Replacing the Holding Tank Monitor

Good reliable data is important when boating. Coolant temperature, exhaust temperature, oil pressure, fresh water level, fuel level, boat speed, wind speed, wind direction, GPS location, battery charge, holding tank level, and others all work together to make the pleasure boating experience, well, a pleasure. The level of the black water (sewage) holding tank tells me when it is time to pump it out. It is illegal in the US to pump the holding tank overboard within three miles of shore. And the state of Washington recently made the entire Puget Sound region a No Discharge Zone, meaning no holding tanks can be pumped overboard, even if you are three miles from shore. Finding a pump out station is fairly easy in the Puget Sound region as there are many of them. The state has made it convenient and easy to comply with the law.

Fiddler has a 50 gallon holding tank. The tank level indicators had stopped working properly after ten years.  The boat came equipped with a simple and certainly robust holding tank monitoring system, considering where the monitor spent its time. The level of the tank is monitored by three level indicators that are basic float switches held by a plate that screws into the top of the holding tank.  When the switches float, they activate sensors that are connected by a wire to a light panel. The float switches are set at certain depths in the holding tank, such as low, medium, and full. As the tank fills, the floats activate the lights that let me know when it’s time to pump out the tank. A premium feature on this model is that when the tank is full, a relay is activated by the “full” float switch that turns off the power to the head, preventing it from flushing and overfilling the holding tank. Here is what the float switches and the old panel look like:

Float switches on the tank level indicator
The old monitor

The Tankwatch 4 system was replaced by the DTM O4 model which is basically the same system, same manufacturer, but with a slightly different monitor and improved float switches. The big advantage of using the updated version from the same manufacturer is that I can use all of the existing wiring.

I started the installation by removing the old monitor and replacing it with the new one. The old monitor required a larger cutout than the new one, so the new model also has the option of a filler plate that matches the old cutout and is precut to accept the new monitor.

Before disconnecting the old monitor, I was careful to note where each differently colored wire was connected in order to properly reconnect them to the new monitor. Fortunately, since the new monitor is an updated version of the old one, the connections were similar and fairly straightforward, despite how the photo below looks.6995D62D-7147-4886-A9E3-9F26A48D876E_1_201_aThe new monitor came with pre-wired plugs that I think are used for original equipment installations.  Since I didn’t have a matching plug to simply plug and play, I had to remove the pre-wired plugs and wire them directly.

The new monitor did not come equipped with the relay to automatically turn off the power to the head when the tank reaches the full level. I was able to remove and use the relay in the old monitor, which is another advantage of using the updated version from the same manufacturer. The part is interchangeable and fit perfectly.

With the wiring completed and neatly bundled out of the way, I installed the monitor and was happy to see it was working.

Now for the part I have been dreading, replacing the holding tank level indicators. This requires opening the holding tank, not a pleasant exercise, and removing the old sensors and inserting the new ones, all as quickly as possible to minimize the impact. After noting all connections, I cut the wires, removed the vent line and plugged it.

The old holding tank level indicator

The float sensors need to be adjusted to fit the size of the tank, as all tanks are different. One float sensor indicates when the tank is empty and low.  Another float sensor is activated at the mid level. The third sensor activates when the tank is full and then the relay shuts off power to the head so it cannot be flushed.

Each float sensor has two small wires connected to it. One wire from each sensor are connected together as a common ground. The other wires from each sensor attach to the wires that attach to the lights on the monitor. As the float sensors are triggered by the rising tank level, the lights are activated on the monitor telling me how full the tank is. Before I installed the new tank level indicators, I temporarily wired it and tested it for proper operation.

After quickly swapping the old tank level indicator for the new, which happened in far less than one minute, I permanently rewired the new tank level indicator and replaced the vent line. The odor was minimal and quickly dissipated. All of that dread for nothing.

Here is the old tank level indicator. You can seen that it is corroded and worn out. It was definitely time to replace it and hopefully the new one will last for at least another ten years.FAD45FB6-E9FF-4502-B56D-A6BE271ABB9D


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