Saturday, 7 February 2015

Vehicle Wiring – Wiring Diagram Master class

Wiring diagrams! Not dissimilar to Marmite! Perhaps I should explain - you either love them, or you just don’t ‘get’ them. If you do have access to a wiring diagram, it can save you a lot of time when diagnosing faults or trying to understand how an electrical component or system works. So on the whole, they are useful, and they can save you a lot of heartache.

The biggest problem is consistency, or lack of it! Many manufacturers have their own idea of the best way to draw a vehicle wiring diagram, different representations that vary with manufacturer is the first of a series of confusing issues, add to this the increasing complexity of vehicles over time, plus the lack of harmonised wiring colours for vehicle electrical systems, and the whole thing becomes more complicated than it’s worth, especially when you can pay someone else to fix it!

In this blog feature, we’re going to show you the basic, most common types of wiring diagrams, and how to read them, so that you can decipher your auto-electrical hieroglyphics.

What is a wiring diagram?

Wiring diagrams are a graphical representation of the vehicles wiring system; they can cover the whole vehicle wiring system, or can just represent sub circuits or systems. With more modern vehicles, that have greater complexity, the wiring diagram is very often split into sections that represent the major vehicle circuit groups. There are different approaches and styles to wiring diagrams that vary according to the manufacturer. In addition, increasing complexity requires a different representation and visualisation than a simple circuit. In order to simplify this topic, lets take a quick look at the different ways of representing wiring systems:

Schematic Diagram:

This diagram type focuses on representing the actual flow of current around the circuit. Simple lines are used to show interconnections, and symbols are used to represent components. The drawing does not really represent to layout of the actual wiring system.

Pictorial Diagram:

Pictorial representation shows the circuit layout and position, using diagram elements that accurately represent the visual aspect of the components and circuit. They’re less useful for understanding how a circuit works, but, a pictorial view can help with understanding and explaining a circuit operation at a simple level

The above methods of visually representing electric circuits are often combined for vehicle wiring diagrams. Over the years different manufacturers have used various approaches and have often devised and developed something unique. It’s well worth familiarising yourself with the most common types your likely to encounter, then, you’ll be able to cope with most things you’re likely to encounter.

Some types of diagrams in use 

Some of the most commonly used approaches, by various manufacturers, for representing a vehicle wiring system are shown below. 

Current flow diagram

By far the most logical and useful type is the current flow diagram, used extensively by German manufacturers, and others (Japanese vehicles). Although the current flow diagram does not resemble the vehicle wiring layout in any way, shape of form, it uses current ‘tracks’ to represent sections of the wiring system, with cross reference markers where wires interconnect. Along the top of the diagram are the main supplies, the current flows down the page to the earth connection running along the bottom. The wiring system is broken into sub-sections, and with this approach it’s much easier to focus on a specific area (e.g. lights, ignition system) when following the circuit path. This type of diagram can be used to represent the more complex systems, such as those found on newer vehicles, in this case, the wiring diagram will be pages and pages of reading.

German Wiring

German based wiring colours are reliable and consistent, you’ll find the same basic colours used on German vehicles (VW group), as well as Fords and Vauxhalls from the 70’s and 80’s. Tracer colours are used to identify individual circuits that are generally fused. Here are the main base colours you’ll come across:
Main beam –White
Dip beam – Yellow
Side, panel and marker lights – Grey
Igniton circuits – Black
Battery feed – Red
Earth – Brown

Schematic Diagram

This diagram type shows the actual layout of the vehicle wiring harness, and the main groups of cable runs. Wires are shown as lines and are often identified with colour codes. The diagram type is more representative of how the wiring system is laid out in the vehicle, and specific, key points of interest, like earth points or main connector junctions are clearly identified as to their position on the vehicle, this can help loads when you’re trying to locate one of these. However, this type of diagram can be difficult to interpret, especially when you are trying to follow the path of an individual cable – my advice...photocopy the diagram and get your highlighter pen out! this will allow you to mark and follow the cable around the diagram. 

Classical wiring diagram

These were the first type of diagrams used to represent the vehicle electrical system, (which were, at the time, pretty simple). They are a combination of pictorial and schematic representation, and were sometimes in colour, showing the actual cable colours used. The diagram showed the actual layout of components, and each component was clearly identifiable as it was drawn as to closely represent the component itself. This meant less reliance on a diagram ‘key’ when using the diagram. This type of diagram works for less complicated electrical systems, but later vehicles (80’s onwards) were a bit too complex for the system to be represented with a diagram of this type.

British Wiring

Most British classics from the 60’s, 70’s and 80’s used BSAU7 wiring colours; this consisted of basic colours for the main function groups, with tracer colours to identify sub-circuits. Later vehicles were appended with additional extra wire colours to cover the additional complexity from extra accessories that were being fitted, like electrical windows, central locking, engine management etc. The basic colours you’ll see are:
Headlights – Blue
Sidelights – Red
Battery Feed – Brown
Fused battery feed – Purple
Ignition feed – White
Fused ignition feed – Green
Earth - Black

Other types

The above diagrams will cover the majority vehicles encountered. However, there are always exceptions to the rule – for example, French, or eastern block cars. This is where the ‘diagnostic skills without diagram’ come to the fore. However, if you encounter a more obscure vehicle, complete with a wiring diagram, then that’s still a good position to be in. Just be aware of the idiosyncrasies that you may come across, for example, French cars tend to use a simple wiring colour scheme with about 4 basic colours, with the wires ‘tagged’ at each end. The problem with this is that half way along a wiring loom; you won’t be able to tell one wire from the other! Note also that the wiring diagram for these vehicles is just as confusing! with many variations and undocumented changes, irrespective of this, something is always better than nothing, and a diagram will always help.


In summary, possessing the correct wiring diagram is a great thing. It is well worth studying the diagram, to familiarise yourself with your vehicles peculiarities and details, even when don’t have any electrical problems. Then, when electrical gremlins hit (and they inevitably do), or, you want to do some mods or upgrades, then you’ve done your homework in advance – forewarned is forearmed! There are some general pointers below that offer ‘best practice’ advice for dealing with extracting and processing the info. you need from your diagram:

  • Photocopy and mark-up – Take your original diagram, store it carefully and keep it in good condition. When you need to use it, photocopy it. You can even photocopy and enlarge the specific section you need to help see the detail. This allows you to keep the original pristine, so that you can mark up or use highlighters when you need to, to aid understanding and use of the diagram
  • Analyse in sections – Don’t try to use the whole diagram at once, break it down into sections (mentioned above). If you’re using the diagram to help diagnose a fault, break the fault down into sections as well. This is a more methodical approach and will help you deal with complex problems
  • Identify cable colours and connection points – Study the diagram for the circuit you are looking at. Try to separate it out and identify the basic colours used for feeds, earths, switches etc. This will help you when you are actively tracing cables on the vehicle

Final Advice

You are always going to come across a situation at some point, where you don’t have a wiring diagram. So, it is also worthwhile when you are using one, trying to make a mental note of how the diagram relates to the vehicle system itself. Also, any general conventions that you may come across – for example, the way headlight circuits are generally fused.
It’s well worth getting into the practice of being able to cope without a diagram and this will help you be more self-sufficient so that you can diagnose faults or understand a cars electricky, in the cases when you don’t have one.

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