The ABCs of electrical diagnostics

Feb. 1, 2019
Just like when you first started learning, mastering the fundamentals has to come first!

We’ve all learned the ABC song, and how to count to 10 as one of our first “organized” instructional classes, even if we didn’t know that’s what we were doing. Now, as a grown up, we’re still learning the same basic golden rules, albeit just a bit differently than our ABCs. As an adult, we follow a flow chart — the basic fundamentals of an electrical circuit — which is like following along with the traditional kindergarten ABC sing-a-long song.

As with the ABC song, nearly every type of repair scenario starts with the right approach, and the right starting point can make all the difference. If you start on the wrong end or somewhere in the middle, it’s like trying do the ABC sing-a-long song backwards. (OK, go ahead, try it.) It isn't so easy, huh? In this article we’re going to go through the ABCs of basic electrical diagnostics from easy to a somewhat complex electrical circuit diagnosis.

Early systems

Throughout the history of the automobile, electricity has been a part of its makeup. For a time, 6V systems were the norm. Then in 1955, the 12V systems became the standard. Positive grounded vehicles were popular for a while due to the fact of the woven fabric covered wire, which had the tendency to absorb moisture. The positive ground reduced the galvanic effect and corrosion that was common on the negative grounded vehicles of that era.

Then, during WWII, a plastic-coated wire (PVC) was developed, which greatly improved the wire quality and integrity tremendously, and the galvanic problems with the copper wires was nearly completely eliminated. This led to the standardization of the negative grounded vehicle.

Computer systems

Now with computer systems and high-tech components, the complexity of the electrical systems in today’s cars have certainly increased, but the basic principles of electricity haven’t changed at all. Voltage, amperage and resistance are still the three main concerns. However, the critical nature of each have been greatly increased and are by far more susceptible to environmental issues and circuit condition than ever before.

Simple circuit diagnosis – beginners only

Let’s use a simple bulb, two wires, and a voltage source as an example. Voltage runs from the battery through the bulb filament and back to the negative terminal lead, completing the electrical path, thus making an electrical circuit in its simplest form. Now, let’s look at what would happen if we took the negative lead off of the battery terminal.

Of course, as you would expect, the bulb goes out because current flow has ceased, but what’s happening to the positive voltage? Has it gone back to the battery and will decide at a later time to flow down the wire? No, not hardly.

This is a unique characteristic of electricity. Each polarity will reach out as far as it possibly can to find its opposite polarity (Talk about opposite attraction!). The disconnected lead is nothing more than an extension of the battery positive terminal (Okay, technically there is a touch of resistance added by way of the bulb filament). Keep in mind, the positive voltage is still at the end of that wire lead, and if that wire lead happens to find another pathway to ground, it won’t hesitate to take it. For the novice technician, when a scattering of electrical spark should appear, it is usually followed by the complementary convulsive reaction to the sparking wire.

Open (incomplete) circuit issues

One common occurrence is the open circuit problem. An open is exactly what was used in the previous example. In other words, an incomplete path of electrical flow. In most circuits, a loss on the positive side (such as a blown fuse) basically brings the entire circuit to complete halt (We’ll cover a blown fuse a bit later.)

On a few occasions, you’ll run across the dreaded “feedback” affect after the original positive signal has been compromised and another leg of the same circuit becomes the voltage source for the remainder of the circuit.

A typical issue would be a digital display picking up a stray or weak voltage from another component, or as in the case of some mid-80’s Chevy Blazers which had two sources for constant voltage for the dome light and cigarette lighter positive signal, if one leg of the positive circuit would blow, the other would bleed current over to its companion fuse, but only when the doors were all closed.

If you’d open the door, the dome light would go off but close all the doors, not only would the light stay on, but you couldn’t shut it off. The only way I found the open fuse was to test the fuses with all the doors closed (Thankfully, that was back in the day when I was quite a bit more flexible). With the driver’s door open and using a test light across the access points of the ATC fuse, all the fuses would check good (The contact points had voltage on both sides because of the feedback. One side would be the source voltage while the other was the feedback.).

However, if the connection point of failure (the open) is on the ground side, it can be an entirely different story. Since the negative is generally a common attachment point for more than one system, it’s likely that you’ll have some other circuit or some unlucky negative signal source becoming the surrogate ground even if it doesn’t want to be. This is commonly referred to as a voltage drop (another later discussion).

When checking these circuits, keep in mind, if your measuring tool, (i.e. multi-meter, test light etc. is connected to a known good ground, and you touch the unconnected ground lead, you will see current flow (Okay, maybe a bit lower voltage or a dim test light bulb), but you will see something because you’re “after” the load has been applied to the circuit (A probe that will show continuity at the same time as the current/voltage is a better choice for this test).

Practice using one of these tools before attempting it on an unknown circuit. Accidentally inducing a voltage or even a ground signal in the wrong part of a circuit such as a computer lead can be hazardous to your pocket book.

Keeping in mind that no matter if it’s a 5V circuit or a 12-V battery supply lead, a loose or disconnected ground lead has the same potential to cause chaos in the systems. For example, if you’re working on an instrument cluster problem where all the gauges are all reading off (or not at all), the most common issues would likely be the instrument cluster main ground. Chevy S10 pickups through the 90’s had the main instrument cluster ground attached with the same bolt that held the parking brake release lever in place, so after a few years of releasing the parking brake, the bolt would work loose.

The stories a customer would tell you about how they were driving at night and when they’d hit a bump, there was this horrific electrical zap by their left knee was more than a little entertaining to say the least. Of course, as an afterthought, they would mention, “And then all the gauges starting working again, even my dash lights (Not knowing the zap, and the gauge problem were one in the same)!"

Now, let’s take that same scenario but incorporate a computer or module with various points for the electrical signals to follow, then create an incomplete path for all of these polarity conscious energy sources, and what do you think will happen? Comparing this to our simply bulb circuit, or the electric light show from one of those old S10’s, and the results are quite different. Now there’s a better chance of having a service code lead you in the direction of the repair rather than a zap from a loose connection.

Electrical diagnostics has sure changed from when I first started in this business. Back then, a simple test light and a DVOM was about all a guy needed to perform nearly every functional test that was out there. That’s not the case anymore. In fact, a test light can be as misleading as some of your customer’s explanations of their vehicle’s problems. Today, it’s a much more detail-oriented endeavor.

However, the basic electrical formulas and fundamental principles remain the same. (You know, those ABC’s) Not that good connections, and a constant supply voltage wasn’t a concern back on those early 50’s 6V systems, it’s just a whole lot more critical in today’s micro circuit-computer controlled contraptions.

Loose connections a few decades ago might bring a customer in with a blinking headlight, or the ever popular “it only works when it wants to” syndrome. Oh, how times have changed.

Shorted (grounded) circuit issues

Here we are at a blown fuse. Yep, back at the fuse box. The common check points for any electrical circuit, and for good reasons, too. Grounded, or as they are sometimes called, short circuits, are just as the name implies. The voltage supply has found a shorter path to follow than what it was designed for. Generally, in today’s electrical systems the positive side of the circuit is fused. In our example circuit, the fuse would be placed between the positive battery source and the bulb (which can be stated as the work that the circuit is performing). But, the big factor is where in the circuit has it be shorted to affect the fuse? The easiest way to remember this is to ask yourself, “Where is the work at?” The work, in this case, is our bulb itself. So, in order to be a shorted circuit (a positive signal going to ground) we have to introduce a negative potential somewhere between the fuse and the work (the bulb). Not between our voltage source and the fuse, or on the opposite side of the work either. The short has to be between the fuse and the load for any chance of the fuse to disrupt the flow. In other words, every fuse in a car can only blow because something prior to the work has caused the fuse to reach its maximum amperage load.

Just to be clear, there are reasons for a fuse to blow other than a grounded positive signal. I’ve seen voltage differences cause an issue. Such as 15 amps getting sent back down a 10-amp power lead. Not as common, but it does occasionally happen, or as in the next section, a loose connection. Or the work is internally shorted.

Loose connections and heat issues

Loose connections are a form of voltage drop, voltage loss and voltage spikes. Something else to keep in mind when it comes to loose connections, as the connection starts to loosen the current draw can increase as the temperature starts to increase. This heat and can lead to the eventual failure of the connection. If you ever looked at a blower motor on a scope, and recorded what the current does when it is first turned on, you would see a huge current spike or ramp-up at the initial startup. This is quite normal for any electrical motor, (not as prevalent with brushless motors these days) this current ramp only last for a quick burst of energy, but if it remained high longer than designed it could certainly be another reason for a fuse to blow.

Too many times in the past, that I’ve had to go back into a fuel pump replacement job performed by another shop that failed to take a closer look at the melted connectors. Note to self: Check both ends of the connector. Not just the end that’s easy to see. Chances are the other end is the culprit, you know, the one that’s harder to get to and everyone else ignores.

Simple tests – not so simple anymore

Not too many years ago a suspected fuel pump problem or a faulty starter motor could be quickly isolated with a couple of smacks of your trusty shop hammer. However, with today’s brushless fuel pumps and their PWM operation that trick isn’t as effective or for that matter recommended. The older brush type motors had the habit of just conking out after a short trip to the store, or just give up going down the road. A quick rap on the tank could jar the motor brushes just enough to get it going long to drive into the service bay. These new brushless motors don’t exactly conk out, they’re more likely to slowly drop their fuel delivery volume until they just stop completely.

But there are similarities between the two types of fuel pump systems. They both have an electrical connection that needs to be checked for melted or expanded connectors. In both types of fuel pump systems (as the pump starts to fail) the current draw will increase. Now, of course, with the PWM type, they’re pretty smart. They’ll set codes to inform you of the situation before it gets out of hand, but that doesn’t mean ignore checking the connector itself. Now, it’s a much better idea to check the STFT and LTFT and see if the percentages are staying close to 0 or at least not above about 5 percent positive trim. But, for the most part the current draw is the main factor that causes the service code to set which informs you of the possible fuel pump efficiency dropping.

Voltage drops

I can’t remember ever using the term voltage drop 20 years ago. If somebody came in with a headlamp with the typical yellow glow, we just called it low voltage or blamed it on a loose connection. Now, that’s commonly called a voltage drop. Today, the loss of one volt on a 5V circuit can be as detrimental as a completely open or grounded circuit. In fact, a loss of more than 50 milliamps on a grounded circuit is enough to cause all kinds of havoc. So the thought of a voltage drop is a dead serious issue these days.

I could shorten this whole thing down to a statement that a voltage drop is more or less a loose connection. True, but not always a correct answer. Take a look at some of the new fuel pump systems, for example. A lot of these new fuel pumps don’t run at 100 percent voltage input, but at a 50 percent duty cycle or even less. Running at a duty cycle means that the pump pressure and fuel volume can be controlled by varying on and off time of the fuel pump. However, with some of these other new systems, it’s not a duty cycle at all but a controlled voltage/current reduction from a PCM or fuel control module.

A voltage reduction basically makes the fuel pump run at a slower rpm when the fuel volume and pressure requests are low. Basically, it’s a forced voltage drop (usually around 9V) by way of the FPCM (fuel pump control module). So now, (for all you voltage checkers out there), if you checked a fuel pump and saw only 9V it may not necessarily mean you’ve got a voltage drop issue. It might actually be in perfect working order. Check the manufacturer’s info for the method they’re using for the fuel system you’re checking before condemning it.

But, let’s get back to voltage drops, the real ones. Yes, a voltage drop is usually found at a ground connection(s), but is just as easily on a positive connection too. Usually at a factory connection or spliced joint. I’ve found factory multi-crimped joints fail that produced a lower voltage on one wire and not on the other leads in the same multi-crimped joint or junction. Even though the term voltage drop is generically used to describe a lower than designed value on a circuit, the same thing can be said about a resistance value dropping or more to the point… increasing its resistance than the designed value.

Hi and Lo CAN quick checks

This is very apparent when you’re working on a high or low CAN lead. Many times the test procedure will say to disconnect the suspected component and recheck the signal. Even though the test is accurate the results may not be, or as in some cases, it will leave you with an assumed correct answer, such as a CAN line that has an extremely high resistance value (60 ohms per terminating resistor – 2 terminating resistors in parallel – 120 ohms in total is the norm). But after disconnecting a component such as the IC or perhaps the PCM your values are all over the place. You could make the assumption the component must be at fault, only to find after replacing the module, nothing has changed.

It’s another case of skipping over some of those boring steps in the middle of the diagnostic procedures and blindly go to the end of the diagnostic tree looking for a quick fix. Now you’re jumping to conclusions without testing for any of the possible causes as closely as possible.

So you head back to that section of the diagnostic tree that said to ohm check each lead, wiggle test each lead, and verify continuity. That’s when you notice any wiggle of the connector the resistance value changed. The solution, find the bad spot, which in this case it was a faulty factory crimped joint where several leads joined together.

Less than 5 ohms per line is considered good to go, but don’t forget to wiggle the harnesses and connections to be sure you’ve covered all the possibilities. Anytime you see a junction or wire connection in regards to a sensitive circuit, you should pay close attention to the procedures to avoid changing good parts for good parts. In other words, don’t jump from one end to the other without checking the middle — do your ABCs. (Another quick tip is to use your thermal gun (temp gun) at these multi-connections. Look for a temperature rise at the connectors.)

How important are the tools of the trade?

Let’s face it, not having the right tool always makes the job that much harder. There are the factory scanners that can cover every aspect of their manufactured models and several outstanding aftermarket scanners that can do some extraordinary and complex issues. A good multimeter (I prefer one with a min/max feature) and of course a good scope. A good two-channel is a great place to start, but it won’t be long before you’ll want a four or six channel.

Having the right tool but not knowing how to use it can be just as disheartening. Too many times I’ve seen guys shy away from repairs because they are intimidated by a scanner. There are classes everywhere that can help you learn how to use your tools more efficiently. Car repair has evolved into literally a college level job and has left the knuckle busting socket jockeys down on the lube rack. Let’s face it, it’s more geek than grease these days.

You do have one advantage today that wasn’t as available back in my younger years — the internet. With one search you can find out a lot about automotive repair and the tools of the trade. But, that’s a whole other story.

About the Author

Scott "Gonzo" Weaver

After more than three decades as the owner of an automotive electrical repair shop in Tulsa, Oklahoma, ASE Master Tech Scott “Gonzo” Weaver now writes and teaches the latest automotive technology. As a storyteller, he has hundreds of published humorous and anecdotal stories that can be found on his website, Gonzo is also the author of the book, Hey Look! I Found the Loose Nut.

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