Need some help with Black Tank Sensors

Did you check the ground?

Yes check the ground because the 1/3 tank lights up, just the 2/3 and full won't light up.

Some type of insulating material has got into the sensor tube. Could be paper or anything.
You should start by jumpering from the common connection to the "bad" sensor and see if it reads your jumper. If it does, then the problem is in the sensor.
You should be able to disconnect the wire, loosen the bottom nut, then pull the sensor out and clean it.
You should be able to test the wire end to end.

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Probes even Horst are just a metal rod that completes circuit when wet

Problem could be with your monitor'

Swapping contacts from 1/3 probe to 2/3 or F probe will test if probe works (provide level is high enough to register)

Pulling monitor and check if wiring is good OR if you can swap at monitor. That would test monitor circuit/lights


I haven't even checked ours in over 7 yrs. Just not worth the frustration or involvement. They are as useless as 'battery status' indicator

Just to clarify My 2/3 and Full Will Not Light Up. Right now showing empty and the 1/3 will light up as normal, however if I fill the tank it will not show up on the panel, just 1/3 full. I understand that if something is on the probes they will light up showing a false reading. This is not the issue I have just the opposite. I hope this helps. Thanks for the advise so far.

If nothing that conducts is getting to the probe, it will not light up!
Have you tried jumpering the conductors?

It may be possible that the resistor pack that the probes connect to (usually encased in a blob of epoxy or plastic or something) has developed some internal fault. It would have to be a rather unusual fault, though, to give a 1/3 reading but not a 2/3 or full reading; a plain short or open wouldn't do that, at least by my estimation.

If it's made by Ventline as many are, there is a 68k ohm resistor between the full and 2/3 sensor and one from the 2/3 sensor to the 1/3 sensor. The wire from the panel connects to the full sensor. On some units the resistors are potted in a small plastic cup close by the tank with one wire leading from it back to the panel. There are also wires from the cup to the sensors. There is another wire from the ground sensor also going back to the panel.
Since the three resistors are in series I don't see how the wire to the display panel or the ground wire could be the issue.
If the resistor between the 2/3 and 1/3 sensor is open that could be the problem.

The display panel has five "LM339 Low Power Low Offset Voltage Quad Comparator" ICs (one for each waste tank, one for the propane tank, the water tank and one for the battery voltage) that are used to sense the voltage drops across the sensors and activate the proper LED. If the black tank LM339 integrated circuit has failed that could cause your problem.