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Broken holding tank sending unit bolt

Discussion in 'Viking Yacht' started by steveg_nh, Aug 28, 2018.

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  1. steveg_nh

    steveg_nh Member

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    Looking for some ideas. I noticed, when working on my holding tank hoses this weekend (1989 35 convertible), that the reason the "Do Not Flush" light never works, is because one of the two wires on the sending unit on top of the fiberglass holding tank is broken right off. The stud is completely gone. The sender may very well be bad too, but this certainly is broken, and not sure of the best way to replace the sender either....

    Looking closely, it looks like there is a metal plate on the top of the tank that "might" be able to be removed by removing 6-8 very old Phillips screws holding it to the tank. This may or may not be easy, as two of the screws are a bit obscured. This removes the whole cover plate. Not sure I want to do that anyway.

    Any thoughts on how to fix this? I assume these two bolt heads are the sender, and it's mounted via them to this plate. I can't just remove the sender with it's own mounting screws. Here's a pic.

    Thanks

    ht.JPG
  2. justold

    justold Member

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    There is no sending unit . What you see is the top of a probe . You can make your own . I drilled and tapped the plate installed a 1/8 pipe to 1/4 compression adaptor a short piece of 1/4 " plastic tube and an 8 " length of 1/8 " stainless steal welding wire for the probe . Just install the plastic tube in the compression fitting then slide the ss wire through the plastic tube and set it to your desired level tighten the gland nut connect the wire and you are good to go . Just a thought in case you didn't notice there are two probes .
  3. MBevins

    MBevins Senior Member

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    What I did was purchased a new sending unit , installed it in the top of the tank. Think holesaw. Then I used the wires from the "do not flush" light to supply the signal to the new gauge I installed in the master head.
    Job done, real time reading.
  4. steveg_nh

    steveg_nh Member

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    Thanks guys. I'm not following the probe setup JustOld. How did it work? You mentioned two probes, I assume the other is the other bolt I can see?

    I would like a little more than "do not flush" so installing an actual sending unit would be good, but how does it hold up being inside all that, well, "junk?" I like the idea of the exterior systems, but not sure the tank is exposed enough.

    Thanks guys.
  5. MBevins

    MBevins Senior Member

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    This is an item number for a sender on eBay. 131928908996
    It's just an example, that one has a threaded mount but they are available with 5 screws as well.
  6. steveg_nh

    steveg_nh Member

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    Thank you. I need to learn how senders and gauges all work. So you buy something like that, and then just make sure you buy a gauge that matches the ohm range? Some type of round holding tank level indicator, vs a digital display I'm seeing out there. How easy is it to cut a hole in the top of the tank? Looks like pretty thick metal.
  7. MBevins

    MBevins Senior Member

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    Viking makes their tanks out of fiberglass so putting the hole in it is pretty easy. I actually cut the hole a little over sized then I made an aluminum plate with the sending unit pattern c/w the tapped holes. I then through bolted the plate down. You could just fiberglass it in place as well.
    As for the wiring, you'll notice the sending unit I showed you has 2 wires , there are 2 wires coming from the old indicater method, they go back to the light in the MAIN stateroom head.
    You'll find your 12v at the light as well. For actual wiring most senders and gauges supply the diagram. If not I can get it to you.
    For the gauge, yes you just match the ohm range to the sender, they're all pretty standard. I couldn't find a simple round gauge that had the right descripter on it so I actually just used a white fuel gauge, opened it up and removed/covered the fuel letters and left it blank, based on location it's self evident as to what it is.
    PM me if you need more details
  8. steveg_nh

    steveg_nh Member

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    Looking closer at my tank top as I've been replacing the hoses, it looks like the top plate is actually plastic. I'm not sure though. So I understand what you're saying about replacing the current setup with an actual sender. I'd like to understand how the probe works too though. I have a lot of projects going on, and if I can easily fix the broken lead to just get the light working again for now, I'd be happy.
  9. justold

    justold Member

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    If you have your OE manual from Viking you will find a wiring diagram that shows an LED a diode a resistor a switch and two probes in series , when the liquid contacts the probes it completes the circuit and the led lights . To check the system connect the two probes .
  10. steveg_nh

    steveg_nh Member

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    Thank you. I do have the wiring diagrams. Didn't click what this was doing. Simple enough circuit, using the fluid as the conductor. kind of gross, but I guess it works and no moving parts. Now I get it. So if I can make contact with the broken stud, I may be able to fix what's there for now...
  11. MBevins

    MBevins Senior Member

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    Yes
  12. steveg_nh

    steveg_nh Member

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    awesome thank you!