• Troubleshooting Basic DC Electrical Problems

    The last few times you used your forward cabin lights you noticed they flickered and were not as bright as they should be. Today when you switched them on the circuit breaker popped. You reset it and it popped again! What now?

    Here is a word of caution before we begin. AC wires on boats use the following color convention, black = hot or current
    carrying, white = neutral and green = earth ground. DC wiring on older boats use red = hot or current carrying, black = ground. So be careful and ensure the black wire you are handling is indeed part of a DC circuit and not the hot part of an AC circuit. In newer model boats they have adopted yellow as the DC ground wire color code which means that if you are handling a black wire on these boats it is likely to be an AC hot wire.

    Troubleshooting DC electrical problems can be a simple affair if you understand the basic concepts and are familiar with the terminology. Before discussing troubleshooting methods lets define a few terms and look at some basic circuit examples. A common misconception is that the current flows from the positive to the negative in a DC circuit, however the reverse is true. But for our purposes we will stick with the commonly accepted direction of current flow, positive to negative. In a DC electrical circuit the current will flow if there is a closed loop or path to ground. What this means is that the electrons leave the positive terminal on the battery, travel through the circuit breaker, then down the positive wire through closed switches to the load then travel through the negative wire back to the negative ground terminal on the battery.

    Positive wire = red wire = hot wire
    Negative wire = black wire = ground wire

    Load
    Load refers to any item using the DC current such as a light bulb, electronic device, heater or pump motor etc. When DC current passes through a load, there will be a voltage drop from one side of the load to the other.

    Ampere (amp)
    A measure of the amount or rate of electrical current (electrons) flowing through a medium. The medium is normally the boat's wiring and in some cases the bilge water and water surrounding the boat. In measuring the rate of current flow, the term amp is used and the conventional symbol is I.

    Amp = rate of flow = I

    Volts
    For the electrons to flow we need a force to cause them to move. This force is caused by the difference in voltage between one side of a load versus the other side and is known as the electrical potential difference. It is also commonly referred to as potential or voltage drop. It is this difference in voltage that is the driving force, not the voltage itself, which pushes the current through the DC circuit. The unit of measurement used to describe this voltage drop is the Volt with the conventional symbol of E or V. We will use V to denote volts since it is more intuitive.

    Volts = force = V

    Voltmeters use simple math to compute voltage. The VOM measures the voltage on the black probe and subtracts that from the voltage measured on the red probe, the result is displayed as volts. In the diagram below you can see the Voltmeter measuring the difference between the two different sides of the load, 12.4 V - 0 V = 12.4 V.

    Attachment 981


    Resistance
    Resistance is the opposition to the flow of electrons. In a conductor (wire), it is commonly caused by the wire's composition, cross sectional area (size) and length. Resistance increases as wire sizes get smaller and lengths (runs) get longer. The standard unit for resistance is the ohm with the symbol omega (Ω) or R.

    Ohms = resistance = R

    This leads us to the Ohm's Law formula:

    V = IR (volts equals amperage times resistance)


    The equation will be discussed in more detail later but if any two values are known, the equation can be solved to find the third.


    To effectively troubleshoot DC electrical systems you need to learn to use a Volt/Ohmmeter (VOM). The VOM is a highly versatile combination meter that will perform many different functions based upon the switch settings. For now we are only interested in the Voltmeter and Ohmmeter functions. We aren't going to concentrate on the specifics of the VOM meter itself; instead we will discuss the concepts and what you can expect to see. For your specific meter, read the instructions very carefully or you can damage the meter.

    Voltmeter
    We will begin by looking at a healthy circuit containing a power source (battery), a switch (toggle) and the load (a light bulb). For our examples we will use a 12 volt circuit but the test procedures essentially remain the same for 24 volts.

    On a DC circuit you can essentially think of it as two sections, the positive 12 volt section (red) and the negative, ground potential 0 volt section (black). A VOM in the volts setting measures potential or difference, so with the leads of the meter placed as shown by meter A you can see 12.4 (positive) - 0 (negative) = 12.4 volts. Apply the same logic to meter B and you will note 12.4 - 12.4 = 0 volts. Note the other meter positions and readings.

    Attachment 982

    Now note the change in indications on meters C and D with the toggle switch selected off. With the switch off, the voltage downstream of the switch drops to ground potential or 0 volts.

    Attachment 983

    Ohmmeter
    The ohmmeter setting on your VOM is also an essential tool for troubleshooting. In the ohm's position the meter will measure the resistance in the circuit allowing you to check the continuity of the electrical components. The ohmmeter has an internal battery that sends a small electrical current through the circuit. The meter measures how much of this current is lost due to resistance and displays accordingly. Warning: power must be removed from the circuit before using the ohmeter or damage will result.

    Attachment 984

    In the diagram above meter C shows 0L or over-limit. This is the way some ohmmeters display infinity or unlimited resistance and means the wire is broken (no continuity) and the resistance value is above the meters limits. Meter D shows a typical resistance reading found when checking various pieces of equipment such as the filament in a light bulb.


    Circuit Examples


    Closed Circuit
    A circuit providing a complete "uninterrupted" path between the positive and negative terminals of the power source, in this case a battery.

    Attachment 985

    Open Circuit
    A circuit that does not provide an "uninterrupted" path between the positive and negative terminals of the power source. An open circuit may be intentional such as with the toggle switch selected OFF in the illustration below.

    Attachment 986

    Or the open circuit may be due to an electrical problem such as the broken wire shown in the next illustration.

    Attachment 987

    Short Circuit
    A short circuit is a circuit with an inadvertent direct connection between the positive and negative where the current bypasses the load, in this case the light. These can be commonly caused by a faulty switch. On older boats you may find wires with chafed insulation that are touching or a screw may have penetrated them as components have been added over the years.

    Attachment 988

    Short circuits normally trip circuit breakers and the load item does not work, for instance in the example above the light is not working.

    Ground Fault (Leak)
    Circuits with a ground fault allow a lower resistance path to ground than the actual ground wire. With a ground fault the current flowing through the positive does not equal the current flowing through the negative. Some of this current is escaping (leaking). This can be caused by a number of things such as chafed or poorly insulated wires. On boats, one of the most common causes of a ground fault is the poorly insulated bilge pump wires sitting immersed in water allowing the current to escape.

    Attachment 989

    It can be confusing at first to understand the difference between a short circuit versus a ground fault. With a short circuit the load does not work at all while an item with a ground fault may continue to work but not as well as it should. Ground faults are not likely to trip circuit breakers unless they are the GFCI type. Short circuits are a severe fire hazard since they may put an excessive load on a portion of the circuit not designed to handle it (smaller wire). For this reason if you find melted wiring thoroughly test all branches of the circuit and check any adjacent wiring.


    Troubleshooting With The Voltmeter

    General Troubleshooting

    If a piece of equipment fails to work and is not providing hints to help us find out why, then a more general troubleshooting procedure can be used. A piece of equipment needs two things for it to work, electrical current and a path for the current to follow both to and from the equipment.

    1. Ensure the power source is providing the correct voltage, set VOM to DC volts and test system voltage at the battery, also test for system voltage at the circuit breaker panel.

    2. Ensure fuses are not blown and all switches/breakers are in the ON position.

    3. Check for the proper voltage at the equipment as shown in the illustration below.


    Attachment 924

    4. If your VOM displays the system voltage then the equipment is faulty.

    5. If the VOM displays a voltage less than system voltage then the voltage loss may be due to a short circuit, a ground fault or excessive resistance. If the VOM shows zero volts at the equipment then you are likely to have an open circuit.

    6. Connect the meter to the positive lead at the equipment and the negative lead to a good ground as shown below, this may require running an extra wire back to the negative side at the power source.


    Attachment 925

    7. If the VOM shows system voltage then the problem is on the negative ground side of the circuit. If the VOM shows less than system voltage then the problem is on the positive hot side of the circuit.


    Attachment 927

    Troubleshooting With The Ohmmeter

    Troubleshooting An Open Circuit

    When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!

    1. Disconnect the circuit from the power source (important).

    2. Set the VOM to the ohms setting.

    3. If the fault was found to be on the positive side of the circuit, begin at the piece of equipment testing the circuit segments (wires, switches etc.) moving toward the circuit breaker panel as shown below. A zero or extremely low ohm reading indicates the segment is good. Any reading larger than this indicates the segment is bad as shown by meter B (over limit) in the illustration below.


    Attachment 926


    4. Note in this illustration we disconnected the equipment from the source downstream of the circuit breaker. To determine if the circuit breaker was faulty we would have to remove it and test across the poles with the breaker ON.

    5. If the fault was found to be on the negative side of the circuit, then the segments on the negative side would have to be checked in the same way.



    Troubleshooting A Short Circuit

    When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!

    1. Disconnect the circuit from its power source; this is normally done at the electrical panel by disconnecting both the positive and negative wires.

    2. Disconnect the load (light bulb, equipment, etc.)

    3. Set the VOM to ohms and place one lead on the positive wire and the other lead on the negative wire.


    In example 1 the meter shows 0L or over-limit which means infinite resistance which it should with the load removed (open circuit). This indicates the circuit is good since no current is finding its way back to the negative side and the short circuit is in the load itself.

    Attachment 990

    In example 2 the meter shows a very small ohm reading. This means that even though the load has been removed (light bulb) current is still finding its way back to the negative side of the circuit. In this case through a screw that has penetrated the wires. A reading of less than infinity or 0L (over-limit) generally means there is a problem with the wiring.

    Attachment 991

    Now that we have determined that we have a short circuit in the wiring, how do we find it?

    Isolating a Short Circuit
    When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!

    1. Ensure the positive cable is disconnected from the power source and reconnect the negative cable.

    2. Ensure all switches and circuit breakers are ON.

    3. Ensure the load is still disconnected and place one of the VOM leads on the positive side and the other lead on the negative side at the load as shown in the example below.


    Attachment 976

    If a short circuit is not present the meter should show an OL or over limit since the circuit should not be able to complete the path. In the example above the meter indicates a short is present (yellow path) with an indication of 0.62 ohms. A lower meter reading normally indicates a more severe short.

    To continue to isolate the location of the short:

    4. Leave the meter connected as shown and move from the load toward the power source to the next switch/breaker in the circuit and turn it OFF. This should break the circuit at that point.


    If after breaking the circuit, the meter indication does not change, then the short is between the meter and the switch/breaker as shown below (see yellow path).

    Attachment 978

    If after breaking the circuit the meter indication changes to OL over limit then the short is on the other side of the switch as shown below. In this instance the switch has broken the circuit.

    Attachment 979If the short is on the other side of the switch as shown above, turn the switch back ON and break the circuit further along, in this case turn OFF the breaker. If the meter indication does not change then the short is between the breaker and the switch. If the meter indication jumps to OL overlimit the short is somewhere prior to the breaker.

    Troubleshooting a Ground Fault

    When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!

    1. Individually switch off all equipment using the power switches on the equipment.

    2. Disconnect any solar panels if installed.

    3. Disconnect the positive battery cable.

    4. With the VOM on the DC volts setting, connect the red lead to the positive battery terminal and the black lead to the red battery cable.


    If the VOM shows any voltage you have a leak, see the illustration below!
    Attachment 992
    5. Now turn the battery switch OFF.


    If the VOM still displays a voltage
    then the leak is on the battery side of the switch. Normally the only thing wired prior to the battery switch is the bilge pumps. In the illustration below you can see the battery switch has been turned off and the VOM still indicates a voltage reading, a faulty connector leaking current into the bilge water is the guilty culprit. When the positive battery cable is reconnected to the positive battery terminal the current flows down the red wire to the bilge water, then into the bilge water where it finds its path to ground via the bad connector on the black ground wire then back to the battery negative terminal. The current could also find its path to ground through anything that is in the bilge water, prop shaft, thru-hull etc.

    Attachment 993

    If the VOM no longer displays a voltage indication with the battery switch selected OFF, then the leak is in the boat's wiring as shown in the following illustration. At some point on the boat past the battery switch the wiring is allowing positive current to find a path to ground. Once again this path to ground may or may not be through the negative wire.

    Attachment 994

    Note: If you are troubleshooting circuits with light fixtures you can get a false positive since the filament in some types of bulbs will allow the path to be completed to ground. The way to overcome this is to turn the light switch off but then you will not be testing the wiring downstream of the switch to the bulbs. The other method is to leave the light switch on but remove the bulbs.

    Attachment 995

    If the leak is in the boat's wiring accomplish the following steps to isolate the bad circuit.

    6. Turn the battery switch ON.

    7. On the DC panel pull one circuit breaker at a time. The VOM should continue to indicate the leaky voltage until you pull the circuit breaker of the bad circuit, then the VOM should indicate 0 volts.

    8. Once you find the leaky circuit you can use the troubleshooting procedures for Isolating a Short Circuit listed above to help find the leak. Since a leak is essentially a minor short circuit the ohmmeter readings will be higher.

    If you pull all of the circuit breakers and the VOM still indicates the leaky voltage, then either you still have something powered on the boat or the leak is in the wire from the battery switch to the DC panel.

    These troubleshooting procedures are just an example of several methods that can be used to troubleshoot DC electrical problems. Hopefully this article will help you become more proficient at tracking down problems.

    Download a pdf version of the article here!
    Jim61554, dsal and gvillalaz like this.

     

    Comments 19 Comments
    1. dsal's Avatar
      dsal -
      very helpful, thank you
    1. TimG's Avatar
      TimG -
      your welcome...
    1. Richard's Avatar
      Richard -
      I believe I have a ground fault problem on my boat (Campion 505BRi). All the systems/switches work, but after one week the battery is dead. I have 4 connectors on the positive terminal and 1 on the negative terminal. Of the 4, 1 is for the motor, one for the radio and float switch, one for the power trim and one for all the switches and guages on the dash. When I do the above test, I never get 0 volts. I get respecively 12.7v, 12.7v, .2v, continually oscillates between .2v and 1.2v (unless I remove the ignition fuse, after which it stays at .2v). Is this normal?
    1. TimG's Avatar
      TimG -
      Richard without a wiring diagram it will be hard for us to say, but if you are checking the voltage on the connectors that go to the positive to a ground you should get 12.x volts etc. It may be easier checking with an ammeter to see if you are getting current flow when you should not be.
    1. Richard's Avatar
      Richard -
      Quote Originally Posted by TimG View Post
      Richard without a wiring diagram it will be hard for us to say, but if you are checking the voltage on the connectors that go to the positive to a ground you should get 12.x volts etc. It may be easier checking with an ammeter to see if you are getting current flow when you should not be.
      Thanks TimG, but how do you connect an ammeter to do this?
    1. TimG's Avatar
      TimG -
      You would want to find a clamp-on ammeter similar to this one Sears.com and all you do is move the selector to amps and clamp it around the wire to take a reading. They come with instructions that will tell you all of the settings and how to use them.

      Just make sure you get one that has a useful range (i.e. can measure up to appx 30 amps). Most are limited below that value, and it will likely take some searching on Google to get an inexpensive one that will handle that much current.
    1. TimG's Avatar
      TimG -
      You would want to turn off everything on the boat, then place the loop around each of the 4 wires (one at a time) and look for an ammeter reading. You shouldn't have any reading if everything is off, if you do have a reading then you have isolated it down to that circuit leaking current. Hopefully the one black wire going to your battery goes to a common ground bus bar that all of your grounds return to on their path back to the battery, this will make it easier to troubleshoot.
    1. Richard's Avatar
      Richard -
      Thanks TimG! I don't have one of those clamp-on ammeters. (I just have a regular multi-meter that measures up to 10 amps but you can't clamp it around a wire.) I'll shop around and see what I can find. Thanks again!
    1. Richard's Avatar
      Richard -
      Quote Originally Posted by Richard View Post
      Thanks TimG! I don't have one of those clamp-on ammeters. (I just have a regular multi-meter that measures up to 10 amps but you can't clamp it around a wire.) I'll shop around and see what I can find. Thanks again!
      Hi TimG. I finally purchased a clamp-on ammeter! Of the four connectors on the positive terminal, the only one that shows a DC current is the connector for the Float Switch and Radio MEM (these are two wires into the same connector). As there is a separate fuse for each one of these on that line, I know that the one labeled Radio MEM is drawing about 0.3 Amps, quite consistently, although I do see it occassionally going to 0 amps and then back-up to 0.3amps. I'm not sure what "Radio MEM" means, but is this normal? Could a draw of 0.3 amps over 1-2 weeks be enough to kill the battery. Let me know.
      Thanks for your help!
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      That is the memory for the stereo in your boat (stored radio stations etc). It most likely goes to 0 occasionally because it's actually charging a small battery and the 'charger' briefly turns off because the battery is full. It is possible, though not very probable, that current could draw a battery down over several weeks. That wire also powers your bilge pump I believe ('float switch') and if it runs a few times a day that certainly could run the battery low.
    1. Richard's Avatar
      Richard -
      Quote Originally Posted by Cap'n Ray View Post
      That is the memory for the stereo in your boat (stored radio stations etc). It most likely goes to 0 occasionally because it's actually charging a small battery and the 'charger' briefly turns off because the battery is full. It is possible, though not very probable, that current could draw a battery down over several weeks. That wire also powers your bilge pump I believe ('float switch') and if it runs a few times a day that certainly could run the battery low.
      Thanks Cap'n Ray! Does that mean that the stero will still work, even if I remove this fuse? I never leave the boat in the water. I have a boathouse with an electric lift, so the boat is only in the water when I'm using it and I've never heard the bilge pump go on, unless I turn it on manually, hence I really don't think it is a problem with the float switch. I really need to know whether a draw of 0.3 amps for the Stero MEM is normal or not, because I don't see any other current draw on the other connections and I'm running out of ideas as to why the battery is dying!
    1. Roger's Avatar
      Roger -
      Quote Originally Posted by Richard View Post
      Thanks Cap'n Ray! Does that mean that the stero will still work, even if I remove this fuse?
      Your stereo will still work just fine, the only difference will be that when you turn it on the clock will need to be set (if you wish) and all of your radio station presets will be lost.

      I would also be surprised if the stereo memory is drawing enough current to drain your battery in a couple of weeks but it's good to eliminate all the possibilities
    1. TimG's Avatar
      TimG -
      I agree that the draw for the stereo memory is not the likely culprit and should not be drawing enough to pull the batteries down, bilge pumps will certainly do that though.
    1. Richard's Avatar
      Richard -
      Quote Originally Posted by TimG View Post
      I agree that the draw for the stereo memory is not the likely culprit and should not be drawing enough to pull the batteries down, bilge pumps will certainly do that though.
      Thanks guys, I appreciate your help! I will remove the fuse for the Stereo MEM and see if the battery continues to die or not (although I'm running out to time as the season is winding down!).

      One more question. Cap'n Ray indicated that there is a battery in the radio. Is it possible that the battery is nolonger able to hold a charge and is therefore charging continuously? Do you know if this battery can be replaced? Thanks!
    1. TimG's Avatar
      TimG -
      Richard that radio battery could be bad and I would think that it may be replaceable. I have a feeling that is not the cause but if you pull the fuse that will help rule it out or not.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Ultimately, if the radio is more than a few years old, you might be better to just replace the radio.
    1. Richard's Avatar
      Richard -
      Quote Originally Posted by Cap'n Ray View Post
      Ultimately, if the radio is more than a few years old, you might be better to just replace the radio.
      Thanks. The good news is that after 1 week the battery was fine. The bad new is that the radio doesn't work at all without the fuse installed. I'll check around to see if the battery can be replaced or if that fails for a radio which will fit in the same slot. Thanks again for all you help!
    1. Roger's Avatar
      Roger -
      Quote Originally Posted by Richard View Post
      Thanks. The good news is that after 1 week the battery was fine. The bad new is that the radio doesn't work at all without the fuse installed. I'll check around to see if the battery can be replaced or if that fails for a radio which will fit in the same slot. Thanks again for all you help!
      Sounds like your radio's main power wire and memory wire are on the same fuse. You could move the wire from before the main switch to after it. This way when you turn off the battery switch you will cut the power to the radio as well. Cost of repair = one crimp connector and a few minutes.
    1. Richard's Avatar
      Richard -
      I don't have a battery switch so I'm not sure it is that simple. I assume that this wire is providing 12VDC directly to the radio, but there must be another 12VDC coming from the dash, because the radio doesn't work unless the ignition is on or in the ACC mode. Hence I'm not sure how to trace this?
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