grey tank level indicators

[mike_b42]

I have a curious problem with my grey tank level indicators. When the tank is empty, I see a green LED. as the tank fills, a second LED turns on, then a yellow LED. There is a fourth (red) LED, which is supposed to indicate that the tank is full. However, that LED never comes on, and when the tank is full, all but the first green LED turn off again.
Then, when I drain the tank, at some level the first two green and the yellow LED turn on again, and subsequently off, until I have just the 1 green LED on that indicated an empty tank.
I looked unter the MH to see if I could replace the sensors, but they are installed in the most unreachable corners of the tank, and they are all of the built-in type, so I would have to drill new holes to put in the new sensors, and I don't think I have the space to drill the holes.
Any idea what this could be? I will fill the tank half way next time we go on a trip and add some borax and soap to see if that will help.

[TexasJeff]

I was thinking the same thing. The top sensor seems to be inoperable and it could be to interference. But usually grey tanks, unlike black, shouldn't have floating objects in them other than really small bits of food possibly, or the like. If you can't clean it out like you're mentioning, this might be one of those occasions you'll either deal with the inoperable top sensor or you'll need to take it in for repair.

I'd be interested in other suggestions here as well. I've seen this happen and it hasn't been a rare occurrence either.