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Old 6th August 2020, 06:40
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1970's BL era wiring is a nightmare of haphazard multicoloured wire.

When they made up the looms they seem to have used any old wire they had laying about the workshop, then published a circuit diagram as a huge office joke.

The only way to be sure of which wire does what is to trace it with a test meter, make all the connections in the circuit you're working on then, say a prayer, hold your breath, connect the battery and be prepared to grab the nearest fire extinguisher.

To answer some of your questions, the two-pull main light switch appears to give you: all external lights on the first click; all external lights plus dash lights on the second pull. All the variations of side, head and main beam are controlled by the left-hand column stalk.

The blue wire with the white stripe is the main beam/flash circuit, and it's one of the few colour combinations that seems to have been universally used across all production years.

Rather than the 'on' switch adding power to a circuit, a lot of the lights have a constant live feed to them with the ignition on, and switching them 'on' connects the earth to complete the circuit. This means that, just to muddy the waters, you'll often find power where you don't expect it when checking with a test meter.

The wipers are on a switch on the dash. I have a late 1500 loom and I can't find any wires that are supposed to connect to either the switch or the wiper motor. Good job I'm using a perspex flyscreen then.

Do you really need a reversing light?

If yes, there are a pair of wires that come out of the loom on the engine side of the bulkhead near to the oil pressure cable. There are then wires of the same colour (purple and white from memory, but who knows?) on each side of the rear lighting loom.

The front ones connect to the reverse light switch on the gearbox if you have one - Spitfire gearboxes have it, Herald ones don't - while the rear ones connect to the reversing lamps. I'm using a Herald gearbox so I'm using that circuit to power my electric fuel pump.

Anyway, if your loom's anything like mine, good luck fathoming out what everything does in your rats nest of randomly coloured electric spaghetti!
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