KiB M1379 Micro Monitor notes

[johnbryanpeters]

I have been poking about trying to understand how the KiB M1379 Micro Monitor in my 2013 R-Pod 171 works. There seems to be little documentation available on the web. A prior post in this forum gave me a starting point, and i'm grateful for it. This is an edit of that post with information from further investigation added. I hope to add to it as I learn more.

The E(mpty) LED is a power available indicator and not really in indication of what is going on in the tank.

The following applies to black and gray water tanks:

When a tank selector button is pressed, the E LED lights and a red lead from the control panel provides 8 Volts DC to the red lead of the K101 pigtail (resistor pack), a red plastic lump with four wires coming out of it. If fluid level provides no current path to ground, comparators in the monitor circuit board do not turn on the 1/3, 2/3, or F(ull) LEDs.

Each tank has four probes.

1. The bottom probe is connected by a white lead to 12 V ground.
2. The next probe up (1/3 full) is connected to the yellow lead of the pigtail.
3. The next probe up (2/3 full) is connected to the green lead of the pigtail.
4. The top probe (Full) is connected to the orange lead of the pigtail.
5. The orange and red leads are connected internally in the pigtail.

As fluid level rises in tank

1. It first passes the grounded bottom probe.
2. At about 1/3 full, fluid contacts the next probe. Current through a 118 KΩ and a 68 KΩ resistor in series causes voltage on red lead to drop from 8 to 6 Volts. This is detected by a comparator in the monitor panel and it lights the 1/3 LED.
3. At about 2/3 full, fluid contacts the next probe. Current through only the 68 KΩ resistor causes voltage on red lead to drop from 6 to 4 Volts. This is detected by another comparator in the monitor panel and it lights the 2/3 LED.
4. When tank nears full, fluid contacts the top probe. This causes voltage on red lead to drop from 4 to less than 2 Volts. This is detected by a third comparator in the monitor panel and it lights the F(ull) LED.

If two or more probes and/or tank wall get contaminated, for instance with soap or grease in the gray tank, the conductance of contamination on the probe and wall reduces the red lead voltage making tank level appear high.

For black and gray water tanks only, you can determine which probe is fouled by removing all the wires from the three probes on the right and connecting them one at a time to the ground probe. The lights should come one properly if the system is working as advertised. (WARNING: DO NOT DO THIS TEST WITH THE FRESH WATER TANK – different circuits)

[sgtlm17]

Fantastic post. One question why is fresh water tank different? My fresh water appears to use the same wiring.

[johnbryanpeters]

Originally Posted by sgtlm17

Fantastic post. One question why is fresh water tank different? My fresh water appears to use the same wiring.

I haven't found that information yet. I'm guessing that fresh water has so little conductivity that the drive circuitry on the monitor board is different; I'm still digging.

[duck_eater]

Has anyone done a schematic of the board? All lights are on in my system, even for a tank that has the wire disconnected which indicates a board fault. I'm guessing the problem is the LM339N, but before doing that replacement want to have a good grasp on the circuitry as it could be a zener.

[duck_eater]

I drew out the schematic, the circuit is really pretty simple. In the end the LM339N comparator was bad. This chip is a quad comparator that takes a comparison of the tank sensor line voltage with the 3 voltages that are set by a resistor string to correspond to 1/3, 2/3 and full tank. If the voltage from the sensor is higher than the comparison voltage it switches on the LED.

Hard part of the fix is extracting a 14 pin chip without a desoldering station. I originally had thought, actually hoping, a small ceramic capacitor was bad and shorting dc to ground. It is never the simplest thing!

[my_bug]

The wiring diagram is available here.

https://www.classacustoms.com/KIB-2-...ate_p_173.html