Electrical troubleshooting for the self-reliant cruiser, Part 2Jun 12, 2008
I have already discussed digital multimeters (DMMs) and the important role they play in electrical troubleshooting. While that is an important subject, let’s step back for a moment and detail a few basic electrical concepts that you may find useful when routing out your own electrical gremlins.
Volts, both AC and DC, amperes and ohms, are all electrical units of measure. While I could go into the scientific and technical descriptions, or use the popular water analogy to help describe these terms, I will opt instead to offer a more practical and, I hope, understandable approach.
There are several types of voltage that may be found aboard a voyaging boat. Battery (or battery bank) voltage is invariably either 12 volts or 24 volts and sometimes 32 volts and it is always direct current (DC). This type of voltage powers everything from your engine’s starters to cabin lights and electronics. Any voltage, whether it’s AC or DC, below about 45 volts is typically considered nonlethal. There are of course exceptions to every rule, so don’t take this as an absolute, always show electricity the respect it deserves. As a rule of thumb, however, you don’t have to worry about being electrocuted by battery voltages (unless of course they are powering an inverter, which does produce shore power-like and thus lethal voltages).
Shore power voltage is typically associated with generators, inverters and, of course, shore power cables. It is usually found aboard a boat in two voltages, 120 and/or 240, and it is always (AC) alternating current. These voltages, which are lethal, are used to power such appliances as air conditioning compressors, battery chargers, microwave ovens, electric galley ranges and some refrigeration systems. AC shore power voltages are the same voltages found in your home or business, and unless you have a thorough working knowledge of how they operate and their potential for injuring or killing people, troubleshooting these circuits is probably best left to a professional. If you are troubleshooting DC electrical circuits that are in the vicinity of AC circuits, at the electrical panel for instance, it’s best to disconnect shore power cables (unplug them rather than simply shutting off the circuit breaker) and inverter. If your inverter doesn’t have a main battery disconnect switch, and it should, either remove the main DC supply fuse or disconnect its 12-volt DC connection to the batteries once again, don’t rely on a circuit breaker or panel control to depower one of these devices.
A measure of work
Amps are a measure of how much work or effort a given voltage is being called on to produce. If your electrical panel is equipped with volt and amp meters, watch what happens as you energize different devices. If you turn on the shore power main circuit breaker and no appliances or branch circuits are on, the voltage gauge will come up to 120 (or there about, anything between about 110 and 125 is acceptable) and the amp gauge will read zero. Once you switch on an appliance, a water heater or air-conditioner for instance, the amp gauge will quickly register 10, 20 or more amps, while the voltmeter will remain unchanged at 120 (it may fall slightly, this is know as voltage drop and it often occurs when a circuit is under load). The more work an appliance is called on to do, the greater its amperage draw, and this is true for AC and DC loads. The average small diesel engine starter may draw 200-amps DC when it’s cranking (on a warm day; cold weather starting nearly always requires more current), while a navigation light may only draw an amp or two. AC appliances, because the voltage is higher, typically draw less amperage (which is why their associated wiring is usually much smaller, the size of a wire is directly related to its ability to carry amperage, not voltage); an air-conditioning compressor may draw 10 or 20 amps AC, while a 5-kw generator may produce 40 amperes of AC current.
Ohms (the symbol often used to represent this unit of measure is the Greek omega) are a measure of resistance.Virtually every DMM will have a selection for measuring ohms. But what is an ohm and why is it important to you as a voyager/electrical troubleshooter?
An example of resistance
Here’s an example, the length of wire between your windlass and its power supply at the house batteries may be 20 or 30 feet long. If the wire is in good condition and the connections are clean, tight and corrosion free, and you use your DMM to measure the resistance or ohms that are present in that cable from one end to the other (you would do this with the cable disconnected from the batteries, resistance measurements must always be made on de-energized circuits) the result should be zero ohms.
That’s right, in the case of resistance, less is more. Resistance is just that, a blockage or impediment to the flow of electricity and thus it’s best, unless a specific device or design calls for otherwise, for every cable, wire and connection aboard your vessel, particularly bonding and safety ground connections, to possess as little resistance as possible, preferably zero. In some cases this perfect score is difficult to achieve so an ohm or two may be considered acceptable, but certainly no more than this (ABYC cathodic bonding system requirements call for no more than one ohm of resistance).
Before making any resistance measurement, carry out this test: set your DMM to the ohms scale and touch the probes together, simulating a dead short. The meter should read zero, or no resistance within the probes, which is normal. If it reads anything else, check the probes for corrosion or the DMM’s battery, it may be fading.
Suppose, however, that the same windlass wasn’t working and you wanted to determine of the aforementioned cable was the culprit. You might perform a resistance or continuity test and find the DMM reading to be open or of unlimited resistance, essentially the reading would be unchanged (when you are making resistance measurements, or any other readings with a DMM, be sure your fingers are not touching the metal portion of the probes, the resistance of your body will skew the measurement), as if the probes were still not touching anything, which would indicate a broken cable, blown fuse, open circuit breaker or separated connection. Or, the DMM may read very high resistance, hundreds or thousands of ohms, which would seriously impede the flow of electricity to the windlass. Thus, ohms are simply a measurement of the ease or difficulty with which electricity flows through a given path. As an aside, contrary to popular belief, electricity does not take the path of least resistance; it takes all paths, with the maximum current flowing through the path of least resistance.
Series and parallel
Two terms worth defining within the realm of electrical troubleshooting are “series” and “parallel.” A connection that is made in series would be one that, if removed from the circuit, the circuit would no longer be complete and it would not work. An example of a series connection is a switch for a light, a horn or virtually anything else. With the switch on (the electrical term for a switch that is on or completing a circuit is “closed”), the device operates; when the switch is opened, the device does not operate. This is a series circuit. Another example of a series circuit may be a vessel’s batteries. Batteries that are connected in series, that is the positive and negative terminals of, for example, two separate 12-volt batteries that are connected together, their remaining positive and negative terminals then become the power source, doubling their output voltage to 24 volts (although their cranking amps and amp-hour capacity remains the same — there’s no free lunch with electricity).
A parallel connection (where the positive and negative posts of each battery are connected to each other, positive to positive, negative to negative), on the other hand, if removed from a circuit, will often not prevent the circuit from operating. Examples of parallel connections include nearly all lighting circuits: each light fixture is connected to the power supply wiring in parallel. If one fixture is removed, turned off or the bulb burns out, the remaining fixtures continue to operate. Christmas tree lights of old were often wired in series, which was why the entire string would go out if one bulb failed. Modern lights, however, are wired in parallel, which means individual lights can burn out while the rest continue to illuminate.
The battery analogy is, once again, useful and important when discussing parallel circuits. Most voyaging vessels use paralleled batteries to increase cranking and house-battery capacity. Two 12-volt, 200-amp-hour, 1,000-cold-cranking-amp (abbreviated CCA, the number of amps a battery will provide for 30 seconds at 0° F) batteries that are connected in parallel will yield 12 volts and 400 amp hours. The same two batteries will also provide 2,000 cold cranking amps for starting purposes.
A final note on the parallel versus series concept, voltage measurements are nearly always made in parallel. That is, the probes of your DMM should access the positive and negative portions of a circuit in order to read voltage correctly (for troubleshooting purposes, voltage measurements may be made in series with a device, a closed switch for instance, to reveal excessive voltage drop).
Conversely, amp measurements are nearly always made in series. That is, the positive or negative conductor of a circuit is separated, usually at a convenient junction, and the DMM probes are connected to or inserted into the circuit. The series and parallel concepts are critically important and it’s imperative that they be clearly understood before you carry out virtually any electrical troubleshooting or installation projects. Don’t feel bad if it doesn’t come easily, it never ceases to amaze me how many professional marine technicians also have difficulty with either understanding or explaining this subject.
More electrical troubleshooting
Although a number of electrical troubleshooting scenarios have already been mentioned, it’s worth discussing a few additional recommendations for, or examples of, electrical troubleshooting. If you are ambitious and serious in your endeavors to become a reasonably proficient electrical troubleshooter, then it’s good to learn how to read an electrical schematic or diagram. The value of being able to trace a wiring run on one of these documents and then superimpose this onto the equipment or vessel on which you are working is valuable indeed. At first, however, this may seem like a daunting task, in some ways it’s like learning another language. There are a number of symbols and designs, which will appear foreign at first. With some practice and a little study (visit www.symbols.net/electrical for a guide to commonly used electrical and electronic schematic symbols), however, these will become more understandable. Many electrical products today offer “light” schematics, these are more wiring diagrams than true schematics, which are geared toward the consumer rather than the electrical engineer. They are larger, clearer and easier to follow and read.
When the engine doesn’t start
One of the more common electrical troubleshooting scenarios involves an engine that will not start. If you are facing this problem, then you should understand the professional terminology that is associated with troubleshooting such a situation, particularly if you are communicating with someone via email, radio or telephone for assistance. An engine that will not crank or turn over is an engine that does absolutely nothing when the key or starter switch is turned or pushed to the “start” position.
Some folks confuse the term “crank” with “run;” an engine that cranks but does not run, is an engine that turns over or rotates when the key switch is turned to the start position, but it will not develop combustion or become self-sustaining. When you release the key switch, it stops turning or cranking. Thus, if you are seeking outside assistance with an engine that won’t start, and a professional asks you, “does it crank?” he or she wants to know if the engine rotates when the key/starter is engaged. This will help him or her determine very quickly if it’s an electrical or a mechanical problem.
If the engine does not crank, then the problem is almost certainly electrical in nature. There is an easy and straightforward test to perform with your DMM to troubleshoot this. With your meter set the DC volts scale, place the negative probe on the DC ground cable where it is bolted to the engine (in some cases this cable may be bolted to the starter, but make certain it’s the ground and not the positive cable). Then, place the positive probe of the DMM on the starting battery positive post. If it shows 12.6 volts or more, this will confirm you have a good ground connection and voltage at the battery. Next, place the positive probe on the large post on the starter, it’s usually about 3/8 inch or 1/2 inch in diameter, about the size of your pinky, to which will be connected a large diameter (1/2-inch or more) battery cable. If you get 12.6 volts or more there, then you have confirmed that the starter solenoid’s primary winding is receiving power.
Finally, place the positive probe on the smaller stud on the starter solenoid, it’s usually about 3/16 inch or 1/4 inch in diameter, a little smaller than the diameter of a pencil. It will usually have one or two small gauge, number 12 or 14, wires connected to its stud. With this connection made, ask a helper to turn the key to the crank position (make certain you are clear of all belts, pulleys and other engine moving parts). If the meter reads 12.6 volts, then chances are good you have a defective starter. If, on the other hand, it does not show any voltage, then the problem is between the starter and the key switch or between the key switch and its power supply, which sometimes originates at the engine itself or at the electrical panel. You would then carry out the same voltage test procedures and process of elimination at the back of the key switch and, if necessary, back to the electrical panel and engine ignition supply.
The technique described above can be applied to any number of electrical troubleshooting scenarios. As you can see, however, electrical troubleshooting can be tedious and time consuming (it’s why many yards and shops charge extra for this and they will only do it on a time and materials basis). The good news is, with some understanding of your vessel’s electrical system and the right tools (either a DMM or a test light could have been used for the entire starter troubleshoot described above), along with a degree of persistence, every electrical fault and gremlin can eventually be tracked down and resolved.
Contributing editor Steve D’Antonio operates Steve D’Antonio Marine Consulting LLC (www.stevedmarineconsulting.com), and offers consulting services to boat buyers, boat owners and boatbuilders. His marine systems book will be published by McGraw Hill in the fall of 2008.