Over time the walls of your RV waste tanks can get a build-up of sediments, which can falsify the sensors on the side of the tanks. Inaccurate readings caused by residue and stuck toilet paper on the walls of the RV waste tanks is a common problem owners regularly face.
Most systems have a main probe on one side and three or four several inches to the side. As the fluid level rises inside the tank, low voltage passes through to the other probes and indicates the various levels. When residue builds up on the sides of the walls, electrical continuity is still present so the probes send as if the tank is full when it is not. Even after completely dumping the RV waste tanks, you can still get a false reading from left over residue and items stuck to the walls where the probes are coming in.
If you are getting false readings, there is an inexpensive and easy way to conduct a quick cleaning of the black water tank walls. Take four to five of the five-pound ice cube bags and dump them down the toilet. The ice cubes will bang around the tank cleaning the sides and probes before they melt. Make sure to do this while driving, otherwise it will not be effective. The longer you drive the better. Once done, the water can be dumped down into a dump station.
Using ice cubes is a quick and easy way to clean residue off the side walls of your RV waste tanks. Hopefully doing this will take care of the false readings you are getting without having to spend a lot of money on replacing the probes. However, if cleaning does not work, it may be best to contact a certified mechanic.
Our issue is our gray tank reading. It registers as 1/3 full even after emptying the tank. Your ice cube suggestion is great for the black, but what about the gray? We have used the Thetford gauge cleaning products several times but no improvement. Is there anther recommendation?
Hello Debi,
Thank you for contacting us.
Using something harsh like Draino works well. Add a jug of Draino or Liquid Plumber and about 8-10 gallons of water to the tank and then travel around. This helps clear the tank of debris and clean off the sensors. This is what a lot of manufacturers recommend to keep the tank in good condition. If that still doesn’t help, the probe itself might be bad and needs to be replaced.
If you have any other questions, please chat, email, or call Customer Service. 
Sincerely,
Sarah
RV Repair Club Video Membership
Not easy to dump ice in a black tank with macerator.
Another podcaster built a holding tank in his truck bed, iced it and proved that this method does not work.
I clean my waste tank every 2 weeks faithfully. My RV does not move as live in it.
Probes consistently show 3/4 full even when waste water is flushed! Any suggestions??
Hello Susan! We’d love to help! What is the year, make, and model of your RV? Thank you!
Where can I purchase new black water tank sensors and find directions on how to replace. Thank you.
Denise,
The probes will have two nuts on them. The outside nut just holds the wire into place. The inside nut it compressed against a washer to pull the rubber end back against the tank, the same principle as a rivet. You just loosen the nut and turn the probe back and forth until it pops out. To install the new probe, place it in the hole and tighten the nut down until you can see the rubber squeeze out past the washer and it looks compressed. You don’t want to over tighten, just enough to add pressure on the probe. When removing the old probes pay attention to how for the screw end is sticking out, this will give you a good idea on how far to tighten the new ones. There are many place to purchase them online. Most local RV parts stores should have them but online is a great source too. They also make some that are supposed to read better since debris can’t get on the ends. They can install into the same holes as the old ones so you don’t have to modify anything. I hope this helps!
https://www.recpro.com/rv-water-tank-probe-sensors-6-pack/
https://www.etrailer.com/RV-Sewer/Valterra/T21301VP.html
Sincerely,
Dan
RV Repair Club Technical Expert
I recently purchased a small travel trailer that does NOT have any monitor for any of the holding tanks, fresh water, gray, or black water. Any suggestions or recommendations as to how to add the probes?
Dear Eileen,
Thank you for your patience. In response to your question-
The probes can installed into the tanks by drilling 4 holes into the side of the tank. One for the ground connection and then 3 for the level indicators, 1/3, 2/3 and Full. You want to try to space them evenly apart and don’t put the Full at the very top, leave a little room for extra space when needed. The ground needs to be at a low spot on the tank. Drilling into the tanks can be hard due to limited area. On tight areas I have actually used a bolt and held it will clamps and then heated it up and pressed it through the tank. They are just ABS plastic so heated bolts eat right through them. After that you place the probe into the opening and tighten the screw down. Once all probes are in place you would have to wire them all. There is a specific wire harness made for the probe system. You attach it to the probes and then run to a monitoring system. This system will need 12v power so make sure it is close to something you can tie into for a power source. You will probably have to drill a hole into the floor to run the wires from the tank to the panel so make sure to seal it well. Adding these systems aren’t very hard, the tough part is gaining good access to the sides of the tank and then running the wires. I hope this was helpful!
Sincerely,
Dan
RV Repair Club Technical Expert
My black water monitor showed full from the first day I bought it new. I should have had the dealer repair it, but thought I could. Had it since 2008, and still give full reading. Everything looks good. No loose connection and seems to be grounded.
Hello Marlowe,
I would start by disconnecting all of the wires from the tank. Check the reading and if it stills says full, the problem is the wire harness or the panel itself. You can cut off the probe wire harness and if it still reads full, it is the panel. If it reads empty when the wires are disconnected, there could be something internal on the tank causing this. There could be some debris on the probes from installation they never came off, replacing the probes can sometimes help this. You can always install new probes around the same area as the old ones too and then connect to them instead but sometimes space is limited. I would take out the probes and see if you can tell what they might be touching. This also depends on what type of monitoring system you have on your RV, the probes with wires is the most common.
I hope this was helpful!
Dan
RV Repair Club Video Membership
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You can also use water softener salt as it will not dissolve as fast and slosh around for a longer period of time. They are also environmentally friendly as it’s just salt in a compressed format.
Any idea on where to find a wiring diagram for these probes? I have a mid 2000 fleetwood wilderness that my dogs decided they liked the taste of. Can’t seem to find anything online. Thanks
Hi Mike. Thank you for visiting the RV Repair Club site and the opportunity to assist with your waste water issue. Finding a wiring diagram for a Fleetwood Trailer built in 2000 is almost impossible as you have found, unless you can talk a dealer into making a copy. The trailer division did not document very well back then and the company has since been sold and the trailer division closed. However, you should be able to repair your wiring system as they used a very similar process as most other manufacturers. At Winnebago we used one 16 ga 12+ wire (purple) coming from the monitor panel to the lower probe and a 16 ga 12 v- ground wire ()green) to the tank ground. The lower probe then had a probe positioned across and then purple, yellow, and blue wires ran from the three going up for 1/3, 2/3, and Full back to the monitor. You should be able to see what color wire is on the main probe and replace it back to the monitor panel.
I guess that you don’t have a “macerator toilet”. Ice might not pass through too well!!!