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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.
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.
Nailed it. Turns out the other circuits on this particular panel also didn't work; I mis-remembered that I've actually only use the other panel lately. Forgot to take my phone in the lazarette with me, but sure enough, the flimsy, unprotected automotive spade terminal attached to the flimsy, poorly soldered bus wire on the panel was corroded and virtually all but detached. It crumbled as soon as I touched it.
Message received, boat: you want your 42-year old electrical system updated.
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!)
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.
Try putting the voltmeter probes directly on the lead posts of the battery terminals and then have someone flip the pump switch. If the battery voltage nosedives, you've found the problem.
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.
IR - voltage drop increases as the current. Bad connections don't usually trip breakers - it just generates heat at the bad connection. I²R - heat increases as the square of the current.
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.
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
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.
Had this problem 2x now. First time it was an internally corroded wire (when cut open a large section had turned green and black almost like tar) which was a pain to find but after a "screw this I'll just start pulling wire out" moment I found it. Original wire on my boat, can't blame it for dying after nearly 40yrs.
This year however I blamed my brand new tiller pilot installation for the same issue. I thought I had a short or at least a bleed in it somewhere. Turned out both my deep cycle batteries finally kicked the bucket. Tell tale signs were there, batteries were "fully charged" as per my shore power charger, after 30mins would drop to 0-9v and everything would shut down, after only 5mins of being plugged into the charger they would be "full" again. 300$ to replace. Luckily not too bad on a small boat.
Anything else running funky other than the pump? Have you tried powering it up while plugged into shore power?
Best was to isolate the ground is to pull a temporary new ground wire to the pump, If everything's hunky dory then its the ground path. If that doesnt work take a new Pos wire to it. Make sure its fused.
If other substantial loads run OK (e.g. bilge pump, starter), it's probably not the battery itself. Bad connection(s) somewhere along the way most likely.
Didn't get a chance to test it yesterday as I'd hoped, but just a little more info: I'm inclined to think it's an issue with the panel the pump and tillerpilot are connected to. I have 2 6-switch fuse panels. Circuits on one of them work fine (cabin lights, nav lights, depth, radio, accessory sockets). Other panel has the pump and tillerpilot, as well as anchor/strobe/tricolor and foredeck ligths. Had to leave before it was dark enough to see if those lights are working, which might confirm my panel wiring theory.
Boat was on the shorepower charger throughout all this testing, so I assume a battery problem -- even if there is one -- isn't the culprit. Hopefully will make it to the boat tonight.
If old syle fuses corrode emery cloth both the ends and replace in scraped cleaned fittings ,.Crud under screws need cleaning too. Think like an electron, if you can't get though, you don't..
I come across this far too frequently. There is a bad, lose or corroded connection somewhere upstream of your pump.
Three weeks ago I had a commercial fisherman who wasted an entire day before calling me. Probably cost him a few thousands dollars. The switch for his two plotters would read 12V until he tried to boot either plotter, then nothing.
All he kept saying was "But I have good voltage." to which I responded "You have good voltage with ZERO current."..
Testing the open circuit voltage uses no current so there is no "load" on the wires or switch or bad connection, so he saw battery voltage. Even a horribly corroded connection can pass a voltage test, but ti does not mean it will pass much current.
His assumption was that if you see battery voltage the switch is working as it should. The minute he tried to boot either plotter the corroded switch could not pass enough current through the corroded contacts to boot the plotters.
These guys get everything covered in rotten fish guts then wash everything down with salt water and call it clean, hence the 8 month old dead corroded auto parts store switch. (D'oh..)
A bad or high resistance connection can come in many forms, a loose screw, corrosion, loose connector, incorrect lug stacking, lose friction fit connectors (bullets or Fast-On's), or just a few strands of copper left passing the current.
In far too many instances we start boat fires with high resistance connections provided they can still pass enough current to get the load working...
EDIT: Just finished reading the thread and see you solved the issue. This is great! Avoid friction fit terminals when ever you can. If you ever remove or unplug one it is best to replace it because they are only good for a few plug and unplug cycles before they loose their clamping pressure. As a marine electrician one of my dreams would be to have them verboten under the safety standards but sadly that will never happen....:wink
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