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Old 04-20-2021, 09:11 PM   #1
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Black Tank stays1/3 full

Hi everyone. Little help here please. I have had issues with my black tank not emptying ever since I purchased my trailer (new). Today I had enough of it so I flushed it repeatedly. Filled it full and dumped. Removed the toilet and visualized the inside. There is no obstruction, no build up nothing in the tank but clear water. The water level is below the drain pipe so I assumed it was empty. I upgraded the sensors and still no change. At this point I used a wet vac to remove the remaining water. The tank had 20 gallons of water remaining! Once the water was removed removed the tank read empty. The only thing I can come up with is that the tank is designed to remain 1/3 full (20 gallons). Is this correct? The tank shape has a belly below the drain pipe and the 1/3 sensor is inline with the drain pipe.
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Old 04-21-2021, 01:54 PM   #2
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Are you level? The drain has to be the lowest point so if your rig is not level or sloped towards the drain you will never get it empty. This is a very common issue (tank not emptying). Mine never shows empty. I just live with it.
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Old 04-21-2021, 01:58 PM   #3
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I would look at this as a good thing. Always having water is a way to not have solids build up.
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Old 04-21-2021, 02:03 PM   #4
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It doesn't seem that the design is meant to leave 20 gallons of water in the tank.
You don't state the model, make, or year of your trailer, but if your trailer is level, climb underneath and check if your tank is level, or better sloping towards the drain.

If as you state there is a "belly" below the drain, I've not encountered that before. It seems like that is a bad idea as it will fill with solids and never drain.

Check with others who may have your make and model and ask if that's normal.
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Old 04-21-2021, 02:06 PM   #5
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I applaud your steadfast and detailed investigations.
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Old 04-21-2021, 02:09 PM   #6
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Sounds like someone cut the drain pipe into the tank at the wrong place.
I have never owned a R/V where the bottom of the drain pipe (where it attaches to the tank) wasn't flush with the bottom of the tank. Some tanks even have a part that extends downward from the bottom of the tank to ensure it empties the contents.

I would NOT look at yours as a good thing, especially if camping without hook-ups is part of what you do. 20 gallons is an awful lot of liquid space that I'd want available.




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Old 04-21-2021, 02:13 PM   #7
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Interesting Picker! I had no idea the black tank sloped that much. I'm assuming that is a typical tank.
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Old 04-21-2021, 02:20 PM   #8
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Quote:
Originally Posted by NJKris View Post
Interesting Picker! I had no idea the black tank sloped that much. I'm assuming that is a typical tank.
Yes, there are others not sloped (or as much) but they usually have a bottom drain.

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Old 04-21-2021, 05:29 PM   #9
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I had a trailer that had a tank with little slope. Bought it used and someone let the tank fill full and then let it sit for long periods until the bottom bulged considerably.

I just cut a couple pieces fo 4X6 so I could run the passenger side up on them at the dump station. Not only did this cause the tank to drain completely, it did it right quick.

New trailer has more slope and no issues with retained "sludge".
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Old 04-21-2021, 05:35 PM   #10
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Eventually, all probe type tank sensors will give you inaccurate readings. It's a design flaw because they always gunk up and can't be flushed out.

Capacitive or ultrasonic sensors mounted on the outside of the tank don't have this problem.
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Old 04-21-2021, 05:54 PM   #11
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Pictures of the sensors:

NOTE: Since the lowest sensor will indicate as soon as water touches it, the 1/3 Full light is ACTUALLY a "NOT EMPTY" light.

The Middle sensor in installed about in the middle of the wall and while it illuminates the 2/3 Full light, it is REALLY a "Half Full" light.

The top sensor is about 7/8 from the top of the tank and REALLY indicates you should be dumping soon.

NOTE: The single sensor on the left is "Ground".

ANY water or sludge remaining in the tank can keep the 1/3 full light on.
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Old 04-21-2021, 05:58 PM   #12
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Oh and for the toilet paper on the sensor crowd.

There is only a "nail head" exposed inside the tank. There is more likely to be sludge shorting it to the wall.

Whichever sensor is sludge shorted to the wall and then to the ground sensor will indicate that that level is full.

For example if the "full" sensor is shorted to the wall, then to ground, it does not matter what the other sensors are reading, the tank will indicate "full".
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Old 04-21-2021, 06:58 PM   #13
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Quote:
Originally Posted by nic7320 View Post
Eventually, all probe type tank sensors will give you inaccurate readings. It's a design flaw because they always gunk up and can't be flushed out.

Capacitive or ultrasonic sensors mounted on the outside of the tank don't have this problem.
A MUCH cheaper solution to the problem was invented by a guy named Horst.

He invented a Teflon probe with a cover that prevents gunk from forming on the probe. It just slides right off. The shield prevents stuff from touching the wire probe at all. 100% reliable and VERY easy to install.

There is a flat on the threaded portion of the shank that must face down so the shield is "up" inside the tank.

eTrailer has the best price I have found:

https://www.etrailer.com/RV-Treatmen...nt=30%20-%2060
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Old 04-21-2021, 10:21 PM   #14
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100% reliable? Really that good?

I bought a set on ebay, but the vendor only sent one probe. If they're that good, I'll try it on my bottom sensor and get more later.
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Old 04-22-2021, 01:31 PM   #15
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Drain

To see if it is an error on where the tank drain was drilled. Try driving the trailer up on ramp w/ the side opposite the drain high. If you get a large additional amount of water out, you may have to take the bottom covering off and shim the tank opposite the drain so it drains better. Other option would be change drain location or replace tank. All assuming you are level. It would be that the factory put wrong tank in or forgot to shim the tank originally.
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Old 04-22-2021, 01:48 PM   #16
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Quote:
Originally Posted by Tundra 2014 View Post
To see if it is an error on where the tank drain was drilled. Try driving the trailer up on ramp w/ the side opposite the drain high. If you get a large additional amount of water out, you may have to take the bottom covering off and shim the tank opposite the drain so it drains better. Other option would be change drain location or replace tank. All assuming you are level. It would be that the factory put wrong tank in or forgot to shim the tank originally.
Black tanks are usually supported at the top. Tank is stepped at the top and the bottom of the top step is supported by angle iron from frame rail to frame rail. Shimming is not an option on these tanks.

Again, most probable cause is a sag in the bottom of the tank.
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Old 04-22-2021, 03:30 PM   #17
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Hi JD14 (the OP)
what if anything did you find out about your situation?
I may have missed it, but what is the make, model, and year of your TT? Did you get it new or used?

If as you state there was nothing coming out of the drain pipe, yet you had 20 gal of water in the tank (nevermind what the sensors told you), I think you need to get a new tank.
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Old 04-22-2021, 10:56 PM   #18
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Black tank

My 2 previous trailer may have been attached at the top, but bottoms were strapped. Either the straps stretched or FR did a poor job of getting the straps tight as both were slipping down. Maybe the TX heat and asphalt melts them. One I noticed at a rest stop where it had ripped out the 3 screws in metal trim due to the bulge. The trim strip had been dragging and ground 1 end down. I had some plywood and sheet metal screws to screw into the cross member to get me to the next stop. I had to redo the strapping. I added an additional band and more screws to all the under aluminium trim that held the plastic Coraplast up. One strip only had 2 screws in the whole span.
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Old 05-06-2021, 08:11 PM   #19
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Quote:
Originally Posted by JD14 View Post
Hi everyone. Little help here please. I have had issues with my black tank not emptying ever since I purchased my trailer (new). Today I had enough of it so I flushed it repeatedly. Filled it full and dumped. Removed the toilet and visualized the inside. There is no obstruction, no build up nothing in the tank but clear water. The water level is below the drain pipe so I assumed it was empty. I upgraded the sensors and still no change. At this point I used a wet vac to remove the remaining water. The tank had 20 gallons of water remaining! Once the water was removed removed the tank read empty. The only thing I can come up with is that the tank is designed to remain 1/3 full (20 gallons). Is this correct? The tank shape has a belly below the drain pipe and the 1/3 sensor is inline with the drain pipe.
I had the same problem on a couple of occasions. Turns out, the lowest sensor (the one that reads 1/3 full) was caked with sewage, therefore reading 1/3 full. I created a little sideways spray nozzle that attaches to a garden hose so I can lower it into the toilet and down into the tank. (You can buy these at an RV store, but I took one look at it and knew I could figure something out.) I had to look at the sensor location on the outside of the tank to know where to direct the spray. Bingo! Problem solved. I've had to do it twice since with success both times.
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Old 05-06-2021, 08:15 PM   #20
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I learned to live with useless black tank sensors long ago. You can look down the toilet if you are worried about filling it. Only ever had that happen once and the recovery procedure got me on the track of using the Mexico approach to TP. Never an issue since. Grey water fills more than black but I just look inside for fresh water tank level to decide if dump and pump needed.

I never hook up to CG water other than to fill my tank.

OP has a different problem though: design, installation, or failure. Not a measurement problem. Get help if you can’t get under to check it out.
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