This Just In - from KIB Electronics
In response to your inquiry, I have attached a file of the M21 wiring diagram, and suggest you make the following tests to determine the source of the incorrect tank readings.
First, remove the monitor panel insert to gain access to the monitor circuit board. There will be a red header connector plugged into the circuit board with 5 wires; we will be interested in the brown wire. Follow the brown wire to where it connects to the coach wiring harness, and disconnect it at this point. Check the monitor panel black tank level, it should read empty.
If the display is something other than empty, the circuit board is defective. If the black tank reads empty, reconnect the brown wire to the wiring harness, and move to the black tank.
With the black tank empty, disconnect the resistor network yellow, green, and orange wires from the level probes, and isolate them from one another. Check the monitor panel black tank display, it should read empty. If the reading is other than empty, there is a leakage path in the coach wiring harness between the monitor panel brown wire connection and the resistor network red wire connection to the coach harness.
If the black tank reads empty, ground the yellow wire to the white ground wire connection on the tank, the black tank level should read 1/3. Next, remove the yellow wire from the ground probe and connect the green wire to ground. The monitor panel should now read 2/3. Next, remove the green wire from the ground probe, and ground the orange wire. The monitor panel should now read full. If the above responses are correct, the monitor panel circuit board, coach wiring harness, and resistor network are functioning as designed, and the problem is a residue buildup inside the black tank. The remedy is to replace all the tank probes with new.
Please contact me with any questions.
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