How do I get RV black tank sensors to work with correct readings?

Dear Dave,
How do I get the RV’s black tank sensors to work with correct readings? —Ray, 2021 Columbus

Dear Ray,
I know we have covered this before; however, there are a few new products available that will reduce or eliminate inaccurate level readings.

According to the brochure available on the Forest River website, your Columbus 5th wheel comes standard with a black water flush valve. I have found many owners that had no idea what that water connection in the service center does.

Why does my monitor panel have false readings?

Your holding tanks have probes inserted into the sides with one 12-volt power wire connected on one side, and 3-4 probes across from them with wires going to the monitor panel. As the fluid inside the tank rises, the 12-volt power probe creates continuity to the other side. As it arches across each probe, it provides 12-volt power to the monitor panel showing 1/3, 2/3, and full. When you empty the black tank, it is supposed to create an open circuit to shut off the lights at the monitor panel. However, most tanks don’t empty completely and there is still “stuff” on the inside of the tank, which gives a false reading.

The black tank flush valve is designed to help clean the side of the tank and especially the probes. Hook up a garden hose to a pressurized city source and the inside valve will swirl pressurized water all over the probes and help clean the side. This should help, but it is also good to do a thorough dump of the black tank, as well. This requires two, sometimes three refills and dumps of the tank. I like to use a clear elbow at the beginning or end of the dump hose to verify how clean it is getting.

Start by dumping the tank initially, and then fill the tank back up to at least 2/3 using the black tank flush valve. Dump the tank again and check to see how clear the liquid is. Sometimes it takes at least three attempts to get everything cleaned out.

What if it still has a false reading?

If this does not, help then the hard water buildup has developed a scale or crust that needs to be broken up. I would then recommend using Thetford Tank Blaster. Empty a packet down the toilet and fill with the appropriate amount of water and let it sit overnight.

Some monitor panels have a sensitivity setting, which is a set screw on the monitor panel that can be adjusted. Check with the company that makes this or a Forest River Customer Service representative.

As a last resort, you might want to install a set of probeless level sensors like the type that Winnebago uses from KIB^®. Just stick the sensors to the outside of the tank—no more inaccurate readings. You can find them on Amazon here.

 You might also enjoy this from Dave 

My holding tank gauges never work. Is there another option?

Dear Dave,
Since the holding tank gauges that detect the levels in the various liquid tanks are so error-prone, what exactly is the gauge? What does it look like? How does it work? How is it replaced? Are there other systems being investigated/researched to make better gauges? Thanks. —James, 2020 Coleman 2515RL

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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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