Re: [OMC-Boats] Volt Gauge wiring.

From: Andy Perakes <aperakes@...>
Date: Mon, 24 Aug 2009 01:13:24 -0400

I'm in complete agreement with Lee here...and I've had relatively little experience with DC generators. DC voltage drop can be huge, even over just a few feet of what looks like large enough wiring. Even though I used 1" multi-strand speaker wire (industrial grade, used for wiring theaters and concert halls), I still saw a noticeable voltage drop on my sailboat when I added an electric start outboard (mid-ship mounted batteries). I ended up having to double what I thought was needed to get satisfactory performance.

Confirming the "helm voltage" theory is easy enough to check, but I'd still start with confirming the battery is good -- disconnect it, measure the voltage (with a multi-meter at the battery), charge it (use a smart charger if you have one so you know its state is near 100%), then measure the voltage again. Once you confirm that, hook it up in the vehicle and measure the voltage at the battery again (engine off). A different "connected" vs. "disconnect" voltage may (but not always) tell if you have any stray (intended or unintended) "leaks" anywhere. If that checks out, proceed to start the engine and again measure the voltage at the battery. If the running voltage isn't higher off-idle, then you most likely have a charging problem. Of course you can also confirm the voltage drop is due to wiring resistance by measuring it at different points along the trail to the gauge, if accessible.

As Lee says, nothing beats an ammeter for telling you the amplitude and direction of power flow real time, but you can generally use voltage as an indicator to aid diagnostics. One note of caution (although I highly doubt it applies to our old boats unless you added a bunch of electrical accessories), I have encountered cars that actually drain the battery (by design, not "malfunction") at idle. This is especially true of cars with many accessories like heated seats, heated mirrors, defrosters, etc. Add all those loads up and they can exceed the alternator output at idle and thus draw off the battery. Since most people don't idle for hours with everything 'on', its not necessarily a problem, but it can mess with an unaware person trying to diagnose an electrical problem. Interestingly, hybrid vehicles are bringing a trend towards much greater instrumentation. At a minimum, there's usually a gauge to tell you whether you're charging or draining the battery, but often there's much more such as operating mode, battery state of charge, etc. And the new reconfigurable displays allow you to customize them to provide about as much or as little information as you could ever want. (Finally, a solution to please both techies and "I'd rather not knowies!")

Since we're on the topic of electricity, I measured the resistance of my forward and reverse clutch coils earlier today. Both read dead-on 6 ohms so its almost certain I'm dealing with a spring tab failure. I drained the lower gearcase lube today, but I don't anticipation starting the disassembly until after Labor Day.


  ----- Original Message -----
  From: Lee Shuster (lib1)
  To: Evinrude & Johnson Boats of the 1960's and 70's
  Sent: Sunday, August 23, 2009 2:46 PM
  Subject: Re: [OMC-Boats] Volt Gauge wiring.

  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-tact).

  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.


          Dallas, TX
          1970 Evinrude Explorer - 155 Buick V6 - OMC Sterndrive

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Received on Monday, 24 August 2009

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