Solving Automotive Electrical Problems
Amperes, amps, or current is the measure of the flow of electricity through a given circuit. It’s the (I) in the Ohm’s law equation above. Amps are different than voltage in that amps are the same throughout the entire circuit, unlike voltage, which starts out high then drops to zero when it reaches the negative battery post. Amps are the same at the beginning and the end of a circuit. If you have five amps at the beginning of the circuit, you’ll have five amps at the end of the circuit.
To continue with our automotive analogy, we’ll equate amps to speed. The higher the amps, the faster the speed. The number of amps a circuit uses is dictated by the load and the resistance in the circuit. The bigger the load, the more amps a circuit will use. A starter motor will use a lot more amps than a window motor because the window motor will not need as many amps to run as a starter motor would. This next fact about amps gets a little weird. The lower the resistance in a circuit, the higher the amp flow.
Sticking with our speed analogy, the more amps you have in a circuit, the faster things flow. Remember when I said that if you had a short circuit you’d blow fuses and possibly burn things up? I was taking about amps. If you have unrestricted electron flow with no load, amps get out of control. It’s like a speeding locomotive that goes off the track. Something is going to break. With automotive electrical systems, it’s usually the weakest link, which should be the fuse. Ever notice how fuses are rated in amps, not voltage? Now you know why.
One last little factoid about amps: You can have high voltage in a circuit and it won’t be that harmful to you. It’ll wake you up for sure, but it won’t kill you. However, if you have just a few amps to go with that high voltage, watch out. As little as 0.2 amps mixed with the right amount of voltage can kill you. In fact, it might even be less than that depending on the circumstances. It’s not the voltage that hurts you; it’s the amps. It doesn’t take too many amps to knock you on your butt, so be extra careful when dealing with amps. To use another common analogy, if volts are pressure, amps are intensity or volume. You can have lots of pressure, but when you add intensity, things can get lively.
I don’t often do amp testing when dealing with automotive circuits, other than when I perform a parasitic draw test. We’ll cover that later in the article. There are two ways you can test amps in an automotive circuit. The first is with an inductive amp meter.
This is a tool that measurers the magnetic field around a wire. The circuit has to be live for this to work. Say you’re looking to test the amps going to the starter via the positive battery cable. Place your ammeter around the positive battery cable and engage the starter to take your reading. You won’t get a reading if you’re not cranking.
The other way to measure amps is by tying into the circuit directly, or tying in ‘series’. This means you open the circuit somewhere and insert your ammeter. All the amps in the circuit need to flow through your meter in order to test this way. This can be tricky, and many a DVOM has suffered from improper amp testing. Don’t worry; most of them are fused, so if you mess up, hopefully you’ll be able to replace a fuse and be back in business. So to be safe when checking high amp circuits, use an inductive lead instead of connecting in series. If you’re checking for small amp values, connect directly into the circuit to take your readings. This video shows amp testing at work, as well as some other useful tests. Remember, be careful when dealing with amps.
Resistance (R) in the Ohm’s law equation is what the electricity works against in an electrical circuit. Resistance is measured in ohms (Ω). Resistance can come in many forms, some good, some not so good. Every electrical circuit needs some form of resistance. If not, you have a short circuit, and we know what happens if we have one of those. Resistance can be as simple as a light bulb or as complex as a PCM. Both create a certain amount of resistance that is accounted for in its circuit.
A few key things can affect resistance. One of them is heat. The higher the temperature, the higher the resistance. The reason for this is that a heated substance has a lot of molecular movement, which affects electron flow. The harder it is for electrons to flow, the more resistance you have. That’s pretty much the definition of resistance.
Distance can also affect resistance. The longer a wire is, the more resistance it has. Think back to our car with a tank of gas. We’re going to use more gas the farther we have to travel to get back to the negative battery post. The size or amount of conductive material that the electricity passes through can also effect resistance. A wide-open path with a very conductive material has little resistance. Conversely, a small wire with a not-so-conductive material has more resistance.
The takeaway here is that resistance can be good or bad depending on where you find it. You need the resistance of the load of an electric circuit in order for the circuit to function, but if you have unwanted resistance, the circuit will not function as intended.
Honestly, I’m not a fan of checking a circuit’s resistance over a voltage drop test. The main reason is because when you’re doing a voltage drop test, you get to see what’s happening on a live circuit. Resistance checks are done on open circuits that are isolated from the rest of the circuit.
When doing resistance checks, you must remove what you’re checking from the circuit first in order to do your resistance check; otherwise, you might damage the circuit or your meter. It’s for that reason I find resistance tests to be inconclusive sometimes. Sure, there are times when testing resistance is the only way to test a circuit, but when diagnosing a problem in a circuit, I prefer a voltage drop test.
Case in point: ignition coil testing. Manufacturers list the resistance you should see at the primary and secondary windings of an ignition coil. I’ve never found a bad ignition coil this way. I spoke to an electrical engineer about this once. He told me that a coil can have good resistance readings and still be bad. The reason for this is that the windings inside the coil are made of very thin wire. You can have a small break in this wire and the resistance will still indicate everything is fine, but the minute the coil is under load, it fails because of the open or short in the windings. On the other hand, if you’re checking a coolant temp sensor, resistance testing is the way to go. My point: Be sure to use resistance checks wisely. But, if you can do a voltage drop test over a resistance check, go for it.
Believe it or not, electricity and magnetism are very closely related. Anytime you have electricity flowing through a conductor, a magnetic field is created around it. Anytime you move a magnet near a conductor, you create electron flow inside of it, which can be considered electricity. In fact, this is how your alternator works.
I don’t want to get too deep into this, but it is a concept that needs to be covered. As I said, an alternator works because it houses a series of electromagnets that are moved near a conductor, thus causing electricity to be generated. The mechanical movement of the engine is used to drive the alternator, which in turn converts the mechanical energy of the engine into electrical energy inside the alternator. Conversely, we create magnetic fields inside the starter motor to produce mechanical energy from electrical energy so we can start the engine. It just depends on how it’s used. Just know that electricity and magnetism go together like chocolate and peanut butter.
This principal is used throughout automotive electrical systems to do mechanical work or to generate the electricity that runs the vehicle. In fact, this is how many hybrids work. They use a starter/alternator assembly on the back of the engine in place of a flywheel. This device makes it possible to use the flywheel as the starter and alternator for the engine. It generates electricity and works as an alternator as long as the engine is running. It also makes it possible to shut the engine off when you’re sitting at a stop light. When you accelerate, this device transforms into a starter and gets the engine going again. It can generate electricity as the engine is running and the vehicle is driving just like an alternator would. It all depends on how the magnetic fields are being used at the time. It’s pretty cool stuff that I hope to make some videos about at some point. For now, we’ll cover electricity and magnetism in general terms and move on.