Joseph Lucas:  The Prince of Darkness.

(1890s translucent overalls whilst undeniably stylish proved sadly impractical..)

So there you are, grubby Haynes Manual in one hand, circuit tester in the other trying to work out why the lights/horn/wipers/cigarette lighter/clock etc. won't work, (apart from the fact that it has 'Lucas' stamped on it that is.)  Tracing the circuit usually works eventually but often you find yourself with several loose wires and no clues as to where they go, in which case you need a clue or two.

There is a British Standard for vehicle wiring number BS-AU7a which defines the colours and allows you to trace where the wires should go from first principles rather than a diagram. It also allows accessories such as electric fans to be added with the correct coloured wires which assists later diagnosis enormously.

None of this helps the fact that the things on the ends of the wires are usually made by Lucas and are therefore of er.. variable quality!   I'm particularly fond of a description of  an E-type Jaguar's headlamp switch as having" three positions: DIM, FLICKER and OFF.

Anyway, that's enough potentially libellous comments (I'm sure all Lucas products are simply fab nowadays and the repro stuff is only crap because it is so authentic :) This info should not only allow you to trace original wires but also to use the correct colours when adding additional equipment like spotlights, ejector seats or whatever.

BROWN is for direct, unswitched, unfused supply from the battery. Seen on wires to alternators, dynamos, control boxes and ignition switches etc. Always live and often carrying heavy current.

YELLOW sometimes used as an alternative to brown on older cars. Also used on solenoids and overdrive switches.

PURPLE Direct, unswitched but fused supply from the battery. On older cars with no purple wires, brown wires with a secondary tracer colour are used.

WHITE  Unfused supply from the ignition switch.

GREEN  Fused supply switched via the ignition switch. Used for things like the wiper motor, indicators and brake lights which only work when the ignition is on.

BLUE  The main colour for front lights- headlamps and spotlamps.

RED  The main colour for rear and side lights.

BLACK Usual colour for wires from components to earth points on the body.

These refer to the main colour of the wire. The second tracer colour which is on many wires is the thinner line and is used to identify the wire. There is a certain amount of logic to them too as you will see.

Below is the colour code table for BS-AU7a which was amended in the 'eighties so there will be a few variations on our '50s to '70s cars..

Main colour is given first , tracer  second

 MAINLY BLACK WIRES
 Black           All earth connections
 Black/ Brown       Tachometer generator to tachometer
 Black/ Blue        Tachometer generator to tachometer
 Black/ Red         Electric or electronic speedometer to sensor
 Black/ Purple      Temperature switch to warning light
 Black/ Green       Relay to radiator fan motor
 Black/ Light Green Vacuum brake switch or brake differential pressure valve to warning light and/or buzzer
 Black/ White       Brake fluid level warning light to switch and handbrake switch, or radio to speakers
 Black/ Yellow      Electric speedometer
 Black/ Orange      Radiator fan motor to thermal switch
 

 MAINLY BLUE WIRES
 Blue            Lighting switch (head) to dip switch
 Blue/ Brown       Headlamp relay to headlamp fuse
 Blue/ Red         Dip switch to headlamp dip beam fuse. Fuse to right-hand dip headlamp
 Blue/ Light green Headlamp wiper motor to headlamp wash pump motor
 Blue/ White           a) Dip switch to headlamp main beam fuse
                               b) Headlamp flasher to main beam fuse
                               c) Dip switch main beam warning light
                               d) Dip switch to long-range driving light switch
 Blue/ Yellow      Long-range driving light switch to lamp
 Blue/ Black       Fuse to right-hand main headlamp
 Blue/ Pink        Fuse to left-hand dip headlamp
 Blue/ Slate       Headlamp main beam fuse to left-hand headlamp or inboard headlamps when independently fused
 Blue/ Orange      Fuse to right-hand dip headlamp
 

MAINLY BROWN WIRES
 Brown       Main battery lead
 Brown/  Blue   Control box (compensated voltage control only) to ignition switch and lighting switch (feed)
 Brown/ Red    Compression ignition starting aid to switch. Main battery feed to double pole ignition switch
 Brown/ Purple  Alternator regulator feed
 Brown/ Green  Dynamo 'F' to control box 'F' Alternator field 'F' to control box 'F'
 Brown/ White  Ammeter to control box. Ammeter to main alternator terminal
 Brown/ Yellow Alternator to 'no charge' warning light
 Brown/ Black  Alternator battery sensing lead
 Brown/ Slate  Starter relay contact to starter solenoid
 Brown/ Orange Fuel shut-off (diesel stop)
 

 MAINLY GREEN WIRES
 Green            Accessories fused via ignition switch
 Green/Brown       Switch to reverse lamp
 Green/ Blue        Water temperature gauge to temperature unit
 Green/ Red         Direction indicator switch to left-hand flasher lamps

 Green/ Purple      Stop lamp switch to stop lamps, or stop lamp switch to lamp failure unit

 Green/ Light green Hazard flasher unit to hazard pilot lamp or lamp failure unit to stop lamp bulbs
 Green/ White       Direction indicator switch to right hand flasher lamps
 Green/ Yellow      Heater motor to switch single speed (or to 'slow' on two- or three-speed motor)
 Green/ Black       Fuel gauge to fuel tank unit or changeover switch or voltage stabilizer to tank units
 Green/ Pink        Fuse to flasher unit
 Green/ Slate       a) Heater motor to switch ('fast' on two- or three-speed motor)
                            b) Coolant level unit to warning light
 Green/ Orange      Low fuel level switch to warning light
 Light green       Instrument voltage stabilizer to instruments
 Light green/ Brown  Flasher switch to flasher unit
 Light green/ Blue   a) Flasher switch to left-hand flasher warning light
                                b) Coolant level sensor to control unit
                                c) Test switch to coolant level control unit

 Light green/ Red    Fuel tank changeover switch to right-hand tank unit or
                   entry and exit door closed switch to door actuator
 Light green/ Purple Flasher unit to flasher warning light

 Light green/ Green  Start inhibitor relay to change speed switch; or switch
                   to heater blower motor second speed on three-speed unit
 Light green/ White  Low air pressure switch to buzzer and warning light
                   Flasher switch to right-hand warning light; or
 Light green/ Yellow differential lock switch to differential lock warning light
 Light green /Black  Front screen jet switch to screen jet motor

 Light green /Slate  Fuel tank changeover switch to left-hand tank unit; or entry and exit door open switch to door actuator

 Light green/ Orange Rear window wash switch to wash pump; or cab lock-down switch to warning light
 

 MAINLY ORANGE WIRES
 Orange            Wiper circuits fused via ignition switch
 Orange/ Blue        Switch to front screen wiper motor first speed timer or intermittent unit
 Orange/ Green       Switch to front screen wiper motor second speed
 Orange/ Black       Switch to front screen wiper motor parking circuit, timer or intermittent unit
 Orange/ Purple      Timer or intermittent unit to motor parking circuit
 Orange/ White       Timer or intermittent unit to motor parking circuit
 Orange/ Yellow      Switch to headlamp or rear window wiper motor feed, timer or relay coil
 Orange/Light green Switch to headlamp or rear window wiper motor parking circuit timer or relay coil
 Orange/ Pink        Timer or relay to headlamp or rear window wiper motor feed
 Orange/ Slate       Timer or relay to headlamp or rear window wiper motor parking circuit
 

 MAINLY PURPLE WIRES
 Purple            Accessories fed direct from battery via fuse
 Purple/ Brown       Horn fuse to horn relay when horn is fused separately
 Purple/ Blue        Fuse to heated rear window relay or switch and warning light
 Purple/ Red         Switches to map light, under bonnet light, glove box light and boot lamp when fed direct from battery fuse
 Purple/ Green       Fuse to hazard flasher
 Purple/ Light green Fuse to relay for screen demist
 Purple/ White       Interior lights to switch (subsidiary circuit door safety lights to switch)
 Purple/ Yellow      Horn to horn relay
 Purple/ Black       Horn to horn relay to horn push
 Purple/ Pink        Rear heated window to switch or relay
 Purple/ Slate       Aerial lift motor to switch up
 Purple/ Orange      Aerial lift motor to switch down
 

 MAINLY RED WIRES
 Red         Main feed to all circuits mastered by sidelamp switch
 Red/ Brown  Rear fog guard switch to lamps
 Red/ Blue  Front fog lamp fuse to fog lamp switch
 Red/ Purple Switches to map light, under bonnet light, glove box light and boot lamp when sidelamp circuit fed
 Red/  Green  Bulb failure unit to right-hand-side and rear lamps
 Red/ White  a) Sidelamp fuse to right-hand side and rear lamps
             b) Sidelamp fuse to panel light rheostat
             c) Fuse to panel light switch or rheostat
             d) Fuse to fibre optic source
 Red / Yellow Fog lamp switch to fog lamp or front fog fuse to fog lamps
 Red/ Black  Left-hand, sidelamp fuse to side and tail lamps and number plate illumination
 Red/ Pink   Sidelamp fuse to lighting relay
 Red/ Slate  Lamp failure unit to left-hand side and tail lamps
 Red/ Orange   Fusebox to rear fog guard switch
 Red/ Slate     Window lift main lead
 

 MAINLY WHITE WIRES
 White             Ignition switch or starter solenoid to ballast resistor

 White/  Brown       Oil pressure switch to warning light or gauge, or starter relay to oil pressure switch. Choke switch to choke solenoid (unfused) and/or choke to
 White/ Blue        switch to warning light, or electronic ignition distributor to drive resistor. Starter switch to starter solenoid or inhibitor switch
 White/ Red         or starter relay or ignition (start position) to bulb  failure unit
 White/ Purple      Fuel pump no 1 or right-hand to changeover switch
 White/ Green       Fuel pump no 2 or left-hand to changeover switch. Start switch to starter interlock or oil pressure switch
 White/ Light green to fuel pump or start inhibitor switch to starter relay or solenoid
 White/ Yellow      Ballast resistor to coil or starter solenoid to coil. Ignition coil contact breaker to distributor contact
 White/ Black       breaker, or distributor side of coil to voltage impulse tachometer
 White/ Pink        Ignition switch to radio fuse
 White/ Slate       Current tachometer to ignition coil
 White/ Orange      Hazard warning lead to switch
 White/Yellow (or just plain yellow)        a) Overdrive   b) Petrol injection    c) Door locks    d) Gear selector switch to start

YELLOW WIRES
Yellow (or yellow/white) a) Overdrive   b) Petrol injection    c) Door locks    d) Gear selector switch to start

PINK WIRE(!)
Pink/ white       Ballast terminal to ignition distributor

That's all folks.  The usual disclaimer applies., ie  that This is correct to the best of my knowledge but you use this information at your own risk. So don't sue me if you blow your alternator or fry all your relays...

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Previous Features:
To save your poor machine having to download all the images from the articles you've already read , these pages are now stored separately. Click below to recall them...

1.      Le Mans Diary 2000  Five Go Mad at La Sarthe

2.    Gentlemen and Cads  The Terrible Turbo versus the Sublime Supercharger.

3.    Hidden Brooklands.  Poking around in the undergrowth.

4.    The Mysteries of  the Lucas Wiring System Explained. Well, you've got to have a system....

5. The Petrol head's Pilgrimage.  USA  Road Trip summer 2001

6.    Brazilian Car Stereo. Choose your weapons.- Bob Marley at 50 Paces.

7. Restoring a Constellation. Never mind cars,  Mike rebuilds airliners. - (admittedly not single-handed.)

8. Allan Winn - Breathing Life into Brooklands. Eoin Young interviews a fellow Kiwi

9. Son Of Mindless. The legends are true! A 4WD monster lurking in the Fens

10. Brooklands Double Twelve 2008. Event of the year.- Or how to compete against your own car..

11. Farewell to the QE2. Last voyage of a great liner