British wiring basics

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British wiring basics

Postby bfleissner » Fri Oct 09, 2009 11:57 pm

I found this info in the 'Q' and 'A' section a 2003 British Motoring pdf and thought it was good to know and something good to share.



British sports cars have a very simple and effective wiring system, and it's pretty easy to understand once broken down into circuits. Some of the reputation for difficulty dates back to the older cloth-braided wiring, which would fade with age to a uniform white color. This made tracing the wiring an interesting guessing game. More modern vinyl wiring is much easier to trace and identify. The color rules are the same for most British cars. The main wire color identifies the type of circuit.

Brown-Battery Circuit: Live all the time. Unfused, so be careful what you touch. These are the wires that go to the alternator/generator and feed the fusebox, ignition, and lighting switches.

Purple-Unswitched Auxiliary Circuit: This circuit is fused and provides power to items such as the lighter socket, trunk, and interior lights (all of which remain powered when the ignition is turned off).

White-Ignition Circuit: Switched by the ignition switch. Unfused, so again make sure that these wires do not get cut. From the ignition switch this circuit powers the electric fuel pump if fitted, coil, and provides an ignition-switched feed to the fuse box to power the auxiliary circuit.

Green-Switched Auxiliary Circuit: Switched by the ignition switch, this circuit is fused and powers the turn indicator, stop lamp, gauges, and windscreen wiper/washer circuits.

Red-Side and Tail Lamp Circuits: Switched by the headlight switch, this circuit also feeds the dash illumination lamps through either a switch or rheostat.

Blue-Headlamp Circuits: Switched by the headlight switch, this circuit feeds the High/Low beam switch, which in turn feeds the headlamps.

Black-Earth (Ground) Circuits: These wires provide the return path down to the chassis.


Many problems can be traced to poor chassis connections due to corrosion or new paint. Knowing these basic circuits, and armed with a test light, many common problems can easily be traced. Problems usually involve poor continuity at switches, connectors, or the fuse box.
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Re: British wiring basics

Postby hoggie » Sat Oct 10, 2009 10:51 pm

Good info. I think I've read about the color descriptions before. Very handy when tracing wiring problems.
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Re: British wiring basics

Postby admin » Mon Oct 12, 2009 9:12 pm

Yes good info...been thinking of adding a "tech tips" to the site, and this would be a good addition. Thanks!
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Re: British wiring basics

Postby rob66sprite » Sat Oct 31, 2009 5:06 pm

I look forward to building a wiring harness for my 66 this spring. I was plannning on doing some research and then building what "seems" right becuase I have worked with electrical systems for many years but will now use this guide to keep the car in line with traditional standards. Nice and simple color codes!
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