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Old 01-15-2019, 05:32 PM   #1
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Black Tank Sensor works - after a while.

So, I though I had a "too dirty" black tank, or a bad sensor system because, after dumping and flushing thoroughly, the tank sensors still registered as close to full. I tried to reset the gauge. That worked once, but the problem cropped up again the very next time I dumped. I ran the flusher for 40 minutes, but the problem stayed. About 30 minutes later, I noticed that, instead of registering as 3/4 full, it now was registering just under half. ????? It was actually empty, or nearly so, but at least it was no longer showing the yellow light on the toilets. Then about 20 minutes later, the sensor was down to about 1/4. Finally about 20 minutes more and it registered accurately at about 10 %. Then it went up as it should as we "filled' the tank. Problem solved, Right? Wrong. Same thing happened the next dump, and the next, and the next. It appears to me that the inside of you tank has to actually dry out after flushing, before the measuring process works. Does that even make sense? At least is works, even if the working is delayed to an hour after dumping. Grey tank does not have the delay. Any ideas? Perhaps my problem is the result of a "partially dirty" tank. Have not done the "Dawn soap on 3/4 tank over a 50 mile drive" technique yet. Next trip I will try that, just for grins. Is anyone else experiencing the same delayed tank measuring symptoms? It would be interesting to know.
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Old 01-15-2019, 06:02 PM   #2
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i have experienced the same thing.....didn't happen until coach was about 7 months old...but has been happening regularly since then....i think the sensors are reading water drops on the inside of the tank.....the worst it's been has shown a reading of 30% after dumping and flushing, however...never enough to cause a problem....

i have also not gotten around to using the "geo" method on the black tank....but i plan to in the spring, before we head out again.....
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Old 01-15-2019, 06:11 PM   #3
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Ours does the same thing. Iíve learned that a minimal flush is sufficient and just ignore the initial ďfalse readingĒ. It always ends up as expected. I donít see it as an issue unless it stays red and wonít allow the flush!
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Old 01-15-2019, 06:17 PM   #4
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I had a class b motorhome do that same thing. Only after it dried out did it show empty. I figured the scale on the sides was sending false ground until it dried out. I poured tide laundry soap in it and filled. let it sit for a week then drained. I saw lots of scale pieces that looked like small pieces of paper come out when I drain it. That solved the problem.
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Old 01-15-2019, 09:49 PM   #5
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You may have the phantom hanging tp curse. For tanks with internal sensors this happens to some. The tp when wet makes the sensor read high until it dries out.

We had a coach with this affliction and just had to guesstimate when we needed to dump. Our current external sensors seem to be working ok, for now.
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Old 01-15-2019, 10:09 PM   #6
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We need to flush/rinse our black tank numerous times to get it really clean. Water, water and more water is what is needed to keep tanks clean and sensors working. A clear section to see what is coming out of tank is important to know how many times you need to rinse tank. Don't leave either tank with fluid if not using coach for a week or more. With all 3 of our Newmar's haven't had any grey or black tank issues like false reading on sensors. Remember water, water, and more water.
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Old 01-16-2019, 02:33 AM   #7
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Your best bet is to use the Dawn. I have heard lots of other remedies, but draining the tank before you leave the campground, fill 1/2 to 3/4 full with rinse water, add a cup of Dawn, drive on and drain when you get there. 99.9% chance it will work.

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Old 01-16-2019, 03:38 AM   #8
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This is the first RV we have every owned (3 previous) that the tank sensors have ever worked correctly.

However, now we have been sitting stationary in our winter spot and a couple weeks ago the black decided to register 1/3 full after dumping and rinsing. I'm going to try adding a little Calgon and rinsing more next time to see if it corrects the issue. If not I'm sure once we start moving around for the year it will be fine.
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Old 01-16-2019, 08:06 AM   #9
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The one time that my external tank sensors would not work correctly on our Winnebago, I dumped a gallon of Vinegar into the tank and filled about 1/2 full before a day's drive. That seemed to clean the gunk off the sides and the sensors worked fine after that.
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Old 01-16-2019, 12:27 PM   #10
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The vinegar is not a bad idea since it's used to remove water spots on glass.
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Old 01-16-2019, 02:12 PM   #11
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Vinegar works for removing scale because most scale is alkaline and vinegar is acidic. If youíre not a fan of the strong smell of vinegar (it bothers me) you can use a bit of citric acid or phosphoric acid. Both are quite safe to handle and are used extensively in the food industry to descale food equipment. If you do go the acidic route donít combine the acid with a detergent as the detergent is alkaline and they will react with each other and neutralize each other.
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Old 01-16-2019, 04:45 PM   #12
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Pour a small bottle of Calgon and some Dawn in the tank, fill 1/3 or so and go for a drive with curves. That will clean the tank.
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Old 01-16-2019, 06:14 PM   #13
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Ok, I have to ask a probably dumb question. How do you fill the black tank with fresh water when all you have are Dometic Masterflush Macerator toilets? Do you fill the bowl and flush them 20+ times??
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Old 01-16-2019, 06:22 PM   #14
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Quote:
Originally Posted by jerry davis View Post
Ok, I have to ask a probably dumb question. How do you fill the black tank with fresh water when all you have are Dometic Masterflush Macerator toilets? Do you fill the bowl and flush them 20+ times??
Just hook a hose to your black water rinse in the wet bay and watch your black tank level. Turn the hose off when you reach the desired %
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