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.

Josh Muckley
S/V Sea Hawk
1989 C&C 37+
Solomons, MD
I'm sharing an electrical problem right now in the hope that someone here
on this list has had a similar problem and solved it...

Over the winter I added an auxiliary 19-gallon diesel tank under the nav
station seat.  I decided to buy 2 new fuel gauges, one for the new
auxiliary fuel tank and the other a replacement of a 30-year old fuel gauge
for the primary diesel tank.  Both (Sierra-brand) gauges are mounted in the
cockpit coaming on the port side with nice labels.

My 30-year old fuel gauge was working fine but it was mounted in the engine
panel and didn't match the color and style of the other gauges--this is
primarily why I replaced it.

I made a nice teak pad (located inside near the gauges) on which I mounted
terminals and bus bars to properly organize the new wiring for all the
gauges.  I completely wired up the new fuel gauges in accordance with the
instructions.  (I rewired so that I could get rid of a lot of old un-tinned
wire.)

When I turned on the ignition to check the gauges function, the needles on
both gauges moved to below empty.  This usually means an open circuit on
the sender side. I've thoroughly checked all the wiring and the gauges and
I can't find any problem with the wiring or the gauges.

I've checked the resistance across the sender on the new auxiliary tank and
it measures 230 ohms--since that tank is empty, the resistance sounds about
right.  The primary tank's sender is more difficult to access, so I
couldn't measure it.

Last night, I checked the tank's grounding by running some extra temporary
ground wires--no difference.

What's weird is that the problem is affecting both gauges the same way.
Any ideas as to what the problem may be?  Its almost like the polarity of
the power input is reversed, but it isn't.

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

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