Black tank says full but it’s empty

[rvshrinker]

Fairly new RV. Have rigorously followed instructions regarding emptying only when full, keeping water in the bottom, using a black tank tablet.

On a recent trip, I was surprised to see the black tank gauge read 2/3 full. I emptied it, a lot came out, then the grey, all good. But when I went back to check the gauges, black was still reading 2/3 full. Grey now empty. I filled the black with several gallons of water and a black tank tab. No change. Within the day, it was reading full.

Now I have not attempted the black tank flush, I assume through what ORV labels the “San-T Flush” because the instructions are poorly written and I want to make sure I get it right. For example it suggests I use a separate hose from my drinking hose, which makes sense, so I have to get one of those.

Please let me know how to trouble shoot this.

[ScottG]

I never, ever look at the black tank sensors. Most black tanks are so big that a few days use just doesn't even come close to filling them up.

[wa8yxm]

First you need to understand what the sensor is.. Basically a nail or screw.
That's all. Just a metal rod that sticks into the tank. THe system assumes that if the rod is wet the tank is "rod" full. (That is wet and shorted to ground by the wet).

Problem is that the sides of the tank get "Slimed" (Think ghostbusters) and the Slime is wet so it grounds out the sensor and the tank thus reads FULL even though if the things are lighed up properly and you use a clear extension on the outlet port you can look down teh toilet and see daylight.

Using the flusher (San-T-Flush) may help (not very likely)

SUing a wand like the Twister tank flushing wand might

Valterra Master blaster is a very well designed want in terms of how it works. but ... I do not think it will last long since the nozzle may well break off first time you use it.. Thinking like the engineers at Valterra I built my own version without the nozzel. Just a PVC pipe. hose fitting (Vacuum breaker and valve)( on one end and a cap on the other with a 7/32" (I grabbed a bit close to 1/4. that was the bit I grabbed) hole in it.. On rare occasions if I "Aim" for the sensors.. I can blast 'em clean.

Basically I don't bother any more. I just ignore the sensors.

[drsteve]

Most of us gave up on our tank sensors long ago.

[gbopp]

pryoclastic wrote:

gbopp wrote:
Try the Geo Method. It may clean the sensors.

Might want to rethink that process:

http://www.forestriverforums.com/forums/f88/concerns-about-geo-method-and-bleach-11962.html

I should have mentioned, I don't use bleach. Just the other stuff or a similar receipe.
Thanks for the reminder.

Often after dumping the tanks I just put in a cup of dish detergent and a couple gsllons of water. It works for me. YMMV.

[Jayco-noslide]

After 3 RVs with those tank sensors, I long ago stopped paying any attention to them.

[pryoclastic]

gbopp wrote:
Try the Geo Method. It may clean the sensors.

Might want to rethink that process:

http://www.forestriverforums.com/forums/f88/concerns-about-geo-method-and-bleach-11962.html

[SidecarFlip]

jplante4 wrote:
If you have a straight shot into the black tank from the toilet, shine a light down there and see if you're empty and just forget about the sensors. Worrying about such things while relaxing defeats the purpose of having an RV.

If you cannot verify that the tank is empty, then the Geo Method works as well or better than any magic pixie dust that Thetford produces.

Mark this day on your calendar folks. I agreed with SoundGuy 🙂

In reality, RV builder use the cheapest sensors available. Immersion sensors are terrible and inacurate and failure prone.

I always fit my tanks with Blue Sea systems EXTERNAL tank sensors. No fouling issues ever. Very accurate and dependable. Too bad, all but the most high end manufacturers don't use them, but, then, they cost more and cost is what is the driving factor in all but the high end RV's.

Having said that, pretty easy to ascertain when the black tank is full, look down the toilet...lol If it's REALLY full you won't have to 'look down' it won't flush anymore....:E

[SoundGuy]

jplante4 wrote:
Mark this day on your calendar folks. I agreed with SoundGuy 🙂

Yikes! :E :B

[jplante4]

If you have a straight shot into the black tank from the toilet, shine a light down there and see if you're empty and just forget about the sensors. Worrying about such things while relaxing defeats the purpose of having an RV.

If you cannot verify that the tank is empty, then the Geo Method works as well or better than any magic pixie dust that Thetford produces.

Mark this day on your calendar folks. I agreed with SoundGuy 🙂

[gbopp]

Try the Geo Method. It may clean the sensors.

[SoundGuy]

Wildly inaccurate black & grey holding tank monitor readings are the RV industry's way of laughing at us all while keep a straight face. :W Until RV manufacturers completely ditch the use of in-tank monitor probes and upgrade to externally mounted sensors RV owners will continue to complain about this. Unfortunately, because manufacturers will always take the least costly approach this will never happen and as a result we'll all be complaining about this same issue years from now. My advice - ignore those tank monitors and simply use common sense to know when it's time to empty those tanks.

[MitchF150]

The only sensor that still works on my 16 year old trailer is the fresh water tank. The black tank is always at 1/2 and the grey tank is always at 3/4, mostly because the wires are broken on the grey tank.. Yes, I know about it and have said "I'll fix it" at some point, but in reality, I never will.

I know my tanks and know my water usage, so I know when to dump..

I can also physically look at my fresh water tank and see it's level, so the monitor is useless for the most part... I only use it for the water pump switch and the water heater switch.

Good luck!

Mitch

[pryoclastic]

rvshrinker wrote:
Please let me know how to trouble shoot this.

Not to worry at this point in time as it is a super common problem. Sensor(s) could have a something clogged up against it making it report a false positive.

I have tried several type of black tank cleaners and this is my favorite:

https://www.amazon.com/Kronen-KHT003-Handle-Holding-Treatment/dp/B00BFTZC3A

Pour some that down into the black tank and let it slosh around. When you pull into your next destination, dump black and see if that fixes the false positive.

If it does, use the holding treatment regularly and especially make sure some is in the black tank as you travel to slosh it.

[Sea_Six]

There is something stuck on one or more of the sensors, such as a piece of toilet paper. Lots and lots of flushing may dislodge it. Or not.

You could get one of those hose attachments that streams a fast jet of water and try blasting the walls of your black tank from the toilet.

Personally I wouldn't worry about it. When the toilet belches when flushed, it's time to empty it.

I had that problem in my last RV. It lasted nearly the whole 5 years I owned it. One day it just started working.

To flush the tank I use a timer. When five minutes are up, I turn off the hose valve which stops filling the tank with fresh water, and dump. Works for me. You can time how long it takes to fill a five gallon bucket with your hose on full blast, to see how long it will take to fill your black tank. Do the math using your black tank capacity. Shave about 30 seconds to a full minute off the time to fill it completely full and use that number on your timer.