Lucas Service Manual — Vehicle Wiring Circuits

This original Lucas Technical Service Manual discusses the various electrical circuits of the “modern British automobile” and defines the wire colors for each circuit to help in electrical diagnosis. This is a wonderful and thorough reference for Lucas wiring.

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The following is the introduction to the Lucas Technical Service manual:

The electrical circuit of the modern automobile can appear most complex to the unuinitiated.

Careful study however, of the wiring diagrams produced by the Lucas Organisation and the Vehicle Manufacturers will allow the student to study the various circuits both individually and as a whole. The study of each circuit separately before tackling the complete job, is perhaps the best method for the student to adopt, as with few exceptions the electrical system of a motor vehicle can be considered as a series of simple circuits, as for example the Lighting, Charging and Ignition circuits. Having grasped the fundamentals of these circuits it is then possible to consider as a whole the complete Wiring Harness or Wiring Loom.

Motor vehicle wiring has been, and continues to be, subject to many changes, mainly to

facilitate speed and convenience on the vehicle assembly lines. With the introduction of snap connectors and separate junction boxes, cable assemblies can now be made up as subsidiaries to the main loom. This considerably assists the manufacturer, whose vehicle in these days of mass production, is often produced at separate branch works in the form of units, i.e. engine, chassis and body, before final assembly. It will be readily seen therefore, that these harness sub assemblies play an important part both in the initial layout and later in service where rewiring of a damaged section becomes necessary.

Let us now consider the means of identification of this multiplicity of cables. As we have

stated, the electrical system is a series of simple circuits, each consisting of the  component, its switch and three wires, feed, switch wire and return. The Lucas colour scheme of identification for British cars is based upon this principle. Feed wires carry braiding of a main colour only, switch wires have a main colour of feed with a coloured tracer woven spirally into the braiding and return or earthing leads are black.

These then are the basic principles which will be enlarged upon in the following pages of this section of the course.

For the administrator and store keeper the sections dealing with cable sizes, their current

carrying capacity, protective sleeving, rubber grommets will be found invaluable as a general guide when considering the materials to stock, particularly where the rewiring of vehicles is an essential part of the service offered.

In compiling this section on wiring circuits, as installed on modern British cars, we have reduced the technicalities of the subject to a level that should be easily understood by all students.


'Lucas Service Manual — Vehicle Wiring Circuits' have 4 comments

  1. October 2, 2012 @ 7:10 am John Zoltner

    I definetely need help in diagnosing wiring problems that arise from time to time. The car is 35 years old after all!


  2. January 10, 2019 @ 10:23 am Dennis

    just purchased a 2 pin ignition switch 68-9304. There are no markings/labels on the pins/ can I tell which is battery?


  3. December 7, 2020 @ 7:11 am Bob Scott

    Please tell me more about your Lucas manual. I have a 1954 XK120 roadster LHD.


  4. August 24, 2023 @ 6:23 pm Doug

    I have recently acquired a 1979 Panther Lima Turbo. It has not been started in over 2 years. I have attempted to start it so that I am able to move it out if its current location and trailer it to my place. Engine cranks fine, but no spark. I swapped the coil (has 12v on the post, does become warm while ignition in) there are three wires, the 12v to the plus terminal, and two on the (-) terminal, INE goes to the distributor, I am not certain where the other goes. I am hopeful there may be a wiring diagram of this vehicle. Thank you.


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