Horst Miracle Probes

Horst Black (l) and Grey Tank Probes

Trailer front end down

This is a description of my installation of the Horst Miracle Probes (HMPs) in the black and grey tanks of my 2010 Jayco EX-PORT 17Z.

I was totally fed-up with the false readings from the factory probes. No matter how many times the tank was flushed / back-flushed the readings would invariably be something other than "Empty".

These probes promised to be more accurate since they had a much greater stand-off from the tank wall, which meant that sludge would not slide down the wall and short the probes thereby fooling them to thinking that there was liquid at that level. Not only that but the probe elements were coated with Teflon™ and the black tank probes, when properly oriented, each has a "roof" over the element so that any floating material would "drape" over the roof and not touch the sensor. More on the sensors from the Horst Dynamics website.

The first step was thoroughly flushing out and draining the tanks prior to starting any work. Since I was doing this at home I used my FloJet macerator pump to pump the tank contents into the backyard sewer cleanout.

The second step was to crank down the front of the trailer as far as it would go in order to maximize the room at the back of the RV.


Black and grey tank outlet pipes face one another

On the 17Z, the tank outlets and sensors are on the same tank wall -- forward wall of the black tank (which is rearmost) and on the back wall of the grey tank which is centred just behind the RV's single axle. This means that there is good access to drill holes if needed and all the work is done in essentially the same location.

Luckily there was no need to drill any holes, as the HMPs were the same diameter as the stock probes, so installation was basically a replacement of each probe, one at a time so that the wires are not mixed up.

Grey tank outlet pipe and sensors

Black tank sensors


Removing the probe using a box wrench and washer

Probe Removal

This is probably the hardest part of the project and it is not hard with this technique. Simply unscrew the outer nut and remove the wire and set aside. All that is left now is the probe and one nut sitting on a plastic insert in the tank wall.

Get a box wrench large enough to fit over the plastic insert (I used a 17mm box wrench) and a large washer with a hole just big enough to fit over the threaded shank of the probe. Unscrew the nut and then place the box wrench and then the washer over the shank, and reinstall the nut. See the figure at left for the arrangement. By tightening the nut you will be evenly pulling the probe out of the tank without damaging anything. The wrench can also be used as a lever to ease the probe out after about 1/4" of the shank is exposed outside of the nut.

The HMP next to a grey tank probe

The hole in the tank. Note the dried scum that helps to foul the probe!

I recommend doing the grey tank first since there is less chance of any smells and after 4 probes replaced you will be an expert!

Seeing the probes side by side you can appreciate why the odds are better with the HMPs. Note the Teflon™ sleeve on the element of the Horst grey tank probe.

Probe Installation

This is the reverse of removal, but easier since it does not involve the box wrench.

Simply remove the outer nut (only) and then push the HMP into the hole. Then tighten the inner nut thereby compressing the washer part of the black insert by say 1/2 its thickness. This is a delicate sensor so don't be ham-fisted with this operation. A nice delicate finger action with the small box wrench is all that is needed. That done, install the wore connector and then the outer nut. Finally, using two wrenches, one to stay the inner nut and one to tighten the outer nut onto the inner nut, making good electrical contact.

For the black tank, when removing each probe have the HMP ready so that you plug the hole as soon as the probe is removed. That way you keep the waft of perfume down to a minimum.

Image from GoogleNow the black tank probes have a notch on the end of the shank to indicate the proper orientation for the "roof". The sensor must be tightened with the flat part of the notch horizontal. That is the only difference between the black and grey probes when it comes to installation.

The 8 probes removed

Last thing to do is check the function of the probes at the monitor. Since my tanks were empty and the probes were brand new, I expected to see that at the monitor and I was not disappointed. The real test will be next camping trip. I did not fill the tanks on my driveway because of the slope which makes it impossible to get the trailer level.