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Electrical 101

What every technician NEEDS to know in troubleshooting electrical faults.
Thursday, January 1, 2009 - 01:00

What every technician NEEDS to know in troubleshooting electrical faults.

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Diagnosing electrical system faults seems to pose one of the biggest headaches for many technicians. But more and more vehicle systems are going electronic, and the ability to handle these types of repairs is going to have a direct impact on your wallet. Maybe it's because you can't see it, maybe it's because you don't quite understand it. In either case, let's see what we can do together to take some of the mystery out of tackling electrical issues.

What Is Electricity?

Simply, electricity is the movement of electrons in the same direction. Direct current (DC) is the movement of electrons in one direction only and is the type of electricity used in the vast majority of automotive systems. Alternating current (AC) is the movement of electrons first in one direction, then in the opposite direction. Alternators produce AC current that is converted to DC for the vehicle's use.

Electron movement occurs when an imbalance is created either by adding or subtracting an electron from an atom. Nature abhors an imbalance, and the atom will try to regain its balance by either shedding the extra electron to a neighbor or stealing one from it. An outside force has to be applied to create this imbalance, and it is called electromotive force (EMF). The most used source of EMF in an automobile is the battery. Here, one pole of the battery has an abundance of free electrons while the other has a shortfall. The difference between the two posts is electromotive potential, or more commonly voltage. Remember this:

Voltage is the measurement of electromotive potential between the two test points.

Don't quite understand that point? Try this experiment: measure the voltage of a battery as you would normally, with your Digital Volt-Ohm Meter (DVOM) leads connected positive to positive and negative to negative.

Now reverse the leads.

Notice how the second measurement is now a negative number? That's because the voltmeter is doing the math for you and telling you the true potential between the two test leads. This is important to understand, as understanding voltage drop testing is based (in part) on this fact.

Magnetic induction is another means of producing EMF and is used as a means of generating AC voltage in the alternator. As a magnetic field passes by a conductor, electron movement is created in that conductor. It is the change in the strength and speed of the magnetic field that affects this potential. Remember the sine wave from high school? That is a representation of AC voltage changing with the magnetic field — first, the North Pole passes by, then the south.

What Is A Complete Circuit?

A battery on the bench does nothing. It has the potential to cause electricity (electron movement) but only if we provide a means to allow the side with an abundance of electrons to get to the side where there is a shortfall. So the first thing we need is a conductor — a path to connect the two sides together.

A conductor is any material that has a minimum number of electrons in its outer ring. That makes adding or subtracting an electron from it easier. Copper, gold and platinum are all examples of atoms with this characteristic, and you are familiar with copper wiring on the cars you've serviced. But what about what is molded around that copper? This is an insulator, a material whose atoms have several electrons in the outer ring, making it difficult (if not impossible) to create the imbalance needed for electrons to flow. No electron flow, remember, means no electricity.

Stringing a copper wire between the two battery posts would certainly allow electrons to flow, but that would be silly — and potentially dangerous! After all, you want your circuit to do something, whether it runs the vehicle lighting system, operates the fuel pump or anything else that needs to happen. The component that is being operated (doing the work) is the heart of an electrical circuit, and is called the load. Every load has its own restriction to electron flow designed into it by the engineers. This restriction to current flow is called resistance. More on that in a minute.

Try this for yourself. Grab an old light bulb and socket, and wire one end to the negative battery post and the other to the positive battery post. Don't worry; it won't care if the polarity isn't correct. It should light up. You now have the minimum elements needed for a complete circuit: a source of EMF, or voltage, a load, or device you want to operate, and a complete path from the source to the load and back again.

Now you need a way to turn off the light, and turn it on when you want to. Since it would be a hassle to hook up the headlights to the battery every time it gets dark, how about installing something in the circuit path to do this for you, like a switch? Any device that is used to open and close the circuit path is called a control. You are likely familiar with basic switches. Relays are typically used as control devices, as are control module drivers. Can you think of any others?

All a control does is open the path, and that prevents electron movement. Have to have a complete path, remember? Because a control opens or closes the path, it can be installed anywhere in that path — either between the source and the load (the positive side of the circuit) or in the return path from the load back to the source (the ground side).

Let's stick in a quick note on the two sides of a complete circuit. The positive side is always a wired connection between the source and the load on an automotive circuit, but the ground (negative) side is a different story. Since the body, frame and engine are all conductive, they are often used as ground points for different components. This saves on wiring and manufacturing costs. Regardless, the path must still be complete from the ground side of the load to the source. And that is typically the battery's negative post. Just because that ground lead in the trunk is good and tight doesn't mean the path is complete. Always verify the entire path when testing an electrical problem.

What about protection for the wiring? After all, if the positive side of the path should suddenly find a way around the load and back to the source (a short circuit), electron flow would be unrestricted and quickly melt down the wiring. Fuses, fusible links and circuit breakers are all means to prevent that from happening, and they are called the circuit protection devices. Their function is simple. If electron flow exceeds the designed limit, the circuit protection device is responsible for opening the path before any more damage can be done. That is one reason it is imperative that any circuit protection device be replaced with one of the same, or lower, rating. Never higher! Because it protects the positive side of the circuit, circuit protection devices will always be between the source and the load on the positive side of the circuit path.

OK, let's review the elements that make up a complete circuit:

A source to provide the electromotive force, or potential, we need for electrons to flow.

A load to perform the actual work we want the circuit to perform.

A control that enables us to turn the load on or off by interrupting electron flow.

A circuit protection device to protect the circuit from excessive electron flow.

A complete path for electron flow to follow that starts and ends at the source.

Specifically, this is the description of a series circuit, one where electron flow has only one path to follow. Automotive circuits, for the most part, are combinations of series and parallel circuits (circuits that have more than one path for electrons to follow). For example, one wire may supply the power to all the brake lights and then split off to multiple wires leading to each individual bulb. This will affect current flow, but will not affect the basics of having full voltage at the load and a good ground after the load.

It can help in your troubleshooting though. If more than one load is affected, look for the problem in those portions of the circuit shared by the loads. If the fault is in only one load, then focus your initial testing on that part of the circuit unique to that load.

I've mentioned electron flow, electromotive potential, and restrictions to flow...now it's time to understand these elements a little more thoroughly.

What Is Ohm's Law?

Ohm's Law is a representation of the relationships between electromotive force, electron flow and restrictions to flow. Before we dive into Ohm's Law, let me first define these three elements for you.

You now know that voltage is the measurement of electromotive potential between the two test points. This is the "source" I told you about that we need in order to get the electrons moving. The movement of electrons, or flow, is called current and is measured in amperes (amps). Opposition to that flow is called resistance and is measured in ohms. Without some form of resistance in the circuit, current would flow as fast and as furiously as the path could handle until it is overloaded and failed...in a matter of milliseconds.

The load (component doing the work) is the primary resistance in the circuit. Control devices, circuit protection devices and of course, the wiring itself should cause no real resistance in the circuit. If any of them do have substantial resistances of their own, that would be a bad thing...as you'll see in a few more minutes.

Back to Ohm's Law. Ohm's Law states the relationship between volts, current and resistance to be:

Voltage = Current x Resistance Or Volts = Amps x Ohms

While there are instances doing the math can aid in diagnosis, the important things to learn from Ohm's Law can be summed up in these two statements:

A decrease in voltage will cause a decrease in current flow for a given resistance.

An increase in resistance will cause a decrease in current flow for a given voltage.

Circuit loads need current to work. If current flow changes from what is normal, the load will not work properly. A weak battery lowers voltage, which lowers current, which keeps the starter from working. A bad ground causes additional resistance, which lowers current, which makes the lights dim. A corroded connector can either prevent full voltage from reaching the load, or add to the ground side resistance. Both will reduce current and the fuel pump (or whatever) will not work the way it should.

Let me see if I can paint a mental picture of how all this works together in an electrical circuit. No, I promise, no "water" analogies and garden hoses.

Imagine a small room packed with people. On either side of the room is an opening that connects to a sealed, one-way corridor leading to and from the room — a complete path. In the middle of the corridor is one of those revolving doors like you would find in an upscale hotel. Since we'll be passing through that door, it becomes the load. The door doesn't move on its own: there is resistance to that movement.

All the people in the room are electrons, when suddenly some wise guy yells, "Fire!" The push you're going to feel as everyone heads for the corridor is electromotive force or voltage. The movement of people through the corridor, to the door, and back is electron flow or current.

Now we are at the door, and we pass through. The push we felt from the crowd is gone as we pass by the other side. This is the same as voltage drop. You follow the corridor back to the room we just left, completing the path and ready to start all over again.

The number of people able to move through the corridor and back to the room is determined by the push of the crowd and the resistance of the door. Greater push, more people. Tighter door, fewer people. That is Ohm's Law!

To further understand actual current flow, imagine this scenario. Ever been part of a "bucket brigade?" Paint a picture in your mind of MILLIONS of people standing shoulder to shoulder, each holding one bucket. The first guy in line is handed a second, and immediately passes off his first bucket to his neighbor, who then immediately passes off his to his neighbor and so on. Now consider that all of these hand-offs happen at nearly the exact same time. As soon as the first guy gets his extra bucket, the last person in line is returning theirs to the "source." The only limit to how fast each hands off their respective buckets is any "restriction" to that hand-off we place in the line, and how hard we "push" the buckets into that line.

What Is Voltage Drop?

Voltage drop is the reference used in describing how voltage (push) is used to overcome the resistance in a circuit. Remember what should be the primary, and only, source of resistance in our circuit? That's right, the load. So, you can measure voltage on the positive side of the load and you should read the same as the voltage potential measured in the battery. Still have your light bulb wired to a battery? Go ahead, take minute to try this. I'll wait.

With the light bulb lit (or any load "on"), now move your positive meter lead to the ground side of the load. What did you measure? It should be darn close to 0.0 volts but not quite. That's because all the available voltage will be used to overcome the resistances in the circuit. Since there is some, very low, resistance in the wiring and connections, you should measure a very small amount of voltage here. If there are substantial additional sources of resistance, like bad grounds or corroded connections, the available voltage will be split proportionally between the individual resistances. That's the basis for voltage drop testing.

If there is additional resistance on the positive side, your DVOM reading will be less than it was when you measured at the battery. If the extra resistance is on the ground side of the path, then you will measure voltage on the ground side of the load when there should be very little. Because this tests the circuit "live," it tests the dynamic resistance of the circuit and that is quite different than just measuring the circuit with your ohmmeter.

But that's a subject for next time!

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