Solving Automotive Electrical Problems
Electricity is the flow of electrons within a conductor. You can get these moving electrons to do things for you in automotive systems, such as starting your engine, charging your phone, controlling vehicle emissions, and opening your windows. Electrical systems are everywhere in modern automobiles. Having a basic understanding of how electricity works can go a long way toward helping you repair automotive electrical systems.
The first thing to talk about is the flow of electricity. For the purposes of this article, we’re going to say that electricity flows from positive to negative. Yes, there are other theories on electron flow, but to keep things simple we’re going to stick with electrons flowing from positive to negative in this article.
To get this electricity to flow and go were we want it to, we use conductors. Conductors are materials that allow electrons to flow freely. Resistors are materials that inhibit electron flow. We use resistors to insulate our conductors so we keep the electricity flowing where we want it to go. Electricity is funny like that; if you let it, it will find the shortest route to ground, which might happen before you want it to. Insulators help keep this from happening.
Then there are semiconductors: transistors, resistors, and diodes, just to name a few. We use these to help control things in automotive electrical systems. Semiconductors can allow electron flow sometimes but not others. They can block electron flow in one direction, but allow it to flow in the other direction. They can also be used to change the resistance of a circuit based on temperature. Semiconductors are used extensively in computers and control systems. As a result, you run into a lot of semiconductors in automotive applications.
Another big component of an electrical circuit is the load. The load is what does the work in an electrical circuit. The load can be a lot of things. It can be a motor, a heating element, a bulb, or any number of other electrical components that do work. The load and the resistance in the circuit dictates the amount of current flow in the circuit. We’ll talk more about that in a minute.
Aside from the wires that carry electricity and the wiring insulation that keeps the electricity on the path we want it to take, we also need the ability to control a circuit. We often do this with switches. Switches come in all shapes and sizes. They can be switches that we activate, or they can be activated automatically when certain conditions are met; they can also turn things off at a given time.
This is a very basic overview of what you’ll find in automotive electrical systems. Now let’s talk about how we label and measure electricity in order to diagnose electrical faults.
We can’t do much work with electricity if we don’t know how to measure it. As I said, electricity is not something we can necessarily see and put our hands on. Because of that, some very smart people came up with ways of measuring and using electricity. Georg Ohm was one of those smart people. He came up with what is now called Ohm’s law. Ohm’s law is written like this:
I = Current Flow or Amps
V = Voltage
R = Resistance
Basically, what this formula says is that amps, voltage, and resistance are interrelated. If you know two of the three in the equation, you can figure out the value of the missing one. If you know the volts and the resistance, you can divide the volts by the resistance and find out how many amps are flowing through the circuit. It’s a handy formula that can give you a better understanding of how electricity really works.
Voltage (V) is the potential energy of electricity. Voltage can be present even if there is no current flow. For example, your battery can have 12 volts in it even if it’s not being used. That 12 volts is still there, waiting to be used, and is the battery’s potential. Think of battery voltage like you think of a full tank of gas; it can get you somewhere, but not until you start the engine and begin to use it.
One really cool thing about voltage is that in an electrical circuit, all of the voltage the electrical circuit started with is used up when traveling through the circuit. For example, if you start with 12 volts at the battery positive and you run it through a circuit, by the time it goes through the load and reaches the negative battery post to complete the circuit, it’s down to zero volts. Why is this cool? Because it allows us to do one of the best tests in electrical diagnostics: the voltage drop.
Voltage drop testing is probably the best way to find an electrical fault. Because the voltage drops as it travels through the circuit, we can tell where it’s being used simply by measuring the voltage at different points in the circuit. Think of a car that leaves the positive battery post with just enough gas to work the load and get to the negative battery post. By the time the car reaches the negative battery post, it runs out of gas and that’s it. That’s how voltage works; it starts at its highest level at the positive battery terminal and ends up at zero when it reaches the negative battery terminal.
Let’s say that somewhere in the circuit, we take a detour and go down a different road and decide not to take the road (circuit) all the way back to battery negative. We call this a short circuit. When this happens, we often blow fuses and sometimes melt or burn up parts. This is because of the unrestricted amp flow that occurs during a short circuit; we’ll talk about that in the next section.
Or, let’s say there is an obstruction in the road (circuit) and we use more gas getting through the obstruction on our way back to battery negative. We’ll still make it back to battery negative, but since we used so much fuel (voltage) getting through the resistance and the load, there’s hardly any fuel left to do any work. This is what happens when we have increased resistance in a circuit. If that’s the case with, say, a window motor, the window motor will still work; it will just work very slowly. Why? Because of the increased resistance we had to go through to get back to battery negative. We ended up using almost all our fuel (voltage) overcoming the resistance in the circuit. Less fuel (voltage) means there isn’t enough to run the window motor at full capacity.
If we did a voltage drop test following the path of the circuit, we could see where the fuel (voltage) was being used. We should see the biggest drop across the load of the circuit, which, in the case of a window motor, is the window motor. While the circuit is operational, we can check the voltage before and after the motor. Voltage drops need to be done on a live circuit; if they’re not active, you won’t get any readings. Remember that voltage is electricity’s potential energy; if it’s not being used, it’s just potential. Before the motor, we should see something close to the 12V we started with. We won’t see the full 12V because we had to use some of the gas (voltage) to get through the wires, and probably switches, going to the motor itself. These things give us a little resistance that we need to account for when checking the voltage before the motor. After the motor, however, we should see some pretty low voltage, because we used all our gas (voltage) to run the motor.
This works for every electrical circuit according to Ohm’s law. Once you know the circuit, you know where the gas (voltage) is going and you can measure it at different points in the circuit to get a good idea of how the circuit is operating.
One last example that might give you some practical insight: Say you have an electrical connector with some corrosion in it. You can’t see the corrosion, but you suspect it’s there. You can take a voltage reading before and after the connector to check the voltage drop across the connector. You should see close to zero volts. If you see a higher voltage than expected, you’ve found increased resistance in the circuit.
Voltage drop testing is your best friend when it comes to diagnosing automotive circuits. Learn how to do it, and you’ll be an electrical wiz in no time.