The Electrical System Roadmap

Jan. 1, 2020
A wiring schematic is a line diagram of an electrical circuit showing all the components needed to operate the circuit and the wiring paths that connect them.

Looking for an electrical fault? The first step is to understand how the system works, and the schematic is one key in doing so.

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A wiring schematic is a line diagram of an electrical circuit showing all the components needed to operate the circuit and the wiring paths that connect them. Back in the early days, a diagram including every circuit on the car would only take up a few pages. Today, it can add up to a small book. That is one reason that many schematics have been simplified into "block" diagrams, focusing on just one circuit and showing only those elements needed by that circuit.

This already makes our job easier, and that is the style of diagram I will show you today. However, if you do use an older style diagram or an OEM diagram that still shows the entire vehicle's circuits on one schematic, the method I will share with you will work just as well.

But before you can learn to read a wiring schematic, you need to understand a few basics (see January 2009 MA, Electrical 101). Specifically, you need to understand the basic elements that make up a typical electrical circuit. They are:

  • A source that supplies the power to the circuit, typically the battery.
  • A load, or the electrical component(s) that actually perform the task the circuit was designed for.
  • A control that allows us to turn the circuit on and off.
  • A circuit protection device that prevents damage to the wiring in the event of a direct short to ground.
  • And a complete path for current flow from the source, to the load, and back again.

The goal of reading the schematic, then, is to identify all of these elements as they apply to the circuit we are troubleshooting.

Let's start with a relatively simple schematic, a horn circuit from a 2002 Buick Rendezvous CX (Figure 1). The customer is complaining that it doesn't work at all. I suggest you look up the same diagram in your service information system, print out several copies and follow along with me as we read it together and try to solve this problem.

Ready, Set...Almost

Take a look at the layout of this diagram. Typically, the power side of the circuit is laid out toward the top of the page, and the ground side of the circuit is laid out toward the bottom. You'll also notice a variety of schematic symbols used to represent the various components. Like a map, there is always a "legend" included in the information to help you identify what these symbols mean (Figure 2), if not already labeled as these are. Component locations on the car may also be indicated. If not, there is usually a component locator table included in the service information electrical section to help you find them. OEM service information often takes this one step further, identifying not only the component locations but the connector and splice locations as well.
Now take a look at any of the solid lines used to represent actual wiring. See the capitol letter abbreviations near them? These are the color codes for the individual wires, and they are the street names in our electrical road map. After all, there is a lot of wiring on a modern automobile, and it would make life unpleasant if they were all the same color! Here, too, there is a table listing these abbreviations to help you decode them. Make sure you look them up, as I once got burned reading a European schematic that listed the colors in German. How was I supposed to know that Gelb meant "yellow?"

One more point to see before we actually begin. I stated that most electrical circuits use the battery as the source of voltage for the circuit. But there is no battery listed on this diagram. Why?

It would be overly redundant. So only the power side of the circuit up to the fuse is typically shown on an individual, block style, diagram. To complete that path, we need a copy of the Power Distribution schematic, listed separately (Figure 3).

In our horn schematic, only the wiring feeding power to the load(s) will be shown. However, that fuse may also be on the power supply side of other circuits, not shown. The Power Distribution schematic will complete this section of the path to the source for us and also tell us if the fuse supplies power to other circuits we aren't aware of. The same applies to the ground side of the circuit. In order to trace the path back to the negative post on the battery, we'll need a Ground Distribution schematic (Figure 4). This also aids in diagnosing some circuit faults, as I'll share in a future article.

OK, Now We Can Start

When I am reading a schematic for the first time, I like to use a highlighter to make the path I want to follow later a little easier to see. Being an older guy, I also have a magnifying glass to see the path better, especially when looking at a more complicated diagram, like one for all the engine control components. With these tools in hand, let's get started.

The first step is to identify the load(s) in the diagram. What component does the work in a horn circuit? The horn(s), of course! Now locate and highlight the horns on this diagram (Figure 5). Notice that there are only two wires coming off of these components — one surely provides the ground path to the source and the other must eventually lead to the source on the power side. If there are more than two wires at the load, you will need to follow them all, but the goal is to find the two providing power and ground, identifying the complete path needed for any electrical circuit to work. If there are any others, we'll worry about those later.

Since there are two loads in this circuit, let's start with the one labeled Right Horn. Next to the symbol for the horn is the location information; this horn is hiding behind the right side headlamp assembly. Now take a look at the wiring shown connecting to this horn. See the "half circle" just before the line makes contact with the symbol? This is the schematic symbol for the connector used to attach the horn to the wiring harness. Notice, too, the letters A and B, as they tell you which connector pin in the plug is which. This comes in handy when dealing with a connector housing multiple wires, sometimes with identical color codes, like a control module connector. Your service information system will often have a listing of these connectors and their individual pin outs to help you locate exactly which one you want to test.

Now we are ready to start tracing our path back to, or from, the source. The block diagram is laid out with ground toward the bottom, so I'm going to start there. Identifying the ground side is usually the easier of the two. Let's follow the wire marked at connector pin B and see where it leads us.

Did you see the abbreviation BLK? That tells us this is a Black wire. Because there are so many wires on the car, many wires have two colors and would be listed something like this: YEL/BLK. The first abbreviation is the base color of the wire, in this case, yellow. The second abbreviation is the color of the tracer, a stripe that runs the length of the wire, separate from the base color. If this were the case, I would be looking for a yellow wire with a black stripe running along its length.

The next stop we come to is a solid dot, labeled S123. This is a wiring splice, and it means that the rest of the path from here on is shared by other components. But since they are not shown on the diagram, they are not related to the function of the horn circuit.

Following the black wire leads us to the symbol for ground. This, too, has a location and identifier listed beside it. Here, it is ground point G100 and it is located on the right side of the engine compartment. But that's not the battery. Our path is not yet complete. Do you have a copy of the Ground Distribution diagram? Look through the sections until you see this same ground point (Figure 6). This ground point is attached to the cross member, and from there it travels through the body to the body ground strap attached to the negative battery cable. That is the complete path you are looking for. Remember, the path is NOT complete until it gets back to the source (the battery, for the majority of the circuits you will be troubleshooting).

Now do the same for the Left Horn. Notice anything different here?

The left horn is grounded to the same cross member, but at a different location — G101.

These grounds are independent of one another. Do you think if one of these grounds was left unattached, that one of the horns may not work? When diagnosing electrical faults, it helps to see if more than one load is affected. If so, then focus troubleshooting on those portions of the circuit shared by all. If not, then focus only on those portions unique to the load that isn't functioning. But I'm getting ahead of myself.

OK, so far we've identified the loads in this circuit, and the path from the loads to the ground side of the battery; half of the complete path we need. We still have to identify the path to the positive side of the battery, the control(s) used and the circuit protection device(s) used.

Time To Go The Other Way

Let's go back to the right horn and look for the path back to the positive side of the battery. There is only one wire left to follow, a Dark Green wire at connector pin A. It won't take you long to follow it to a box with its own connector identifier, A4-C2 (Figure 7). This means that this component has more than one connector going to it, and the one with our wire is Connector 2. The specific pin in connector 2 is A4. All of this is entering a box, outlined with a dashed line. The dashed line means that NOT ALL of the components contained in this box are shown, only the ones that relate directly to the horn circuit. Can you tell me what this component is?

That's very good. It is the Underhood Fuse Box, and according to our schematic it is located on the right side of the engine compartment, near the battery. So this is one of the connectors plugged into the fuse box. But we still haven't gotten to the battery, so let's keep tracing.

Next, we come to another connector pin, labeled X7. This connection is part of a component outlined in a solid line, meaning everything is shown. What is this component? Looking off to the side of the block, it is identified as the Horn Relay. Continuing our trace we come to a stop at the schematic symbol for a switch. Is that a control? Does it turn the circuit on and off, by opening or closing the path we need? It sure does!

On a schematic, controls are shown in their normal, at rest, state. In this case, the switch is open and no current will flow. That makes sense, doesn't it? Don't want the horns on all the time, do we? It also has additional wires attached to it, meaning we will have to eventually follow them all to see if they are part of this horn circuit. But for now, we want to focus on finding the path back to the battery and complete it first.

Continuing the trace through the switch, through the connector pin identified as X5, we next come to a solid dot connecting more wires to our trace (figure 8). The dot means these wires are spliced together, and that leaves several paths for us to follow, and we're still looking for the one that leads to the battery. Think of the dot as a stop sign at an intersection. Follow all the paths from that dot until you identify the one back to the battery. Making a left turn leads to the relay again; that's not the battery, but may still be important. Let's save that path for later. A right turn straight up leads to the Horn Fuse, fuse No. 23. What role does the fuse play? It is the circuit protection device for this circuit; protecting the harness from meltdown should a direct short occur. The schematic tells us this should be a 15-amp fuse, contained in the same Underhood Fuse Block as the horn relay, and that it is HOT AT ALL TIME. Still not the battery, but getting warmer. What diagram would you use next to complete that path?
Boy, you are getting good, aren't you? The Power Distribution diagram, of course! Picking up where the horn schematic leaves off reveals that this fuse is powered directly by the battery (Figure 9). If our horn diagram had listed this fuse as HOT IN RUN, HOT IN ON or HOT IN START, we would see power first routed through the ignition switch before arriving back at the battery.
Let's see what we've accomplished so far (Figure 10). We have the loads identified, the circuit protection device identified, and the path from the source to the load and back again identified. That only leaves the control device, but we found that already, didn't we? It's the switch incorporated in the relay, isn't it?

How About That Relay?

Yes, we did indeed locate the control for the horns. The circuit path to the horns is completed when the switch in the relay closes. But what makes that happen? Is it a manual switch on the dash the driver has to operate? There were two more wires attached to that device we still need to follow.

If you're ahead of me, you have already found the power supply to the relay. That was the first turn we took just before we got to the fuse and it enters the relay at connector pin W5. From there, it passes through the schematic symbol for a coil winding and out to connector pin W7. Let's continue our trace.

From this connector pin, the wiring exits on the Underhood Fuse Box connector C3 on pin A8 as a Black wire, through the Turn Signal/Hazard Switch, the Inflatable Restraint Steering Wheel Module (the clock spring) and finally through the Horn Switch to ground. Wait a minute? Power in one side of the coil and ground on the other side (Figure 11)? The coil in the relay must do some kind of work, huh? If it were a straight path from the fuse to ground, without a load, that would be a direct short and the fuse would blow, wouldn't it?

Anytime you find power in one side and ground on the other side of a component, that component is a load. A relay is an electrically actuated switch, and it can be considered both a load and a control, depending on which portion of the relay you are testing. The same is true for control module drivers. Did you notice a second path from the relay at A8 to the Body Control Module? Looks like another control device, doesn't it? Since there are two distinct paths on the ground side of this relay, that tells us that either the horn switch or the BCM can turn on the horns. The driver in the BCM is the control, but the BCM itself is a load. And like any load, it has to have the same elements that any other circuit does: a source, a circuit protection device, a control and a complete path from and back to the source. Now we can tell how this circuit works.

Putting It All Together

With the horn button on the steering wheel depressed (turned on), the path through the relay coil is completed and current flows. This current builds a magnetic field in the coil, and when the field strength is strong enough to overcome the spring tension of the switch contained in the relay, it closes.

The BCM isn't needed to operate the horns manually. Without doing some digging, I would hazard to guess that this is part of the antitheft system on this car, allowing the BCM to sound the horns in the case of a detected break-in.

With the relay switch closed by either control, the current path we first outlined to the primary loads, the horns, is now complete, and they sound off.

Back to our complaint. If neither horn worked, on which side of the circuit do you think the problem might be? How about if only one horn worked? What would your first troubleshooting step be?

But that's a topic for another time...stay tuned!

An Electrical Challenge

Think you've got reading schematics down pat? Then try this one on for size — a 2003 Chevrolet Malibu LS Cooling Fan Circuit. This is a dual fan, dual speed circuit and what I want to know is, "How does the low speed circuit work?" You can go to the Technical Discussions group at the Motor Age community, AutoPro Workshop, at: Just click on the "Community" tab, and then go to the Technical Discussion message board.

The answer will be posted there.

Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier directly at

About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels. 

Connect with Pete on LinkedIn.

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