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Old 04-21-2012
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Voltmeter wobbles?

I've got a puzzle I need some help figuring out.....my trusty old Adler Barbour cold machine stopped working recently, and in the course of trying to figure out what happened I used my voltmeter to check the voltage at the breaker. Putting the positive lead on the positive wire to the fridge at the breaker and the negative lead on the negative wire to the fridge at the ground bus I noticed that the needle on the voltmeter wobbled pretty steadily back and forth from 13 to 11 volts. When I moved the negative lead on the voltmeter to any other negative lead on the bus bar the oscillation stopped and the meter reads a steady 12 volts.

When I checked the voltage at the fridge between the positive and negative leads I got the same oscillation from 11 to 13 volts. (BTW - while I was there I also noticed that the thermostat wire had gotten pulled loose from it's connection on the fridge control box, and when I plugged it back in the fridge came back to life. Cold Beer again!). Connecting the thermostat wire didn't do anything to stop the wobble on thevoltmeter though.

The oscillation persists whether or not other DC breakers are on or off or whether the AC system is connected. Very puzzling to me but electrical isn't my strong suit. The wiring on the boat is 31 years old but seems to be generally in good shape.......

Any thoughts on what is causing the oscillation in voltage, and if this could be a sign of a serious problem?
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Old 04-21-2012
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Re: Voltmeter wobbles?

Loose connections would be my first guess. The module that runs the compressor is very sensitive to amps, volts, and resistance It could be a problem with the module itself too.
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Old 04-22-2012
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Re: Voltmeter wobbles?

I suspect that the oscillation is just due to the nature of the load at the 'fridge.
If it works, I wouldn't worry about it.
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Old 04-22-2012
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Re: Voltmeter wobbles?

Voltage drops like that can be introduced by loose connections, corrosion, and inadequate wire size. It could also be from using a wire gauge that is too small for the current and wire length, which induces a voltage drop across the entire length of wire.

Denise is probably right, though. Loose connections generally cause that kind of voltage fluctuation while inadequate wire size and corrosion typically introduce a constant voltage drop (but appears to fluctuate if the load varies, such as when a compressor turns on and off).

So what should you do? Clean and tighten all connections, inspect the entire length of wire for corrosion if you can (but at minimum the wire ends), and ensure that all wire is tinned and suitable for marine service.

Just out of curiosity, what is the current draw, approximate wire length, and wire size you have, preferably in AWG?

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Old 04-22-2012
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Re: Voltmeter wobbles?

I would go with a corroded connection. Assuming the wire size is correct. It is very easy for a thin layer of oxidation to form at any connection, any terminal block. A loose screw, or a bad crimp forms oxidation very quickly.

Start at the battery with full load, no load. If voltage is steady, go to the next connection, etc...
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Old 04-22-2012
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Re: Voltmeter wobbles?

Thanks for the input everybody....

I cleaned the terminals at each end of the positive and negative leads.....they didn't look too bad but I buffed them a bit anyway. The wires are #12 AWG, about 6 feet long, and the max load when the compressor is running is about 7 amps. The voltage fluctuation was still there with the terminals cleaned....the voltage fluctuation is regular and constant, like a metronome, oscillating back and forth between 11 and 13 volts.

I'm guessing it must have something to do with the action of the compressor or controle module?

The fridge seems to be working OK, though, and I'm not seeing anything out of the ordinary in any of the other circuits on the boat.

At least I was able to puzzle over this in the cockpit with a cold beer today!
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Re: Voltmeter wobbles?

I just reread your original post. You saw a difference between one grounding point and another! You have grounding issues. Try running a separate wire from the negative lead of the reefer to your gounding bus bar (i.e. the negative bus bar) and see if the voltage fluctuations go away.
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Old 04-23-2012
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Re: Voltmeter wobbles?

Andy, if this is a digital meter? I'm guessing the fridge has some component that is charging and discharging, probably a capacitor, or some other impedance factor is making the fridge look like a dead short for an instant. And the digital meter is capturing that and averaging it out as a low voltage from the high power drain.

But I'd second what Tom said, check the wiring all the way back to the battery. You can only get that behavior if something in the fridge, even for a short while, is not getting as much power as it wants to see.

If you can temporarily run heavy cable (i.e. battery jumpers) directly to the fridge to feed it gobs of power, that would make a fast easy way to test this. Or a call to Adler might get you a fast answer, that it is nothing to worry about.
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Old 04-24-2012
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Re: Voltmeter wobbles?

Thanks for the tips everybody....I'll check the condition of the ground bus and connections again, and since I have some leftover #6 from a previous project I'll try going directly to the fridge and see if that makes a difference.
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Old 04-24-2012
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Re: Voltmeter wobbles?

Thinking about the pulsing voltage, two things come to mind. The compressor is not getting enough current so the compressor stalls and kicks out, first taking current and then dropping out causing the voltage pulse. The second, which I think more likely, is the battery charger is regulating the voltage to the battery by varying the length of the pulse. On some battery chargers, you will see a yellow LED (light emitting diode) flick on then to another green LED back and forth. This is a common way for an intelligent charger to regulate the voltage. Although somewhat controversial, the idea is the pulsing type of intelligent charger can reduce sulfonation better than a constant voltage type of intelligent charger. Even if there is no ground fault or resistance in the wiring, the current from the charger is sufficient to cause the voltage pulse. Disconnect the shore power and see if the pulsing still exists. There may not be a problem in this case. If pulse still exists, consider compressor stalling.

However, in your situation, the voltage drops to 11 volts, which is lower than what the house battery would be capable of supplying, it should be above 12 volts. Take the volt meter and put it on a ten volt range or maybe 12 or 15 depending on the meter, place one probe on the house battery positive post, the larger post, and the other at the wire going to the frig. If you see a voltage reading of more than half a volt, you have a high resistance somewhere between the battery and the wire to the frig. Next thing is to narrow down the problem with a test from the battery to the post on the positive side of the circuit breaker, or the fuse, then across the fuse or circuit breaker. Surprising, many times there is a high resistance in these circuit protection devices so replace as needed, being sure to check for corrosion where the fuse of circuit breaker plugs into the power distribution panel. You should also test the ground doing the same thing by holding one probe on the negative battery post (smaller post) and the other end to the ground at the panel with the fuses or circuit breakers (the DC panel). It might be that the wiring is undersized. Some designs have as much as a 10% voltage allowance. Others are about 3% voltage drop. If you have no voltage drop across the terminals to wire connections, consider undersize wiring if you get a reading above half a volt. Sometimes the terminal is not crimped correctly to the wire and you get a significant voltage drop.

The reason you are seeing a constant voltage of 12 volts on some of the other grounds might be that you are measuring the voltage from the starting battery. Most marine battery chargers will have more than one output, one for the engine battery for starting and the other for the house battery, which is everything else. If the battery charger is setup correctly, you should be seeing about 13.4 volts on the other ground where the engine battery might be fully charged and therefore you see no pulse, otherwise you would also see the charging pulse. Most likely your engine battery is not hooked up the charger and has deteriorated from sulfation. Buy a hydrometer like this
Amazon.com: Customer Reviews: Dorman 9-1302 Hydrometer Amazon.com: Customer Reviews: Dorman 9-1302 Hydrometer
If you see little bubbles when you draw fluid into it, tap it a few times to get rid of the bubbles as it can influence the reading. If you buy the little cheap plastic hydrometer, the bubbles will not come loose from the plastic with tapping. The bubbles will come loose from glass. If the battery is more than half discharged, buy a new one as it is not possible to get a full charge into the battery, unless the discharge is recent, say within several months. If you are using an automotive battery charger, the boat is likely not isolated from the AC shore power and you could get electrolysis, a severe corrosion problem. In this case you could consider an isolation transformer which would allow AC power in the boat for other purposes besides an automotive battery charger

Last edited by LakeSuperiorGeezer; 04-24-2012 at 03:49 PM. Reason: Return ground also has voltage drop, lowered voltages
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