Reading a schematic

This Mazda OEM diagram (taken from MotoLogic) highlights power and ground paths for you, depending on the key position you select.

Wiring diagrams are like road maps. You wouldn’t take a cross-country trip without consulting a map or a GPS, would you? But many techs will dive blindly into an electrical diagnosis without first consulting the schematic. Unfortunately, wiring schematics don’t come with a GPS navigation option. (The one neat exception I’ve seen is the OEM Mazda diagrams in MotoLogic. They are interactive and show current flow in the circuit under different key positions.)

No, for the most part we have to do it the way techs have been doing it for years. And you’ll find it isn’t all that difficult if you read through and follow all the steps. Just like reading a road map, the first place to start is with the diagram information, providing you with the position, arrangement of devices and terminals. OE diagrams often are all-inclusive, that is they show everything on one big diagram that is broken up into segments. Most aftermarket service information providers provide simpler diagrams, referred to as block diagrams that show only the components/wiring needed by an individual circuit. Most block diagrams also start with the power source at the top of the page and follow the path to ground, ending at the bottom of the page. This is followed by a color code chart, so you’ll know what colors the diagram abbreviations are referring to.

Just like reading a road map, you need to know what these symbols stand for to understand what “town” you’re in.

Think that too simple? Check out an OEM German schematic where all the colors are listed in Deustch! Next is the abbreviations list that will clue you in on what the component abbreviations stand for. Last, but not least, review the symbols that will be used in the diagrams so you understand what they mean.

Once you’re comfortable with the basic layout of the schematic (map) you’re using, it’s time to locate the specific address you want to investigate. On a wiring diagram, that would be the electrical component, or load, that is giving you a problem. If that load isn’t working, it’s because it has an internal problem or the current flow isn’t flowing the way it should. That means we need to identify the basic elements the load needs to operate: the source of power, the control(s) that determine when the load is on and the path that connects it all together.

This is the schematic of a Corolla headlight system. That’s a lot of wire!

If you are not as comfortable with tracing wiring diagrams as you’d like to be, I would recommend you Google “Wiring Diagram Color Coding by Jorge Menchu.” Menchu has been teaching wiring diagram color-coding seminars for years and offers his resources at AESWave.com. In his seminar, he covers the basics that many have missed in our industry. Menchu explains how to color code the wiring diagram in five basic parts using red to represent power all the time, green as grounds all the time, orange as power only when the circuit (control device) is closed, yellow as ground only when the circuit (control device) is closed and blue as a variable voltage. Most electrical faults can be found easily when you know what test results to expect, and Menchu’s method of color-coding helps you do just that. I’d also recommend you visit the Motor Age website and read The Electrical System Roadmap article in the February 2009 issue.

Something To Get You Started
For an example of how to get started improving your wiring diagram navigational skills, I’m going to use a typical block diagram schematic of a 2007 Toyota Corolla CE headlight circuit and show how to use it to troubleshoot a missing driver’s side low beam. If you’re typical, the first thing you’d do is replace the bulb. Hey, I would have done the same thing. And most of the time we’d probably be right. But what would you do when the new bulb also fails to work? 

Go make a copy of the diagram and follow along.

Now pick a wire and see if you can follow it back to power or ground. This is the power side.

Lots of stuff on this diagram, isn’t there? But don’t freak out quite yet. You already know that every electrical circuit has to have certain basic elements in order to work. The first thing we need is a load, or component that will do the job we want the circuit to perform. What other reason would we have for wiring stuff in? In this case, it’s the headlights, so let’s find them first and give them a little color so we can find them again easily.

With the load identified, it’s time to move on to what that load needs to work — power and ground. The object of the next step is to locate and identify the wires at the load that supply both. In this case, there are only two wires to choose from, so we’ve got a 50/50 shot. Seriously, though, in those cases where the load has more than two wires, just pick one and start following it to see where it goes.

I’m going to start with the one on top. In block diagrams, power wires generally head up the page while ground wires generally head down. This might not always be the case, and it doesn’t really matter. Like I said, I’ve got a 50/50 shot. This red wire with a white tracer (that is, a stripe that runs the length of the wire) first enters the fuse box where a 10-amp fuse provides the protection against shorts to ground for this circuit. A fuse will only be located on the power side of the load so I know what side I’m tracing now.

While this isn’t the path all the way to ground, I really don’t need to go any further. I’m looking for a problem that would affect only one side and anything past this will affect them both.

It’s also easy to see that just past the fuse, the power feed to the other headlight is spliced in. The two bulbs may have separate fuses, but they share the same power source. Onward and upward.

The red trail now changes from one with a white tracer to one with a black one. The path leads to the headlight relay, and by the look of the relay symbol, it’s on the switch side of the relay. What does that make the relay?

Control or Load?
A relay is an electrical component typically used by a control module to turn a high current load on or off. That means the relay can be considered a control device or a load all its own. In this case, we are on that part of the relay that acts as a switch and we would treat the relay as such. Am I worried about the solenoid side of the relay at this point? No, not until I suspect the relay’s not doing its job as a control device. Until then, I’ll stay focused on finding the power source I’m looking for.

The left low beam is the only light not working, and this is the part of the wiring schematic unique to it. The problem has to be here.

And we’re almost there. The red path now turns completely red as we exit the relay on the other side of the contacts. From there, it’s a short walk to the Main Headlight 40-amp fuse, and from there it ends with a notation that reads “Hot At All Times.”

What does that mean?

The source for the majority of automotive electrical systems is the battery. Eventually, that’s were we want to end up. In most block diagrams, the notation Hot At All Times means this point in the harness is a straight line back to the battery. You can verify exactly how it gets there by referring to a diagram called the Power Distribution schematic. This shows every circuit protection device on the car, the route from the batter to the device, and all the circuits that device protects.

“Hot In Run,” “Hot In Start” and similar phrases indicates the path back to the battery first goes through the ignition switch. You’ll find the details for these routes in the Power Distribution diagram as well.

Before you start randomly probing the harness, take the time to plan your route using the electrical road map, the schematic.

For our purposes, though, we have succeeded in identifying the power side of the load back to the source. On to the “Dark Side.”

With one half done, let’s go back to the headlight and find out how current passing through it makes it’s way to ground. The only wire left to trace is … what, another red wire with black tracer? But didn’t we already trace that wire up at the relay? We can’t have two power wires going to the headlight, can we?

No, of course not. Just goes to show you that wire colors aren’t necessarily unique to the one you have your hand on.

So let’s continue. It doesn’t take long to see that both low beams come together on this path. Joined, they both continue along to the Daytime Running Light Relay (DRL). Sure are a lot of wires coming off of this thing, aren’t there?

I’m So Confused!
Here is where a lot of techs start to lose it. They find themselves tracing a diagram to a complex component on a wire they want to believe is one thing but the colors are telling them it’s something else. Trust what you know. You’ll find, as Menchu is fond of saying, that you know more than you give yourself credit for.

The red/black wire at the DRL has to be the ground path for the low beam headlights, doesn’t it? With the complaint of only one low beam not working, do I care what the DRL does or how it does it? Think about that for a minute.

Test the fuse by checking for voltage at the probe points. A fuse can fail and still look intact.

Both headlights share this part of the path, don’t they? Anything that happens after the point where the two headlights meet would affect the operation of the both of them. And since we aren’t having any problems with the right side, I’m going to focus only on that part of the schematic unique to the left one.

So which side has the problem? That’s easy enough to find out. Turn the headlights on and measure for voltage with your multimeter at the red wire with white tracer. Take this measurement right at the headlight connector and ground your meter right at the battery. This makes sure you check the entire path you just traced on your diagram. Yes, I know I said I’m only concerned about that part of the wiring unique to the light, but testing the entire path is a best practice and a good habit to get into.

If there’s no power, then there has to be a problem between the bulb connection and the point where the power feed splices together. The fuse would be the next easiest place to check. Just be sure to check it on both sides. I have seen fuses that looked fine on a visual check but were still open. No power there either? Only one stop left, and that’s at the red/black wire that brings the power to the two low beam fuses.

What if there is power? Then we move our multimeter lead to the ground side. If we read the same voltage here as we did on the power side with the lights turned on, there is an open circuit between the bulb and the ground splice point. If we read a perfect 0 on the meter, the bulb filament is blown or the bulb is not making good contact in the socket. No current is flowing. Any reading between the two over, say, 0.50 volt indicates excessive resistance in that short section of wiring, But if that were the case, the bulb would be lit, just dim.

A relay can be a switch or a load, depending on what part of the circuit you are looking at.

And if Both Sides Worked?
You would still perform the same two tests, measuring the amount of voltage on both the power side and ground side of the bulbs as close to them as you can get. The only difference is where you would focus your efforts on the side with the problem. Because both bulbs are not working, you’d look only at those parts of the circuit the two have in common. You might even have to do some reading on how that DRL works, but it’s a sure bet it completes ground somehow.

That’s where your skill, training and experience come into play. Just like a good driver, you are comfortable with the idea of the basic operation of the system you’re taking on. If not, you’ll learn about that system first, won’t you?

And that’s what separates you from the amateur.

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