Lucas Wiring

From Moss Motoring, the quarterly newspaper of Moss Motors. Reprinted with permission in the Austin-Healey Magazine, June-July 1987

Lucas wiring systems used on virtually British cars since the 1930's are a source of frustration and bewilderment to a great many sports car enthusiasts. In fact, Lucas wiring is clearly engineered around a standardized color code and cable size formula. This system is used on all British sportscars and once understood is very simple.

The following detailed explanation has been excerpted from a Lucas technical manual which dates from the mid 1950's. The professional mechanic or die-hard enthusiast may wish to clip out this article for future reference. Alter all, this information could be invaluable in sorting out the 'Manumatic' gearbox wiring of your 1957 Borgward Isabella estate wagon!

With few exceptions, the electrical system of a motor vehicle can be considered as a series of simple circuits, each consisting of the component, its switch and three wires: feed, switch wire and return. On earth return systems, the return circuit is provided by the frame of the vehicle, although in the case of components insulated from the chassis, an earth lead is also necessary. Some variations are to be found, such as fuses, two-way switching and so on, but the principle of feed wire, switch wire and return remains, and it is upon this principle that the Lucas color scheme is based. The insulation on feed wires carries a main color only, switch wires have the main color of feed with a colored tracer running the length of the wire, while return earth leads are black.

Where components are switched or controlled in the earthed side, that is, with the switch wire on the return side of the unit instead of on the feed side, this is normally indicated by the use of a black tracer.

Main colors, of which there are seven, are allocated to the circuits as shown below. The practice of feeding certain of the accessories through the ignition switch and auxiliary lighting circuits through the side-and tail lamp switch is recommended, so that the side and-tail-lamp switch and ignition switch wires become feeds to other circuits or, in effect, master switch wires.


  • BROWN: Battery circuit. From battery or starter switch to ammeter or control box and (with compensated voltage control) feeding lighting and ignition switches (and radio, when fitted) from control box terminal. Also from starter switch to electric clock, inspection sockets and battery auxiliaries fuse (from which are fed electric horns, cigar lighter, interior lights, etc.)

  • YELLOW: Generator circuit. From Generator Terminal to corresponding control box terminal and to ignition warning light.

  • WHITE: Ignition circuit and all requirements essential when ignition is switched on but which do not require fusing, e.g., electric petrol pump, starter solenoid switch, etc.

  • GREEN: Auxiliary circuits fed through ignition switch and protected by the ignition auxiliaries fuse, e.g., stop lamp, fuel gauge, direction indicators, windscreen wipers, etc.

  • BLUE: Headlamp circuits. Fed through terminal on lighting switch.

  • RED: Side and tail lamp circuits. Fed from terminal on lighting switch. Included in these circuits are fog lamps, panel lights and other lamps required only when the side lamps are in use.

  • BLACK: Earth circuits. If a component is not internally earthed, a cable must be taken to a good earth point on the chassis.

Hopefully, the above information, combined with a proper wiring diagram for your car, will help turn that multi-colored mass of spaghetti into an understandable wiring system. Don't get discouraged; Lucas really did make an effort to use logic in their wiring.

Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

5 views0 comments

Recent Posts

See All

By Norman Nock Originally published in the Austin-Healey Magazine, June 1994 The little black box called the voltage regulator (or "control box") is mounted to the passenger side firewall in the engin

By Norman Nock Originally published in the Austin-Healey Magazine, August 1990 The colors of the electrical wires on Austin-Healeys tell us a story. If you are looking at a group of wires that are goi

by Mark Bramfitt Originally published in the Austin-Healey Magazine, December 1988 The Lucas electrical system has been the focus of jokes since the beginning of time. Most of the time, however, probl