12.4V DC open but drops to near 0 when load applied - SailNet Community
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12.4V DC open but drops to near 0 when load applied

Title says it all. I'm replacing my broken freshwater pump (now realizing the breakage was probably related to this until now undiscovered electrical problem). I checked the open voltage at the supply wire before splicing it to the pump's wires, and it reads 12.4V. So far, so good.
So I butt connect and shrink wrap the pump to the wiring, screw it to the bulkhead, and go flip the switch with great expectations of hearing it prime itself. But nothing. I know the pump works because I tested it on my car battery and bypassed the pressure switch. So maybe the fuse on the panel just blew? Nope, good there. Snip the wires and check again. Still 12.4V at the wire, no change. Huh?
So I twist the pump and supply wires together, go flip the switch, and check the voltage across the pump: 0.5V, or something like that. A-ha! Wait, what? I mean, even if the pump suddenly developed a short, at least the fuse at the panel would blow, right?

Ideas, anyone? I'm not sure what to look for or test when I go back to the boat and poke around behind my electrical panel. I mean, if there's 12V at the supply point I don't expect to see anything awry at the panel. But now I realize my recently defunct tiller pilot circuit is probably suffering the same fate...and those two switches are neighbors on the fuse panel. 12.4V at the tiller pilot's cockpit receptacle, but zero response when I plug the pilot in. But other circuits I've been using on the panel are fine (cabin and nav lights, radio, etc.), so it shouldn't be a panel problem. Hmmm.
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Re: 12.4V DC open but drops to near 0 when load applied

Sounds like the supply wire - either positive or negative - has a bad connection, probably at the panel. It only takes one strand of wire connected for the voltage to pass, but it will not carry the required current for the pump to run.
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Re: 12.4V DC open but drops to near 0 when load applied

High resistance between battery and the pump.

Could be the entire wire suffering corrosion inside the insulation.
Could be corrosion at a connection.
Could be as above, a wire that is nearly broken at a connection (or in the middle).

We are assuming that other loads are not having the issue and thus proving that the battery itself isn't dropping to near 0 volts under load, which is possible for a bad battery or a battery that needs water (distilled only!)
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Re: 12.4V DC open but drops to near 0 when load applied

Sounds more like a dead battery. If there was that high a resistance in the conductor, the fuse should blow (if there is one) or if it's a 20amp fuse on a 5amp pump or that last strand should melt.
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Re: 12.4V DC open but drops to near 0 when load applied

Wiring issue, probably a ground connection if the wire itself is good Try checking 12 volts with an old fashioned car head light.
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Re: 12.4V DC open but drops to near 0 when load applied

Had this exact problem. Turned out to be corrosion under the connection to the ground bus. Both the pump and the main feed from the battery. Can't figure the concept, but it would show good voltage, until there was a draw.


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Re: 12.4V DC open but drops to near 0 when load applied

Start with the battery, checking for full voltage.
Then, check ALL the 'ground' (NEGATIVE) wires & connections from the batt to 'the device' — for high resistance.
Then, check all the POSITIVE wires & connections from the batt to 'the device' — for high resistance.

You'll need a good digital volts/ohm meter.
Better - once you check for high resistance & if you have a quality 'clamping' volt/ohm/amp meter, also re-check for unduly high 'amperage' in each wire and across each connector.
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Re: 12.4V DC open but drops to near 0 when load applied

Poor grounds can be confusing. Had an old Plymouth. started and ran well but beep the horn and engine stopped. Battery terminal would pass small amps and big amps but horn was just the right draw to sizzle
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Re: 12.4V DC open but drops to near 0 when load applied

Quote:
Originally Posted by Minnewaska View Post
Can't figure the concept, but it would show good voltage, until there was a draw.
IR - voltage drop increases as the current. Bad connections don't usually trip breakers - it just generates heat at the bad connection. IČR - heat increases as the square of the current.

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post #10 of 15 Old 21 Hours Ago
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Re: 12.4V DC open but drops to near 0 when load applied

Ohms law, E = IR, where E is the voltage drop, I is current and R is resistance.
You have a high resistance connection some where in the circuit.
Disconnect the battery, both the positive and negative leads.
Measure the resistance from the positive wire that WAS connected to the battery to the positive side of the pump. It should be a few ohms because of the fuse and maybe the switch. Any more than a few ohms start measuring back towards the battery connection. Measure at the fuse, switch, ect.
If the above measurement is around a few ohms, measure from the negative wire that WAS connected to the battery to the negative side of the pump. Again it should be a few ohms, any more than that measure back towards the battery until you find the bad connection or corroded wire.
More than likely it is a corroded connection, a bad switch or corroded wire(s).

The other way of finding the problem is to leave the battery connected, turn on the pump, attach the negative lead of the volt meter to the negative battery terminal then start measuring the voltage along the circuit. Measure positive battery terminal, should be the same as when the pump is off. If the voltage across the battery is good, move on down the circuit (wires) until you get to zero. Such as you measure the battery side of the fuse and it is 12.4 volts, the pump side is zero, all the voltage is dropping across the fuse so its either bad wire connections to the fuse, or the fuse it self is bad but not blown.
Note I suggest the negative lead of the meter to be connected to the battery, not the ground of the engine or pump.

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Last edited by Skipper Jer; 21 Hours Ago at 12:40 PM.
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