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Odd troubleshooting results: disobeys ohms law

  1. Sep 11, 2016 #1
    I have a DC powered pump, that has suddenly stopped working.

    During troubleshooting I came up with the following results, that I just cannot reconcile:

    Input wires, not connected to pump: 12v

    Input wires connected to pump: 0v

    Sounds like a short in the pump right? However, connecting the wires does NOT blow the fuse (though the fuse DID blow before I started troubleshooting). Also the fuse box, has fans and stuff, that I can always hear go on, when the pump’s load is applied, and I no longer hear them.

    Furthermore, an ammeter test, connected in series (along the hot line) with the pump, show ZERO amps flowing to the pump, when it’s connected.

    That’s what I just don’t get:

    How can it take that input voltage of 12V, and knock it down to zero without any current flow?

    Any thoughts on how these results might make sense?

    (The ohmmeter shows a resistance across the pump that is at least several Mega-ohms, when power is not connected)
  2. jcsd
  3. Sep 11, 2016 #2
    There's something else going on there. Probably an issue with connectivity. If there's no current there's no circuit meaning a fault somewhere. Sometimes things like that can be hard to find. It could be a problem with the motor windings where sometimes they short and sometimes they are open.

    Some power supplies are fault tolerant where if they detect an over-current condition they will shut down. It's possible that may be happening as well.
  4. Sep 11, 2016 #3
    Well @Glurth it's hard to diagnose simply by text but one thing is for sure , keep searching because I can assure you no laws are broken in your case.
    One of my friends , he was an electrician used to say that in electronics there are only 3 types of faults.
    1)There is a contact which shouldn't be there
    2)There is no contact where it needs to be
    3)Looking in the wrong place.

    If the power supply would supply it's output to a very low resistance load aka short circuit then the voltage could drop but surely there would be current as with no current there is no reason for the voltage to drop in the first place.So check you power supply , either it's smart and detects a fault or maybe it has already taken damage and doesn't function anymore.

    To be honest there aren't much way in which an electric motor can go wrong , either the windings loose their insulation and short circuit themselves resulting in a very low resistance at the input terminals or the winding/s burn open and then there is no resistance and the ends are open.
  5. Sep 11, 2016 #4
    Thanks for the feedback Craig!
    I don't understand how a fault in the windings could cause these odd symptoms of a short (0v) with no current(open circuit). Can you explain?
    Power supply cutoff: this makes a lot of sense, but I would have expected an over-amperage to blow the fuse. I guess I could test this by intentionally shorting the input voltage wires, and see if the fuse blows. This sounds unwise: perhaps you could recommend a different way to test this? Would it be equally unwise to simply connect my ammeter between these two wires, without a resistor? (Alas: I don't have any resistors to test with...)
  6. Sep 11, 2016 #5
    an ammeter is used in series with a load , it's not meant to be the load itself , i'm not sure I don't know how big of a resistance a typical ammeter has but I assume it's small unlike that of a voltmeter.
    The proper way to do this would be to use a low ohm resistor in series with your ammeter and then see what it reads.
    if your source voltage is 12v then take some common resistor , say 4.7ohm or about and test it.If you don't have a ready resistor you can use some wire coil as you only need low resistance since you are dealing with a low voltage.

    I don't know your power supply and it's capabillities but if it''s a car battery or anything alike shorting it's output intentionally is not a wise thing to do.Anyway even if it's a low power smart supply don't simply short it , it could damage the supply.maybe it's already damaged , take a resistor and measure see what it shows.
  7. Sep 11, 2016 #6
    oops @Salvador we must have posted at the same time:
    " I can assure you no laws are broken"
    hehe sorry for being so sensational in my title. I agree, I'm just missing something, like the power supply stuff you guys mentioned. I think that's where I'll start, just not sure yet how to safely test that.
    Hmm, is is possible the megaohms of resistance that I measured at the pumps inputs, is NOT what the power supply would detect when voltage is applied?
  8. Sep 11, 2016 #7
    for any practical electric motor measuring megaohms of winding resistance means only one of two things , either while measuring the probes or anything in between had a bad electrical contact or there is a bad possibly burnt contact somewhere in the motor , maybe the windings are gone , it could be a number of things but anyway it's not normal.
    especially for a 12v rated motor.
    megaohms would mean literally no current which means no magnetic field which means the motor doesn't run.

    Just curious what type of motor is it ? is it a brushed universal motor ? maybe a BLDC? is the 12 volts AC or DC ? what kind of application is this ?
  9. Sep 11, 2016 #8
    Guess I'll just have to go into town, without a shower, to pickup some resistors.
    We are living in an RV motor home.
    The motor is a fairly new seaflo water pump. It is powered by 12v DC, input on a pair of wires. I have no idea what the internal construction is.
    The power supply is a DC converter, that takes 120 AC power, and uses it to provide 12vDC power to components on demand, and charge the connected the 12v car battery.

    regarding the motor analysis: I agree that should lead to no current flow. Which brings me back to my confusion: no current-> no power supply cut-off, so the inputs SHOULD remain 12v DC when connected to the motor.

    I'll let you guys know the results of my tests with a resistor on the power supply. Many thanks!!
  10. Sep 11, 2016 #9
    Ok now that you said this , most definitely the power supply is a SMPS type and is either smart enough to shut itself down or has already blown itself because of the short circuit you said about.
    As for the motor who knows it could be either BLDC or a more classical brushed motor.Probably brushed anyways test out your power supply first and let us know.
  11. Sep 11, 2016 #10
    Issue was combination: power-supply AND pump.

    Unable to find a resistor, I WAS able to find a twelve volt device that could connect instead.
    This device failed to turn on when connected to the power supply. (Proving your intuition correct !)

    I moved my pump input input line to a different circuit, on the power supply, and connected a fuse.
    When I connected my test device it worked. (I guess this means that original circuit is dead now. Wonder why the fuse protection was insufficient.)

    I then connected the pump, but no luck. Poking around and jiggiling some "internal" wires however, got it to start. Unfortunately, sufficient water-pressure in the pipes is no longer sufficient to turn it back off automatically. Regardless, the point is, the pump was indeed suffering from both some kind of loose connection, and without turning off, a possible cause of high power draw.

    I've got it on a manual switch for now- and will activate the pump warranty on Monday.

    Many thanks for helping me figure this out!
  12. Sep 12, 2016 #11


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    A loose connection can act like a high resistance. So when you disconnect the pump the circuit is open and you see 12v. With the pump connected it drags the voltage down to 0v. The missing 12v is dropped across the loose connection. The fuse doesn't blow because the current is limited by there loose connection and anyway there is no short circuit in the pump. All makes sense.
  13. Sep 12, 2016 #12
    Why would 12v potential get dropped to 0 volts across a high resistance load? That doesn't make sense to me or am I missing something?
  14. Sep 12, 2016 #13
    You're not missing anything.

    As mentioned you never want to short a supply to see if it's working. The supply you are using is likely fault tolerant, but you don't want to find out by causing a fault on purpose. It depends on the fuse, but it is possible to over-load a power supply causing a shutdown without blowing the fuse if it's rated higher than the power supply.

    Likely the motor is having trouble, either an intermittent connection or a short or both. There's a couple different failure modes for motors. For both brushless and brushed motors they can overheat and burn up the windings. When windings fry they can fail short or open. For brushed motors they can burn up brushes where they can also fail short or open. As motors wear they can start running hotter and hotter until they break down. It's not uncommon for motors to draw excessive current when they become heavily worn which gets things hot enough to do further damage. It may not trip a fuse or put a power supply into shutdown until the condition gets severe.

    It's possible a failed motor can burn up both the supply and the windings or brushes if the motor has them. For brushless motors they additionally have a motor controller that can get taken out by a failed motor. If there's a controller inline with the motor then you would have to make sure you're measuring winding resistance after the controller or you will get a high reading like several meg.

    If you have another load handy, use that to check the power supply. If it checks then I would replace the motor and the controller if it has one. Brushless motors always have controllers. Brushed motors may or may not have a controller. At the least it has a pressure switch controlling it for a RV water pump. An over-current due to motor failure could damage that switch as well.
  15. Sep 12, 2016 #14


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    OK so the symptoms are..

    "Input wires, not connected to pump: 12v"
    "Input wires connected to pump: 0v"
    Pump works when connected to known good supply.

    In my post #11 this is the circuit I had in mind that could explain these symptoms.

    Poor connection.jpg

    You might be measuring 0V at the motor because 12V is being dropped across a loose connection that has high resistance but is not totally open circuit. When you measure without the motor present you get 12V because as I said the connection isn't totally open circuit.

    The loose/poor connection could be in the +ve supply or in the ground/return. Ground/return faults are quite common on vehicles.
  16. Sep 12, 2016 #15
    oh right i got it now , sure if you have a loose connection somewhere it's basically like the load isn't attached at all , electrically so measuring at the load side will return no voltage.
    I was just under the impression that you are speaking of a situation where there is a high resistance load attached and all electrical connections are ok and then you get no voltage which would seem odd.
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