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Old 09-02-2010, 09:39 PM   #1
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Non normal holding tank sensor problem

I recently took out the black tank so I know the tank is clean. The problem I have is all the lights come on (shows the tank full) even when none of the sensors wires are connected to the tank. They also all come on when all the wires are connected or even when some of the wires are connected. It seems to make no difference how many are connected. Gray works fine. My coach shows lights when momentary switch is depressed. Any ideas?
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Old 09-03-2010, 04:05 AM   #2
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If it is only the black tank then you probably have a bad resister pack. It is what the different sensors are all connected to one line going back to the monitor.
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Old 09-03-2010, 12:01 PM   #3
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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 cap close by the tank with one wire leading from it back to the panel. There is another wire from the ground sensor also going back to the panel. There are also wires from the cup to the sensors. There is no connection between the 1/3 resistor/sensor connection to the ground sensor terminal other than fluid in the tank.

On others all four wires go into the wiring harness and I don't know where the resistors are located. Maybe on the display panel?

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.

Since the resistor pack is open ended (the bottom resistor does not connect to anything except the 1/3 sensor) I don't see how it could cause your problem if the problem persists when the pack is not connected to the sensors as you said.

I suspect that the LM339 on the PC board may have gone bad.
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Old 09-03-2010, 12:34 PM   #4
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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 cap close by the tank with one wire leading from it back to the panel. There is another wire from the ground sensor also going back to the panel. There are also wires from the cup to the sensors. There is no connection between the 1/3 resistor/sensor connection to the ground sensor terminal other than fluid in the tank.

On others all four wires go into the wiring harness and I don't know where the resistors are located. Maybe on the display panel?

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.

Since the resistor pack is open ended (the bottom resistor does not connect to anything except the 1/3 sensor) I don't see how it could cause your problem if the problem persists when the pack is not connected to the sensors as you said.

I suspect that the LM339 on the PC board may have gone bad.

Your description fits what I have. Is there a reason that the LM330 on the Pc board would go bad just from the removal and replacement of the black water tank? If the wires are grounded accidentally will the LM339 short out?
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Old 09-03-2010, 04:06 PM   #5
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I don't think so. There is a resistor on the board that is connected from the LM339 to the wire going to the resistor pack and I think that would protect the LM339 from shorting wires in the tank area but maybe not.

I can't really think of how taking the black tank out could have hurt the board but at the same time I am always suspicious of coincidences.

This might be a good time to investigate a non contact sensor system such as the SeeLevel unit made by Garnet . SITE SeeLeveL RV Gauges by Garnet Technologies Inc.
If you end having to buy a board you might as well put the money towards a better system.
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Old 09-03-2010, 09:06 PM   #6
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Well, I often have said you don't have to pass an intelligence test to own a motorhome. I just proved it. After working on installing the tank for six hours in the storage compartment between the rear tires yesterday, I was really tired and obviously not thinking clearly. So when I tried to hook up the sensor wires, I just couldn't figure out why they were always on. So late today I went at it again. This time I READ the monitor correctly, the gray tank which I had filled with water for leak test was the one I was reading as the black. Reading the correct tank and hooking up the wires resulted in every thing working as it should.
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Old 09-03-2010, 10:08 PM   #7
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Well, I often have said you don't have to pass an intelligence test to own a motorhome. I just proved it. After working on installing the tank for six hours in the storage compartment between the rear tires yesterday, I was really tired and obviously not thinking clearly. So when I tried to hook up the sensor wires, I just couldn't figure out why they were always on. So late today I went at it again. This time I READ the monitor correctly, the gray tank which I had filled with water for leak test was the one I was reading as the black. Reading the correct tank and hooking up the wires resulted in every thing working as it should.
I've got to take my hat off to you for confessing that! We all do it....or at least I do it too and just have to laugh...

Glad its a "simple fix"
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Old 09-04-2010, 04:00 AM   #8
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Originally Posted by scoutmstr View Post
Well, I often have said you don't have to pass an intelligence test to own a motorhome. I just proved it. After working on installing the tank for six hours in the storage compartment between the rear tires yesterday, I was really tired and obviously not thinking clearly. So when I tried to hook up the sensor wires, I just couldn't figure out why they were always on. So late today I went at it again. This time I READ the monitor correctly, the gray tank which I had filled with water for leak test was the one I was reading as the black. Reading the correct tank and hooking up the wires resulted in every thing working as it should.

If it ain't broke; don't fix it
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Old 09-04-2010, 09:25 AM   #9
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I have made worse mistakes!
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Old 09-04-2010, 09:30 AM   #10
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I have made worse mistakes!
Oh yeah! Mine would be getting distracted while the hose was running on my San i flush black tank fitting. I remembered it when... while sitting on my couch inside the coach... DW asked my why all of that dirty water was pouring off of our awning!

Move over Robin Williams.
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