The sender doesn't have a polarity.  It is just a resistance.  Is this what
the back of your sender looks like?  Which wires do you have attached where?

Josh
On Apr 4, 2015 10:49 PM, "Robert Boyer via CnC-List" <cnc-list@cnc-list.com>
wrote:

> When you connect the sensing wire to ground, the needle goes positive all
> the way as it should do but it does not register the fuel level.
>
> I now believe that one gauge is working correctly because the needle moved
> slightly positive when energized--but the tank is completely empty.
>
> The other gauge is hooked up the same way but I think the wires inside the
> tank sender is reversed, which makes the needle go in the wrong direction.
> I remove the ground from the sender and reverse the sender polarity
> tomorrow.
>
> Wal, would I also have to reverse the power supply polarity on the gauge
> to make it work properly?
>
> Thanks!
>
> Bob
>
> Bob Boyer
> S/V Rainy Days / Annapolis MD
> 1983 C&C Landfall 38 - Hull #230
> email: dainyr...@icloud.com
> blog: dainyrays.blogspot.com
>
> "There is nothing--absolutely nothing--half so much worth doing as simply
> messing about in boats."  --Kenneth Grahame
>
> On Apr 4, 2015, at 10:40 PM, Marek Dziedzic via CnC-List <
> cnc-list@cnc-list.com> wrote:
>
> Josh,
>
> This is certainly not sailing related, but if i remember it correctly,
> they are all really ammeters (the current flowing through them creates the
> magnetic field that moves the needle).
>
> Marek
>
>
>
> Josh Muckley via CnC-List wrote:
>
>  All analog meters are just measuring the "voltage drop" across a
> resistance.  The key is to have the appropriate voltage input +12v.  If the
> +12v and the sensing line were reversed then the "voltage drop" would be
> "negative" driving the needle to the empty peg.
>
> Josh
> On Apr 4, 2015 7:08 PM, "Wally Bryant via CnC-List" <cnc-list@cnc-list.com>
> wrote:
>
> I thought they were Ohm meters. Anyway, I just Googled it to make sure,
> and see a ton of pages on testing marine fuel sender resistance.
>
> Wal
>
> you wrote:
>
> I agree with your diagnosis.  It's gotta be reversed polarity...of the
> sense and gnd.  Fundamentally the fuel gages are just voltage meters.  They
> respond proportionally to the voltage drop across the variable resistance
> in the sender.  I would disconnect the leads and use an ohm meter to
> measure and validate the resistance of the senders.  Switching the polarity
> of the sense and gnd monetarily probably won't break anything.  Before
> doing so validate that your wiring looks like the attached picture.
>
>
>
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