How to Read Wiring Diagrams

Reading wiring diagrams is an essential skill for any classic car owner even if car restoration and repair; i.e. Electrics by making available as many wiring diagrams as possible.

We have a repository of Free Wiring Diagrams for many popular classic cars and are actively adding to the archive but how to you read and interpret them so you can make repairs with confidence? In this guide we show you how stepping through a common type - the Lucas standard.

Wiring Diagrams - Basics

Most classic car wiring diagrams are simply a schematic of a series of simple circuits, each consisting of a component, switching and fee and returns cabling. On the majority of vehicles the return is an earth return systems meaning the vehicle body or chasis itself provides the return. These are easily identifable by multiple wiring points terminating against the vehicle body or substantail metal component e.g. top of the suspension mount.

On top of this basic construct sits additional electrical circuit complexity that is evident to various levels depending on the vehicle such as fuses to protect wiring circuits from harm, two-way switches, return feed switches etc.

A wiring diagram is simply a schematic representation of these elements usually roughly laid out aligned to component location in the vehicle, usually front to back - but not always.

Cable Colours

Each wiring diagram should come with a colour-coded legend of key so you can identify wires when your dangling your head underneath the dashboard with a torch in your mouth.

Lucas Vehicle Wiring Chart

Wire colours vary by publication but for UK classic cars build in the 1960s and 1970s the Lucas S.M.M and T. Standard was a common standard notation used across several manufacturers with a simple notation of single letter to indicate colour for example 'B' for Brown and 'P' for purple.

Identifying Wiring Diagram Components

Battery

Identify the battery on a wiring diagram to orient yourself to the main engine and ignition system components like the starter motor and ignition circuit.

Starter Motor

Starter motor, usually fed via the starting solution

Switch (One-way)

Basic one-way switch. Could be retractive, steering column stalk or button

Fuse

Circuit protection, typically part of a fuse box containing several fuses in line.

Light

Any filiment bulb device such as headlights, brakelights, interier illumination, dashboard lamps etc

Heater

Resistence element e.g. plug heater, cigeratte lighter.

Horn

Single or Twin horns

Rheostat

A rheostat is a device for measuring and adjusting electrical output based on resistance for example level of fuel in a tank or dashboard illumination

Solenoid or Relay

A switching device used to control current flow to a component e.g. starter motor or washer wiper mechanism