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Holding Tank Monitor


Terence Thatcher
There have been some discussions on this topic before. My 3/4 full light in the head has never worked. Before I start spending time and money on new sensors, here is my question: Does anyone know how it is supposed to work. I assume the water and filth filling the tank is supposed to close a circuit and turn on the light. I don't need a fancy monitor, so... has anyone ever got the original 3/4 full light to work? Might my light's failure be something simply repaired like a loose connectioin, a bad ground, a burned out bulb? Thanks for any advice. If I can't get it to work, I will probably go with a Tank Tender, which works well for my fuel, but is quite expensive. This didn't used to be a problem, since I have a Lectra-San, but Puget Sound is now a complete no-discharge zone. That means I cannot even discharge treated sewage a gallon at a time, even though all the Puget Sound littoral communities discharge millions of gallons of treated sewage a day. I have long been an environmentalist, but this doesn't make sense to me. I guess my principles are malleable, once the shoe pinches my own foot.


Mark Pearson
Staff member
Ha! Yeah that's kind of crazy, Terry. When things are "working well" those communities dump tens of millions of gallons of "treated" sewage a day. When there are flood events, they dump tens of millions of gallons of "raw" sewage a day! I don't think recreational boaters make much of a difference.

I know you know all this, but for newer owners: As our boats came from the factory, the holding tank monitoring is tricky, because the top of the holding tank, where the tank level 'sender' would normally be, is also periodically flooded because it's the bilge. Some folks have moved the holding tank to a different location, which has other pros & cons but makes monitoring easier. Some have constructed senders that are waterproof (for a while) on the top too. Some just make do with not being certain of the levels. We have been in the latter category for the last 10 years.

This wasn't the main reason we just bought a composting toilet, but it will be nice not to have to worry about it. The main reasons were the dead simplicity (no valves, head fittings, tanks, piping, pump-outs, etc.) and lack of odor with the composting toilet. We don't have it set up yet on Zia, but it makes a great conversation piece in the middle of our living room. ;)
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Richard Dowe
Hi Terry, I repaired that system 4 years ago on my boat. I was able to find a exact replacement float on Amazon.
The wires from the switch are very fragile to have in the bilge. I went to the hardware store and picked up a 1 1/4 threaded to 1 1/2 pvc male adapter and about 1 foot of 1 1/2 pvc pipe. I believe I took a piece of 1/2 pvc and threaded the inside so the float could screw into it. I am a little sketchy on the next step but it can be done several ways. The 1/2 " pvc has to be held in place in the male adapter and still allow enough room for the 1 1/2 pvc to be glued into the bell of the adapter. I think I made a round blank out of something and drilled a hole that would allow the 1/2 pvc to pass though it. I than filled the bottom threaded portion of the adapter with the 1/2 pvc inserted with epoxy or 4200. After it was dry I added heavier wire to the switch and threaded it up through the 1/2 pvc. I also glued the foot of pvc into the bell. I was than able to screw the unit back into the bilge deck. Doing it this way I was able to dictate how far down into the tank the float was and having the extra pipe topside keeps the wire out of the bottom of the bilge and also a way of gripping the sender to remove it.
If you go to the search bar and type in 130831 it will bring you to the thread that has pictures I have posted. Just not sure how to transfer them to here.
The circuit is simple: 12 volts through a fuse to one side of the float, from the other wire on the float to the light in the bathroom and than back to ground from the light.