The four horsemen of electrical circuits

Feb. 2, 2016
There are only 4 ways and electrical circuit can fail. Knowing that puts you one step closer to fixing the problem!

It’s easy to feel intimidated when you are tasked with solving an electrical problem on a late-model vehicle. Modern automotive electrical systems are complex and only getting more so with each new model year. To make matters worse, lots of technicians struggle with basic electrical theory and have not acquired diagnostic strategies to assist them in conquering these problems.  Many would rather be doing brakes or engine work, but there aren’t any systems left on the vehicle that don’t have electrical components. You can run, but you can’t hide!

If this is describing you, take heart. The electrically-challenged technician should always remember the following:

1.     Electricity always works the same, no matter how complex the system.

2.     There are only four possible failures that can take place in an electrical system. These are the open circuit, high resistance, short-to-ground and short-to-power.

3.     Understanding the customer concern and simple visual inspection can often narrow down which of the four failures you are dealing with.

So, take a deep breath while we walk through the basics of what you need to know to become a top-notch electrical troubleshooter! 

The basics

Let’s do a quick review of electrical fundamentals. First off, what is electricity? Scientists are always coming up with new information on that, but we can assume that electricity is the flow of electrons through a conductor. Electrons are one of the basic building blocks of matter, and are an integral part of the atom. An atom is the smallest particle that an element can be broken into and still retain the properties of that element. An example of an element is copper, which has free electrons that can be moved from one atom to the next, much like a rock skipping across the water.

Elements that are capable of conducting electrical current are known as conductors. The best electrical conductors are silver, copper, gold and aluminum, in that order. Materials that block electrical current flow are known as insulators. Commonly used insulators include plastic, glass, rubber and bakelite. No surprise that we coat copper wire with plastic insulating material, as we don’t want the electricity to leave the wire and go somewhere we don’t want it to.

Voltage is electrical pressure—the force that causes current to flow in a circuit. You can have voltage with no current flow, but current flow will not happen without voltage.
If voltage (E) stays the same but resistance (R) decreases, current (I) will increase. If resisitance increases, on the other hand, current will decrease.

What makes water flow through a pipe? There has to be a pressure difference; a higher pressure at one end and a lower pressure at the other. Water will naturally flow through the pipe away from the end with higher pressure and towards the area with lower pressure. If there is no difference in pressure, water will not flow. In the same manner, electrical current will not flow without a pressure difference. The “pressure” behind the electrical current is known as voltage, and is measured in volts. Voltage is also known as potential difference, or electromotive force (EMF). While it is possible to have voltage (pressure) with no current flow, it isn’t possible to have current flow without voltage.Electrical geniuses like Edison discovered long ago that if you could make electrons flow, you could do a lot of useful work. Electron flow is known as current, and is measured in amperes.  Think of it like water flowing in a pipe. In order to increase the rate of flow, the pipe will have to get bigger in diameter. Electricity is no different. More electrical current can do more work, and the more current you want to move, the bigger the wire will have to be. All the biggest wires on your car are made to flow more current and do more work, the best example being the cable running from the battery to the starter motor.

A partially closed valve or damage to a water pipe that reduces its diameter will increase the pipe’s restriction to flow. Increased restriction to flow, in turn, will reduce water flow. Electrical resistance works the same; increased resistance results in reduced electrical current flow.  Electrical resistance is measured in ohms and can be caused by loose connections, frayed wires or corroded terminals. An important principle to keep in mind concerning resistance is that the closer you get to zero ohms, the less the resistance in the circuit. In turn, an energized circuit that has zero amperes of current flow would have infinite resistance.

You’ve likely heard of Ohm’s Law, which is named after a German physicist and mathematician named Georg Simon Ohm. Ohm’s Law is central to this discussion because it describes the relationship between voltage, current and resistance. In simple terms, Ohm’s Law states, “one volt of electrical pressure will cause one ampere of current to flow through one ohm of resistance.”

Ohm’s most important contribution to a technician’s understanding of electricity is that when circuit resistance rises, current flow decreases. The reverse is also true; if circuit resistance is reduced, current flow will increase. This explains a lot of what we see in the field everyday as technicians, helping us understand the why and how of many electrical failures. When failures take place, either current isn’t flowing at all, current flow is reduced, or the current is flowing somewhere that we don’t want it to.

What goes wrong

We all know armchair automotive technicians that attempt to diagnose electrical problems by hearing the story and then telling you that a “short” is causing it somewhere in the system. Not helpful! I don’t do this personally, but if you were to ask them exactly what a “short” is or how to narrow down where it is taking place, they would be hard pressed to give you an answer.

Despite their good intentions, calling every electrical failure a “short” is misinformed. There are a total of four possible failures that can take place in an electrical circuit: an open circuit, high resistance, short-to-ground and short-to-power. So what are the symptoms of each of these failures, and how do we go about locating them?

Once you've verified the customer concern, do your visual inspection and basic tests. Here's an open circuit that you should catch without breaking out your DVOM.
Automotive electrical systems typically locate the main fuse box near the vehicle battery. This ensures that the majority of the wiring harness is protected by a fuse, fusible link, or circuit breaker.

The technician needs to start by getting a solid understanding of the customer concern. Ask clarifying questions, including whether or not the problem is intermittent. This will give you some clue as to which of the four possible failures you are dealing with. Try to have the customer present as you verify the concern, and if necessary, go for a test drive with the customer at the wheel.

The next step is to perform a visual inspection and basic tests. This is the simple stuff like bulbs and fuses. Don’t pull the fuse; instead, check both sides for battery power with your voltmeter. Is the fuse melted? If so, there has been excessive current flow caused by reduced resistance in the circuit. If the fuse is OK, circuit resistance is either normal or too high. A significant percentage of electrical problems are solved at this step, so be thorough.

1.  Open Circuits

When the customer states that a device is not working at all, you should start thinking open circuit. An open circuit is a broken connection that stops electric current from flowing. Because a connection is broken, circuit resistance is infinite. An open circuit is also known as a non-continuous circuit.

If the fuse is OK, this further reinforces the open circuit diagnosis. Your next step is to gain access to the malfunctioning device and turn the circuit on. With the negative lead of your voltmeter connected to a good ground, measure voltage on the power side of the device. You should see battery voltage on the meter; if not, you will need to measure voltage at points upstream from the device to narrow down the location of the open circuit. Keep working your way back towards the battery; when you suddenly find battery voltage, the problem is between there and the last place that you measured.

Always use your voltmeter
If there is a cardinal rule of electrical diagnosis, this is it: Almost all of your electrical testing should be done using a voltmeter with the circuit energized. The other functions of your DVOM (digital volt ohmmeter) have their place, but you will get more done in less time by using your voltmeter on live circuits. There are a number of reasons why this works best. First, your voltmeter will give you more useful information about the status of the circuit, because it also gives indirect measurements of resistance and current. Second, even if you don't know what you're doing, you are not likely to cause damage when using a digital voltmeter. This is because it has very high internal impedance and won't cause a short circuit.

What if you measure voltage on the power side of the device, but it still isn’t working? Your next step is to measure voltage on the ground side. If you happen to measure battery voltage on both sides of the device, the device is good and there is an open on the ground side of the circuit. Connect a jumper wire between the ground lead of the device and the vehicle frame to confirm your diagnosis.

If there is battery power and a good ground available at the device but it still doesn’t work, it’s pretty obvious that the device itself has failed.

2.  High Resistance

When the customer states that a device is working poorly, this immediately tells you that you are dealing with high resistance. This could be a dim lamp, a motor turning slowly, or an electric heater that isn’t getting hot enough. There is a poor connection somewhere in the circuit, and increased resistance is reducing the voltage available to the device.

This is caused by a phenomenon called voltage drop, which takes place anytime electrical current flows through a resistance. Low resistance connections allow current to flow unopposed and cause very little voltage drop. A poor (corroded or loose) connection, on the other hand, will actually consume voltage and leave less for the device to use. A voltmeter connected across a connection or wire will tell the story. If the connection is good, only a few millivolts of drop will show on the voltmeter. A poor connection will show more than just higher voltage drop numbers, it may also show signs of getting hot. This can melt plastic connectors and burn insulation over time, and is a sure visual sign that you’ve found the source of your high resistance!

No need to check the fuse on this one...it is good. Turn the circuit on and take a voltage measurement across the device. You should see close to battery voltage, but odds are good in this situation that it will be significantly less. If you are seeing more than 1 volt below battery voltage, it is time to go looking for a high resistance failure.

There could be high resistance on the power side, on the ground side, or both. Connect the negative lead of your voltmeter to a good ground and check the voltage at the power side of the device. If the voltmeter is now showing close to battery voltage, the power side is good. However, if the voltage there is low, you will need to work your way upstream looking for voltage drop. If you see battery voltage at a point in the circuit, the high resistance is somewhere between there and the last point you checked.

If the power side showed close to battery voltage, the failure is taking place on the ground side of the circuit. With the negative lead of your voltmeter connected to a good ground, check voltage at the ground wire of the device. The voltmeter should show just millivolts of drop if it is good. If the reading is higher, repair the poor ground connection and verify correct device operation.

A telltale sign of a high-resistance failure is a melted connector. Once this terminal started getting hot, it was game over for the pigtail and the switch connected to it.
Here's an example of a failure in a ground that affected more than one device. Voltage drop across this connection caused malfunctions in the rear wiper, rear washer, and rear defrost circuits.

3.  Short-to Ground

Here is where it gets interesting. If the customer tells you that a device is not working at all and you find that the fuse is melted, a short-to-ground is a possible cause. A short-to-ground is a low resistance path to ground, causing excessive current flow. This is often caused by a wire rubbing through its insulation and making contact with the frame of the vehicle (copper-to-steel).  However, it can also be an internal failure of a device which can only be repaired by component replacement.

If you happen upon a melted fuse, start by replacing it and then operate the circuit. It is possible for the fuse itself to be the root cause of the problem, but if the new fuse blows right away, you’ve got something else going on. Did the fuse blow the moment you replaced it, or after the circuit was switched on? That tells you right away if the problem is upstream or downstream from the switch.

Take a careful look at the wiring diagram to determine how many circuits are powered by the fuse that failed. Which circuits are most likely to cause a short-to-ground? Is it possible to disconnect circuits that you suspect may be causing the problem? If the fuse stops blowing when a certain component or section of the circuit is disconnected, you are that much closer to solving the problem.

In my experience, a short-to-ground can often be found where some kind of monkey business has taken place. A really good example of this is trailer wiring that has been spliced into the vehicle harness. Loose wires that rub against the frame can cause an intermittent short-to-ground that affects lighting circuits. Another possibility is an accessory that was installed in the vehicle recently. Is it possible that a mounting screw was driven into a wiring harness?

A more sophisticated method for locating a short-to-ground is to use a tone generator. This diagnostic tool has a transmitter that connects to the fuse holder (or another point on the wiring harness) and sends a signal through the problem circuit. The technician follows the circuit with a receiver that emits a tone when it senses the signal travelling through the wire. When you get close to where the short-to-ground is located, the sound from the receiver turns off. These tools can sometimes be used to assist the technician with finding open circuits as well.

A good fuse will measure a small amount of voltage drop, which will increase as current flow rises. If you measure battery voltage across a fuse, it is open (melted).
The milivolt (mV) range on your digital volt ohmmeter (DVOM) is handy for measuring voltage drops across fuses and connections.

4.  Short-to-Power

A short-to-power could be reported to you in a couple different ways. First, the customer could tell you that a certain device on their vehicle won’t shut off. Another possibility is the customer saying that they have to get their vehicle jump started after it sits overnight. A short-to-power is a failure where a switch is being bypassed; either the switch itself has an internal short circuit, or a wire downstream from the switch is making contact with a wire from another circuit. In all likelihood, the fuse will be OK, but the circuit will be powered up when you don’t want it to.

If the customer says that a device won’t shut off, start by pulling the fuse for that circuit. If the device is now off, the problem is isolated to that one circuit. Reinstall the fuse and then disconnect the circuit switch; if the device turns off now, the switch has an internal short circuit.  If the device stays on when the fuse is pulled, it is getting power from another circuit. Inspect the wiring harness carefully, looking for sections that may have rubbed through and caused a copper-to-copper failure.

If the customer says that their battery is going dead overnight, you likely have a parasitic drain to deal with. Avoid pulling fuses if you can. Instead, use your DVOM to measure voltage drop across individual fuses with the ignition key off. Normal circuits will display very low millivolt readings. If you find a fuse that has a much higher millivolt reading than the others, you’ve likely found the circuit that is killing your customer’s battery!

Get after it

We can talk all day about electrical troubleshooting, but the only way to really learn it is to get out there and start fixing cars. Get yourself a good quality DVOM, start looking at wiring schematics every time you diagnose a vehicle, and get some solid repairs under your belt. In good time, you will have customers asking for you by name to get their electrical problems taken care of.

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