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  #1  
Old 07-01-2008
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DC Ammeter Question

The DC ammeter on our Cabo Rico 34 pegs the needle sometimes when we turn the battery switch on. Switching the DC switches for the lights, fans, etc. will cause it to drop to more normal current draws; there is no rhyme or reason to which switch causes the drop or return to a pegged condition. Is the ammeter bad or is something else going on?
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Old 07-01-2008
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You meant switching ON your fixture will cause a drop ? Probably a rusted needle mechanism, and drop might be caused by switching mechanical vibration. Find a multimeter and select volts or milivolts, then check the two conection on the back of your ammeter (there you'll see volts) and see how it goes. Should see a linear increase ....
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Old 07-02-2008
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artbyjody is just really nice artbyjody is just really nice artbyjody is just really nice artbyjody is just really nice
loose grounds at the switchboard or the batterie(s) will cause the same symptom....tighten all known grounds especially at the switcher... been there done that...

Also, lot of times you can wire like those blue sea systems panels wrong. by default they have the (-) ground and shipboard ground (unless you connect the two ground buses to each other). Multiple panels, it is easy to wire the actual neg ground to the shipboard ground - which leaves you with effectively a floating ground. Which will produce the same exact symptom as a loose ground and the same symptom. Because if you have lights on a (-) to battery ground) but the panel gets its ground from a common ground connection on another panel - the (1) battery ground will prevail and thus will result in less voltage (actually true voltage) because floating grounds means there really is no ground so the energy is just going through the system and thus you see higher than expected voltage...

I state that from experience btw - and those symptoms were exactly what I had and then I realized tracing things through - there was a missing (- to battery) ground on one of the panels ...
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Last edited by artbyjody; 07-02-2008 at 12:25 AM.
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Old 07-02-2008
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Clarification

I failed to mention the needle pegs when a light switch, fan switch, etc. is turned "on". I'll investigate the ground questions and let the group know what I find. thanks for the help!
Charlie
CR 34 Arriba
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Old 07-02-2008
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DC Ampmeters usually show both + and - flow from the alternator to the batteries.

Is is pegging to + or - ? Is the engine running?

I would investigate since the purpose of the meter is to tell you if something is going wrong.
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Old 07-02-2008
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Wink meters and troubleshooting...

DC ammeters that are rated to carry more than about thirty amps have to have a 'shunt' resistor. Given the fact that all of the current flowing to or from the battery needs to be measured, this shunt resistor is usually very close to the battery physically.

Take a look at your battery's hot leads. They're probably coming from a 1/2/both switch. Follow the common lead from the switch and see if it leads to some kind of metallic looking block with the large battery cable from the switch attached at one end, and another large battery cable wandering off, probably toward your circuit breaker panel, starter motor, whatever. The big metallic block (especially if it looks like a heat sink with fins, etc,) is your meter shunt.

What it actually is can be described as a very small value, very high current resistor. Normally it will be less than one ohm if you have a meter. On either end of the shunt resistor, you'll probably find a small wire, and these go off to the meter. What they are doing is reading a very small 'voltage drop' across this resistor. This, in turn, can be easily read directly by a meter, and the meter is calibrated to read in amps, rather than voltage.

In the case of boats, corrosion is your enemy. Bouncing meter needles is usually an indication of bad connections somewhere. I'd start with the batteries. Remove and use an old fashioned battery terminal wire brush to clean the posts and the battery connectors, then reconnect everything, making sure it's good and tight. Finally, put the anti-corrosion stuff of your choice on the terminals. (I like Corrosion Block).

After checking the battery ends, move to the other ends of the heavy cables. Make sure that there is no rust, slimy green stuff or whatever, growing on the main ground, which should be on your engine block. In all likelihood, there will be a bunch of wires there leading off to your bonding system, electronics (radios like REALLY good grounds!), etc. Clean all of these connectors up, reinstall and tight them, and slather them up with anti-corrosion stuff.

Next stop is the alternator. Make sure everything is snug. Look for connectors that have melted or even gotten really warm. Factory wiring is almost always undersized, or at best, the minimum they can get away with. Take a hard look at the output of the alternator and make sure the wire is up to the task of charging a nearly-dead battery at whatever current the alternator can put out. Bigger is always better with wire, no matter what the yard or factory people tell you.

Check the leads going to the main DC circuit breaker and make sure they're tight. Moving the wiring harness to look at stuff, vibration, and our old friend corrosion all conspire to make those leads a high-resistance source.

Finally, you can tighten up the leads on the meter itself, plus wherever they seem to come from.

There are ways to test the meter, but in most cases, there's no way of knowing exactly how much voltage it takes to get full deflection on the needle. Sometimes it's as simple as connecting a AA battery across it. Other times that would blow the meter up. Without the right test equipment, it's easy to blow something up.

Last, but not least, if you do happen to have a good digital meter, you can disconnect the common lead from the battery switch and put the meter into the 'AMPS or MA' load. In this case, the meter goes into the circuit IN SERIES with the cable. Make sure you have a fairly small load, like a VHF or something, and nothing else turned on and see if the digital volt meter's reading agrees with the boat's built-in meter. (Plus or minus about ten percent, anyway.) Remember that your DVM will have a limited current capacity--usually 3 amps, so don't try to fire the engine up, or something like that. Just a small load will do the job.

If the DVM and the boat's meter agree, but after putting everything back like it was, the meter still pegs, you've got a problem. If you have a built-in battery charger that works from shore power, test it. I had two of them go bad. One of them went berserk and hit my brand new Rolls batteries with 51 volts and nearly twenty-five amps of AC current when one of the main bridge rectifier diodes went out. The other one was harder to find: it never dropped from it's mid-level charge of 13.6 volts, and slowly cooked the water out of the batteries.

Which brings me to one last comment: if you have an old battery charger, bite the bullet and replace it. The old Consta-Volt systems that a lot of older boats have are simply junk. If you're using an automotive charger that is over ten years old, relegate it to the garage and get a new charger. The same is true of very old voltage regulators. If it's over ten years old, you might consider getting a new one with at least two-stage charging abilities built into it. It'll keep you from frying your batteries. Spending the bucks on a new charger will pay in the long run, because you won't be boiling the batteries constantly.

Good luck!

Cap'n Gary
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Old 07-02-2008
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Good 4th post !! Welcome Gary ....
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Old 07-05-2008
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Most current meters on boats are voltage meters that can read very low voltage drops across a low ohm shunt. The one that I installed was a 75milivolt at 300 Amps. There are other combinations as well. The digital display was verified for accuracy prior to installation. You can hook up a volt meter across the shunt and check the response of the analog meter to the volt meter to see if there really is a large surge.
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