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.
Figure 1
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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
Figure 2
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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.
Figure 2 continued
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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?
Figure 3
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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.
Figure 4
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