KIB black tank level - 2013 Greyhawk 31DS

[KC1SA]

The fluid level sensors have been discussed many times. Our fresh and grey water tank level sensing seems to work OK and don't doubt the indicators too much. We can (wife and I) can boondock 3 days without too many worries. The black tank sensor is hit or miss. Seems like on hot dry days it works well but on damp or rainy days it always indicates full. I just did a good cleaning of the tank so there should be no residual stuff hanging on the sensor. I measured the four leads. The top sensor is tied directly to the red lead, the middle sensor has a resistance to it and the lower sensor also has a resistance to it as well. The white wire (bottom most) doesn't seem to have a connection to frame ground. I measured from the crimp to the chassis spring mount and no continuity. Trying to locate the schematics for the KIB system and it appears the white wire, what they consider ground, is brought back to the control panel. All the wires buzz out ok connected to the tank. When all else fails we just look down the toilet to get a sense of fullness on the black tank. Any suggestions are welcome.

 

[SmokeyBear-JAY]

I use a soft wood stick like a dip stick. Works every time. No guessing. I even put marks on it for "guessing" 1/3, 2/3, 3/3 but it's still a guess. Wne it gets to 2/3 or more, time to dump. However, it rarely gets that full.

 

[JFlightRisk]

Flushing with a wand down the toilet has worked for us for (going on) 13 years. I did buy both sets of Horst Miracle Probes when our gray tank was stuck at 1/3. The black sensors are still in the package waiting for the time we need them.

 

[Kevin Cooper]

There is a video on YouTube at Myrvworks channel that explains how the sensors work and the ohm values to check. There should be no voltage to the bottom sensor. They work by the resistance changing when liquid level gets to the sensors above. The electric goes through the liquid.
One thing that can cause sensors in black tanks to not work correctly is Struvite. It is crystals that stick to the tank and build up a layer. So, when you drain the tank, the Struvite remains wet and acts like liquid because it is wet.
Struvite forms from the two things that your body put out and hard water. So, if you take out any of those three things, it cannot form.
Mine were not reporting correctly when I bought it used. Here is the fix if the sensors test correctly.
Mix 1/2 to 3/4 cup Borax or Calgon into hot water to dissolve it. Then put in some detergent, 1/2 cup of Dawn, or clothes or dishwasher pod (this is what I do, less foam) into the tank and go on a trip. Leave it in until tank is full and dump it. This may take a few times before they work correctly. But it will get them working again.
Also, never use bar soap in an RV. Liquid soap only. No soap scum!