Holding tank sensor


Well-Known Member
May 28, 2013
Boat Info
2004 420 DB, GHS Hydraulic Lift
Garmin 8600/Garmin 1222 plus
AB Mares 10 VSX with 30 hp Tohatsu
Cummins 450C 8.3 L Turbocharged
So I found out this weekend that my holding tank only reads empty when it truly is empty. After that its anyone's guess what the levels are as it doesn't read half or full. So my question is...does the problem exist with the sensor display in the control panel or is it the sensor in the tank itself?
I'd bet it's the sensor in the tank.

+1. Easy to confirm once you get the sensor assembly out and hit the connections with a multimeter.
May be electrical part of the sensor or may be all kinds of nasty stuff messing with the float. My second light won't come on until it is very full. I use a flashlight and some rocking to check during routine bilge checks. A bit of that blue WM mint stuff can help to see it if its mostly water.

I hope you use good TP.
You need to pull the sensors from the tank and clean them. There are 3 of them empty, med and full. They havea float on the sensor that goes floats up and down. I just figured that out after calling SeaLand.
Will give it a shot. Thanks!
Had exactly the same problem on my 38DA since taking ownership back in April.

Pulled the sensors out yesterday cleaned them and now all working as they should. Tested them by turning each sensor upside down before installing into the tank. If you remove the floats, as i did, (by removing the plastic circlip on the bottom of the sensor stems) make sure you install them the correct way as otherwise the panel light will be indicated when it shouldn't. I discovered this on one of my sensors during my testing before re-installing into the tank - removed the float flipped it around and all good. They do have markings on the floats - N O on one side and N C on the other indicating normally open and normally closed, respectively - however these markings weren't visible until after I had cleaned them.

If you pre-test them you will know the full sensor (shortest sensor) is working as you will hear a distinct click as it also serves to shut off the vacuum pump to prevent further flushing of the toilet. This is also event in the head where the indicator light switches to red rather than green.

Now I just need to replace those duckbills......after my nose hairs grow back!!
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I agree the sensors are all mocked up . One thing to do before is FLush the tank as much as u can prior. Fill it , empty it and just keep repeating ... Ull thank me later !!!!

Will do. After winterizing this past weekend I didn't have the dexterity to tackle this job so I'm leaving it for the spring. I went to the gas dock to fill up and do a final pump out for the winter layup and the dock hand thanked me for taking the bullet since the incident in which we found out the sensor wasn't working. At the time I think my words to him was " you may want to take a step back".

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