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Reading a schematic

Reading an electrical map means little if you don’t know how to drive.
Thursday, June 20, 2013 - 10:21
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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.

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