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How to read automotive wiring diagrams

Learn how to navigate the electrical circuit roadmap
Friday, January 27, 2017 - 08:00
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Wiring diagrams and road maps have much in common. Road maps illustrate how to get from point “A” to point “B.” However, instead of connecting interstates, highways and roads, a wiring diagram shows major electrical systems, sub-systems and individual circuits, all inter-connected. Another feature they have in common are layers of detail. For example, if you look at a road map of California, you won’t be able to locate a street address in Los Angeles. You might find a city or town, but you won’t find a specific address. In order to find the exact location of a particular residence or building, you would need a detailed street map or go online and use Google Maps or the GPS feature on a smartphone.

While this wiring diagram for a 1979 Ford Mustang is dated, the skills required for using it to diagnose an electrical problem are no different than when viewing an online diagram from a late-model automobile. Unfortunately, there are no instructions as to how to actually read, and/or interpret most wiring diagrams whether in print, on a DVD or online.

Wiring diagrams and road maps have much in common. Road maps illustrate how to get from point “A” to point “B.” However, instead of connecting interstates, highways and roads, a wiring diagram shows major electrical systems, sub-systems and individual circuits, all inter-connected. Another feature they have in common are layers of detail. For example, if you look at a road map of California, you won’t be able to locate a street address in Los Angeles. You might find a city or town, but you won’t find a specific address. In order to find the exact location of a particular residence or building, you would need a detailed street map or go online and use Google Maps or the GPS feature on a smartphone.

The same is true (to a lesser extent) of wiring diagrams. Vehicles made before the 1970s usually had their wiring diagrams contained on one, or two pages in a service manual. By the 1980s the complexity of automotive, on-board electronics changed and most vehicle manuals had multiple pages of wiring diagrams to show all of a vehicle’s electrical system. In the 1990s printed service manuals started to disappear and now manuals, and wiring diagrams are found on digital media or online. There is one aspect of wiring diagrams that has unfortunately remained constant. They lack directions regarding how to actually read them. Similar to a map, wiring diagrams will have a legend where symbols and naming conventions are spelled out but no “how-to” instructions.

While online automotive service manuals are written with the “professional” technician in mind, every technician had to learn to read and interpret wiring diagrams at some point in their career. The design and layout of wiring diagrams do not accommodate intermediate, or entry-level technicians by starting with easy-to-understand circuits that become progressively more difficult to read and understand. This article will take a different approach and begin with simple circuits and wiring diagrams, then move on to diagrams with more complexity. This step-by-step process not only makes learning to read a wiring diagram less painful, it promotes a greater understanding of how electrical circuits operate. Becoming more proficient at anything, including reading wiring diagrams, involves practice and there are some challenging questions included for that purpose as well.

A light bulb powered by a battery illustrates the 3-Things that all 12-volt electrical circuits must have to operate — Power, a Load Device and Ground Return. While this may seem obvious, locating the 3-Things, plus whatever controls the circuit, on a wiring diagram that spans many pages is not a simple process.

3 Things

The simplified wiring diagram of the battery, light bulb and wires is easy to understand. However, if this same circuit was more complicated and included several relays, multiple power sources and a computer controlling the entire circuit, the resulting wiring diagram would be far more challenging to read. A quick review of basic electrical circuits will make understanding how they are depicted in a wiring diagram easier to understand. Every electrical circuit on an automobile has to have 3 things to operate: 1) a power source, 2) load device, and 3) a ground return. The charging system and battery function as power sources and are extended throughout the entire car by way of numerous wires. Load devices are simply anything that does electrical work and can include lighting, starter motor, on-board computers, relays, power windows, keyless entry and many other components. The ground return completes the electrical path from the battery positive terminal, to the load device and back to the battery negative terminal. If any of the 3 things are missing, the circuit won’t operate and wiring diagrams provides a “map” to assist in determining which of the three is not present.

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