Tank Level Monitor

Project date: 2002

It just seems like I am finding any excuse to start a new project. Recently, I discovered I could only tell when my holding tank was full under dire circumstances. I even inspected the tank the day before - just to see how full it was. Could not really tell.

Through research, I found out that there is an optional tank monitoring system for my boat. Unfortunately, this was not ordered with the boat. However, I came to find that the manufacturer uses a standardized wiring harness - regardless of what options are ordered. As a result, the wiring harness, power, and ground wiring already existed in my boat for the factory optional tank monitor.

I think the boat's documentation is above average. I found part numbers for the holding tank sending unit, and the tank monitor gauge. I made a quick call to the gauge manufacturer and we were able to determine the proper part numbers for the items.

There are other after market tank monitoring systems available on the market, some of which do not require insertion into the tank. However, since I was able to locate the OEM components, the installation was going to be fairly easy.


When ordering the system, you purchase the display unit and each sending unit separately. When ordering the sending units, the recommendation is for them to be 1.5" shorter than the tank depth to prevent them from bottoming out. The sensors are available in 1" increments, so you should be able to find the proper size. And depending on what mounting method you need, the sensors are available either threaded or flanged.

I installed the connectors on each sending unit and gauge.

I was able to locate a source for the connectors in the wiring harness. Connectors are needed for the gauge and both sending units. The obvious advantage to using the existing connectors is new ones won't have to be installed in the boat's wiring harness. The sensors units came with pig-tails, and I could have just as easily used crimp connectors, but I thought the plug-in connectors would be nicer.




This view of the holding tank shows the blank plug installed in the tank where the sending unit needs to go. The plug had some silicone sealant around it, which made it difficult to "break" the seal. However, in short order it came out.

One word of advice. Get the tank pumped out before starting this step. Then, put a lot of tank treatment in - about 10 times the normal. Believe me, this will cut down on the odors while you have the tank open.


The sending units come with an o-ring, which is about worthless. I could not get any degree of snugness on the unit before the o-ring deformed, and stuck out the side. I determined this was due to the lack of a groove in the flange of the sending unit to properly contain the o-ring. Therefore, I removed the o-ring, and put a bead of silicone sealant in its place. Caution: Do not use 3M 5200 or a similar product. This is an Adhesive/Sealant, and you will not easily remove the sending unit if it needs servicing.

After installing the sending unit in the tank, simply connect it to the wiring harness. Use cable ties to secure everything.


In a similar manner, install the sending unit in the fresh water tank.

Although not a big deal in the holding tank, be careful that you don't drop any silicone residue into the tank by removing the plug. I had to remove both forward and rear engine hatches to get enough clearance to angle the sending unit into the tank.


The stock location of the factory installed gauge is in the head, just to the right of the light switch. Although I had ideas of mounting the gauge in another area, the wiring harness terminated here.

Before drilling any holes, I connected the gauge to the wiring harness and tested the system.

After verifying the system worked, I taped the fiberglass panel and marked the center of the hole. I used masking tape in the belief that drilling the hole would not unduly scratch or chip the gel coat.


I used a 2in hole saw to drill the hole. The instructions indicated to use a 2 1/8in hole saw. Although I had this size, I was concerned I might chip the hole, so I drilled the hole smaller, then used a file to enlarge the hole just a bit. My thought was if I made a chip, I could file it out as I enlarged the hole. As things go, I actually put a chip in the gel coat by using the file - so this was not the proper procedure. However, the chip was small enough not to show when the gauge was installed. If I did it again, I would simply have used the correct size holesaw.

The last step was to line the cut fiberglass with epoxy. While there is no coring here, it is a good practice to seal all of the holes you drill in a boat.


Here is the finished product. When you depress either the Holding Tank or Water Tank buttons, the current level is shown on the gauge for a period of 2 seconds. This technique only draws battery power when the tank is being "read", which conserves power.

For reference, I used the sales brochure for the boat which showed the location tank monitor. Looks just like the factory installation.



Summary. This was a relatively inexpensive project, and should only take 4-6 hours to complete - well within the weekend project timeframe. The only "leap-of-faith" aspect of this project is drilling the hole for the gauge. This is a "point-of-no-return" decision. Although a hole drilled in the wrong spot can be fixed, a hole this size would take some significant repair.

Project Update


Since this article was written, the manufacturer (Fireboy-Xintex) has discontinued this system. An improved system, employing pneumatic sensing probes has replaced the original system. The display unit looks the same as well as the sensor mounting method in the tank. I have not tested the new system, but it looks to be a direct replacement.

For further information go to: Fireboy-Xintex Tank Monitor System




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