How To Replace Holding Tank Sensors On An RV

RV holding tank sensors are a hotly debated topic in nearly every sphere of the RV world. From people who believe that they will never work properly to people who rely on them heavily for holding tank maintenance, everyone with an RV seems to have strong opinions of sensors. Here at Unique Camping + Marine we are in the camp (pun intended) of those who find them to be a helpful tool in managing your RV holding tanks. When sensors DO misread, it is usually a matter of needing to clean the holding tank to remove any debris or build-up from on or around the sensor. However, sometimes even with extensive cleaning, RV sensors can still misread. When this happens, there is usually an issue with the wiring or with the controller that shows the liquid level in the tank. This guide will help you troubleshoot your misreading RV sensors so you can know if you need to replace components, or if you simply need to clean your probes.

In this guide we’ll be talking about the process of replacing sensor probes when you’ve determined they need to be replaced. This is most common with the freshwater holding tank where hard water and mineral deposits can form on the sensor heads, causing them to no longer read accurately. This article will walk you through how to replace sensors on the freshwater tank, as well as on the gray water and black water tanks.

The style of sensors that we are replacing are the most common style, which measure in either thirds or quarters. This style of sensor can either be the OEM button style or aftermarket Horst Miracle sensors. We won’t address other types of sensors, such as SeeLevel II or Horst Miracle Gauge in this article. 

Recommended tools for the job

• Gloves
• Adjustable wrench or 3/8″ box wrench or ratchet 
• Channel lock wrench
• New set of sensors
• Colored tape (if you are installing Horst Miracle style sensors)

Step 1:

In order to replace the sensors in any holding tank, you will need to access the exterior of the tank, which is very easy on RVs with an exposed undercarriage. Unfortunately,  many RVs have a covered undercarriage which can make this a bit more complicated.

Sometimes this cover is chloroplast sheets; other times there may be insulation that is either laid out in fiberglass rolls or that is sprayed in and hardens over time. For these RVs, finding the sensors and getting access to replace them is more difficult, so many RVers are not comfortable with this process. If this is the case for you, we recommend either taking the RV to a service center or having a certified mobile technician replace the sensors.

But if you do have easy access, or you’re up to the challenge of removing the cover/insulation, the actual task of replacing the sensor is relatively easy. Let’s jump right in!

Step 2:

Before getting started, make sure that whichever holding tank you are replacing the sensors in is completely empty.

This is best accomplished by opening the low point drain from the tank and allowing any water to completely drain out.

If you are replacing the sensors on either the black water or gray water holding tank, be sure that you have dumped the contents of the tank at a dump station or a location suitable for receiving wastewater.

It is good practice to turn off the water pump and make sure the RV is disconnected from a water supply source. Also make sure that the RV is not connected to shore power, and that any onboard power sources such as batteries have been disconnected.

Step 3:

It is best to begin by taking a picture of the existing sensors with all of the wires attached. You will refer to this when reinstalling the wires.

The different levels of the holding tank have different resistors in the wire harness, these are color coded on the insulation of the wires. The specific resistances help the controller to read the correct liquid level in the holding tank, and the wires must be in the same order that they were in before the sensor replacement. And instead of removing all of the wires at once, it’s better to replace one wire at a time to keep things in the proper configuration.

Step 4:

There are two ⅜” nuts that sandwich the ring terminal that goes to the wire, providing a solid connection to the sensor. Remove the first nut and slide the ring terminal off the sensor end.

With the wire removed from the sensor, loosen the second ⅜” nut to remove the tension on the rubber bushing that expands to create a watertight seal between the sensor and the holding tank. Once the second nut has been loosened, back out the nut on the threads to allow a wrench to fit in between the nut and the washer. Using the holding tank as a brace for the wrench, pull the sensor free from the holding tank. The rubber bushing that makes the water tight seal will bulge and it will take some force to pull it free from the tank. Since this bushing will not return to its original shape once it is removed from the tank, cleaning the sensor and reinstalling it is not recommended. 

Step 5:

Sliding the new sensor into the tank is very easy.

While holding the sensor flush with the tank, tighten the second nut to expand the rubber bushing, which will make the water tight seal. Being careful not to overtighten the nut by comparing the thread depth of the other sensors and matching the depth of the nut as best as possible. Place the ring terminal with the wire attached on the sensor and thread the first nut to sandwich the ring terminal between the two nuts. Make sure the nut has been tightened to a level that will hold the ring terminal in place. From there you can proceed to the next sensor.

If you are installing Horst Miracle style sensors with the hoods installed on them, be careful to them with the hoods oriented in the correct position. The hood should be oriented to the top of the holding tank. The best way to ensure that this style of sensor has been installed correctly is to add some tape on the rubber bushing that makes the water tight seal. This shows the direction of the hood from the outside of the tank which will help when tightening the screws.  

Step 6:

Once all of the sensors have been replaced, double check that the wire colors match the sensor levels from the picture that you took of all of the sensors at the beginning of the installation. While you still have easy access to the sensors, restore power to your RV and test the sensors. It is a good idea to start by testing the tank on empty, and then slowly add water to the tank, checking each level one after another. Make sure to check for leaks at the individual sensor bushings. If leaks do occur, remove power from the RV and tighten the second nut on the leaking sensor to expand it further to stop any leaks. Restore power again, and continue testing. Once you have confirmed that the sensors are working and there are no leaks, you can replace any covers that were over the holding tank. Keep in mind that when you empty a tank, there may be some residual water or liquid remaining on the walls. This may cause a temporary misreading of the liquid level at the control panel. So it is a good idea to allow some time after dumping before checking the control panel. It typically doesn’t take much time for the liquid to streak off the walls and sensors. Things should be good to go within an hour. [please adjust time if needed]

Congratulations! You have successfully changed the sensors on your holding tank!

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