Re: [OMC-Boats] Volt Gauge wiring.

From: Lee Shuster (lib1) <lib1@...>
Date: Sun, 23 Aug 2009 12:46:16 -0600

The Ammeter vs Voltmeter question will get us into "heated" debates as
to which approach is better. I'm from the old school (yes, I had
vehicles with DC Generators, not AC alternators.)

Here's the basic distinction: A voltmeter and ammeter are to an
electrical system what a pressure gauge and a flow gauge are to an oil
pressure system: A voltmeter measures electrical pressure in volts; an
ammeter measures electrical flow in amps. "Pressure" or volts is
merely the potential and should reflect the battery's specific gravity
or "relative" ability to perform it's job. Flow is much more a matter
of real-time, what's happening at this moment in time.

OMC boats used direct-reading (non-shunted) ammeters. They were
usually 20 - 0 - 20 amp-rated and were place in series with the
current flowing to the helm. In the position, they do not read the
heavy current flow required to operate the engine starter motor or the
tilt motors which are sourced directly to the battery. Only their helm-
switched remote-control relays will reflect discharge current through
the ammeter, as originally installed at the factory.

IMHO, (again, old school experience talking her) ammeters are more
useful because they reflect REAL-TIME electrical activity, based
current or flow. This extends to monitoring the alternation and it's
diode/rectification and solid-state regulation. In other words, if you
pay close attention you can see the current discharge of a bilge pump,
a shifter coil, a horn, or essentially anything controlled from the
helm, as it occurs. I like to have immediate feedback on the status of
my alternator and a POSITIVE flow or charging rate on the ammeter is
the only reliable way to show that.

Voltmeters on the other hand, are good at overall, slower responding
battery potential or state of charge. Voltmeters essentially reflect
the chemical specific gravity or potential of the battery, like
pressure. This change tends to occur more slowly and it is extremely
dependent on the point of measurement as well as ambient temperature.
If you are obsessive-compulsive like me about battery care and
maintaining electrical connection integrity, then a voltmeter is
really not going to give you much short-term warning. (IE, A good
(fully-charged) marine battery should have enough reserve to get you
out and back over an extended day, without a correctly operating
charging system.) So again, IMHO, the voltmeter is more or less
useless on the boat, AND THIS is especially true of the typical 10 to
16 volt range analog/dial gauges, where it's nearly impossible to
distinguish between 11.8 and 12.4 volts. Trust me, if you have a 12
volt battery that only truly has a resting-state voltage of 11.5
volts, then it has 0% state of charge. A "fully-charged" battery will
show 12.70 to 12.80 volts and that's hard to distinguish with most
analog dial meters.

So why has the automotive-marine instrument industry shifted away from
ammeters to voltmeters? Cost-cutting and the fact they probably think
the public doesn't understand the difference. A decent DIGITAL
voltmeter needs to read out in tenths of a volt, perhaps hundredths to
be useful in my opinion. And the point at which you measure the
voltage is extremely important, due to wiring resistance/voltage
drops. You see the effect of that Bill because most likely you didn't
re-wire your voltmeter directly to the battery. Instead you are
reading the voltage under the helm. Try running separate positive and
negative wires from the voltmeter directly to the battery, but fuse-
protect this circuit. Your voltage is dropping due to the resistance
in where you connected your + and - voltmeter leads. You may have
other issues as well. (see below).

Bill, did you replace or test your Prestolite-OMC regulator? If you
simply threw in a replacement alternator without verification of the
status of the regular, you could very likely still have a no-charging
situation. I would also recommend that you trickle charge your battery
and take a direct reading of both the battery voltage and specific
gravity (if it's not a sealed AGM type). With a properly wired
voltmeter, measuring the battery voltage as supplied by a properly
charging alternator-regulator I would expect to see 14.10 volts at
1500 RPM (with no parasitic electrical loads other than the ignition
coil). All "12-volt" devices are designed to run most effectively
between 12.8 and 14.8 volts. Below that level everything has to work
harder and far less effectively. This effects everything from starting
to fuel efficiency and environmental impact.

BTW, if you want to replace an original OMC DIRECT reading (shuntless)
ammeter with a modern ammeter (using a shunt) it can be done. I use
both digital ammeter & voltmeters on my boat, but my digital voltmeter
is in the engine compartment and I only look at it before launching to
verify my batteries haven't lost their "Mojo" between outings. A nice
advantage of going digital (besides their greater readability/
accuracy) is I can have programmable threshold alarms on charging/
discharging rates, as well as low/high voltages.

But frankly most people can tell by the way their starter motor sounds
or their tilt motor reacts whether their battery is fully charged.
(Assuming everything electrical is totally clean and corrosion-free,
rarely the case on any 40+ year-old boat with it's original wiring in-

So for my money (analog) voltmeters are fairly useless. Call me old
school -- but voltmeters are a total cop-out. OMC engineers (yet
again) had it right when they included ammeters as standard equipment
on all their boats. The marine industry seriously compromised when
they gradually moved over to voltmeters as replacements for ammeters.

Further recommended reading:


On Aug 23, 2009, at 10:32 AM, BLDFW wrote:

> Had another test run yesterday with disappointing results. I'm
> still having performance issues which I will list those out in an
> email later this afternoon
> In the meantime, I have a question related to my new alternator and
> the new Volt gauge on the dash. As part of my restoration, I
> replaced the original gauge set with an expanded set from Faria.
> Nice looking, white background, yada, yada, yada.... The original
> dash had an AMP gauge, the new one a Volt gauge.
> On a car, the Volt gauge shows charging and discharging. When
> charging, it shows clearly over 12v and when discharging, it shows
> clearly below 12v. My new volt gauge shows consistently
> 11-11.5volts. If I hit a switch on that dash it will flicker
> downward but other than that, it doesn't show more than 11.5v.
> Some restart problems after being out on the lake for a while
> suggested the alternator was not charging up to par so I had it
> checked and was advised it was bad. With Lee's help, I found a
> source for a new replacement alternator and installed that. The
> results....the volt gauge still shows a flat 11-11.5volts (gee, was
> the original alternator really bad as told??). That makes me wonder
> if I have the gauge wired wrong and it's just showing the level of
> the battery as opposed to what the alternator is doing.
> I've looked at the wiring chart that Lee provided which documents an
> AMP meter setup. Does anyone have a schematic reflecting a volt
> gauge or can someone enlighten me as to what the wiring should be
> like for one? Maybe I don't have it wired properly.
> Thanks.
> -Bill
> Dallas, TX
> 1970 Evinrude Explorer - 155 Buick V6 - OMC Sterndrive
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Received on Sunday, 23 August 2009

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