hole bolt pattern. Any number of sensors will fit that pattern. What we need to know is the kind of gauge it has.

There are sensors that use a floating arm to show the fuel level. These are subject to 2 kinds of failure, the float becomes saturated with fuel and sinks or crud in the diesel collects in the hinge causing it to stick. The problems are not mutually exclusive. A good alternative to the floating arm sender is a Kus Sender, they are available on Amazon, the link is to one I recently purchased.

The other sensors are capacitance senders, they could be from ScadTech. Other than that there is not much I can tell you.

 

[Arbee]

Thanks, I've just discovered completely by accident that the water/waste tank parts are not listed as "tank sending units" on the Hunter Owners Store, they are listed separately as "water sensor moda". Looks like an exact match so I've ordered them. Apologies for the low res image, best I have at hand, but the multi-sensor panel ("Hunter Marine" make) is, I believe, standard issue on these boats.

 

[jssailem]

Your Waste tank issue may be waste material on the inside of the tank that has attached itself to the tank side or built up on the tank floor, giving a false sensor signal. A good tank flush may help.

The Water Sensor units work well on plastic tanks. Metal tanks need an internal float to work. These floats can get stuck because of internal debris. They can be a mess to replace/clean.

Regarding your installed sensor, I would use a DVM to check if they are sending a signal to the display. Clean the contacts, and you may not need to replace them. Good luck

 

[Peggie Hall HeadMistress]

SCAD offers an internal sensor for metal tanks that consists of putting the sensor into a sealed PVC tube that allows it to "read" like the external sensors do without any contact with the contents.

That said, metal is fine for water and fuel, but urine is so corrosive that it'll turn a metal waste tank--even 316 stainless steel into a colander (think: spaghetti strainer) in an average of about 10 years.

--Peggie

 

[Ralph Johnstone]

Perhaps it's the poor resolution of the photos. but the external foil attachments appear to be less that tightly adhered to the plastic tank walls. Check on the installattion instructions and I think you'll see mention of the foil needing to be tightly adhered to the tank walls

 

[Arbee]

Your Waste tank issue may be waste material on the inside of the tank that has attached itself to the tank side or built up on the tank floor, giving a false sensor signal. A good tank flush may help.

The Water Sensor units work well on plastic tanks. Metal tanks need an internal float to work. These floats can get stuck because of internal debris. They can be a mess to replace/clean.

Regarding your installed sensor, I would use a DVM to check if they are sending a signal to the display. Clean the contacts, and you may not need to replace them. Good luck

Thanks. As much as I can tell with a bright torch, while the waste tank sides have become quite opaque over time, there doesn't appear to be any debris on the sides or the bottom. In the five years we've had the boat we have a "number ones only except emergencies" rule and we bin the paper these days. The tank also gets an oxide additive every time we leave the boat (we don't live aboard, just weekends), so it's about as clean as we can make it. I was wondering if the water sensors might need re-calibrating, not sure if that's even possible though with the standad Hunter multi-sensor gauge. As someone else has just responded, the cause might be just a slow breakdown of the conductive tape and adhesion.

Good point re the diesel tank. When I last filled the tank it was a high pressure diesel hose which overflowed, so I wondered if the pressure might have dislodged/broken the sensor inside somehow. But I'll do the obvious first as you suggest and ensure there is current.

 

[Arbee]

Perhaps it's the poor resolution of the photos. but the external foil attachments appear to be less that tightly adhered to the plastic tank walls. Check on the installattion instructions and I think you'll see mention of the foil needing to be tightly adhered to the tank walls

Thank Ralph, yes it makes sense that over time the tape/adhesion has degraded and hence the readings with it. Are you aware of any calibration required (and how) when I replace them?

 

[dlochner]

Thanks. As much as I can tell with a bright torch, while the waste tank sides have become quite opaque over time, there doesn't appear to be any debris on the sides or the bottom. In the five years we've had the boat we have a "number ones only except emergencies" rule and we bin the paper these days. The tank also gets an oxide additive every time we leave the boat (we don't live aboard, just weekends), so it's about as clean as we can make it. I was wondering if the water sensors might need re-calibrating, not sure if that's even possible though with the standad Hunter multi-sensor gauge. As someone else has just responded, the cause might be just a slow breakdown of the conductive tape and adhesion.

The external sensors you have are measure capacitance not resistance. The two leads have a small electric current applied to them, when the tank level is below the level of the leads the current does not pass between the leads, when the fluid level reaches the lead level a current passes and the gauge shows the level. The tank level gauge is capacitance meter.

The diesel tank sender is a resistance sensor. There are a couple of ways to do this, the easy to describe way involves two parallel wires submerged in a tank with a conductive float between the two. A current is passed through the wires and the level of the float, as the float position changes the resistance in the sensor changes affecting the voltage. The different voltages are translated into a gauge which shows the fluid level. Basically the fuel gauge is a voltmeter, the higher the voltage the less resistance which means the float is higher. (Note the European and US senders are the opposite of each other, on one high resistance is empty and on the other low resistance is empty. I can't remember which is which.)

The most common failure in the resistance sensors is cruded up floats or saturated floats. To be sure the problem is the float, disconnect the sense wires and measure the voltage. It should be 12.5v or so. Changing the sensor is easy, remove the 5 screws, pull out the old sensor, install a new sensor connect the wires.

@Ralph Johnstone is correct about the capacitance sensors. They must have good contact to tank walls. If the tape is failing, remove the tape, clean the surface tape them down using the same kind of tape.