The Lucas motorbike electrical system

A series of articles and step-by-step guides to maintaining and improving the electrical systems on classic motorbikes.

The Lucas logo in a headlamp

Joseph Lucas, ‘Prince of Darkness’, alleged inventor of the world’s first self dimming headlights and intermittent windscreen wipers. Who’s light switches usually have three settings – dim, flicker and off. And who apparently still holds the patent for the ‘short-circuit’. The list of Lucas jokes is endless, but that really doesn’t help with keeping your old bike on the road does it?

In reality, the electrical systems on classic bikes (be they Lucas, Miller or otherwise) are pretty simple and straightforward to maintain and work on. Electronics has always been one of my interests so the dodgy wiring, temperamental Lucas E3NL dynamo and intermittent Lucas MCR2 voltage regulator on my old Matchless was more of a pleasant opportunity for some tinkering than a big headache. However, I realise that all this electric-trickery isn’t everyone’s cup of tea, and so that’s why I’ve put together the following series of articles and guides looking into various aspects of a motorbikes electrical systems.

Most of these articles have been compiled and written with respect to my cherished 1951 Matchless G3LS, but they are equally applicable to many other makes and models of motorbike from around the same era. Most of the principles and techniques described are also equally applicable to cars and other vehicles, as well as to other electrical manufacturers besides Lucas such as Miller. If your bike has an alternator charging system then that’s a very different beast to a dynamo, but you may still find the guides on fuses, wiring, switches and lighting useful. So please scroll down and see what takes your interest.

General system checks

This is my basic beginners guide to diagnosing potential faults with the Lucas electrical system of your classic motorbike, which covers:

  • Checks using the headlights.
  • Verifying the ammeter readings.
  • Testing the battery voltage.
  • Checking the wiring and connections.

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Testing a Lucas dynamo

If you suspect that your Lucas dynamo is at fault, this article will guide you through a series of simple checks and tests you can perform, including:

  • Disconnecting the dyanamo.
  • Checking the output with a multimeter.
  • Measuring the winding resistances.
  • Testing the dynamo under load.

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Repolarising a dynamo

Lucas dynamos can sometimes lose their residual magnetism over time or be incorrectly polarised, so this is a step-by-step guide to repolarising which includes:

  • How the Lucas dynamo works.
  • When to repolarise the dynamo.
  • Determining the correct polarity.
  • How to ‘flash’ the dynamo safely.

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Converting to 12 volts

Many classic bikes are wired with 6 volt electronics, but converting to a modern 12 volt system has many advantages. This guide includes:

  • Disadvantages of 6 volt electronics.
  • Advantages of converting to 12 volts.
  • Selecting a modern solid-state reg.
  • Upgrading the battery and fuse.

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Replacing the regulator

This is a step-by-step guide to replacing the old Lucas mechanical voltage regulator unit with a modern solid-state electronic device. It covers the following topics:

  • Selecting a replacement reg.
  • Making the new connections.
  • Selecting between 6 or 12 volts.
  • Installing a fuse for protection.

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Replacing the ammeter

Old ammeters often become inaccurate or fail completely over time, so this guide explains how to replace the gauge with a shiny new one, including:

  • Removing the old ammeter.
  • Installing the new gauge.
  • Checking the wiring and connections.
  • Verifying the readings are as expected.

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Installing a fuse

Most classic bikes never had a fuse fitted as standard, but fitting one is simple and can save you a lot of smoke and fried wiring! This article explains:

  • Why a fuse is so important.
  • Where to fit the fuse(s).
  • The different types of fuse available.
  • Selecting an appropriate fuse rating.

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Installing brake lights

Brake lights were still not fitted as standard to many bikes right up until the 1950′s, but they are essential on today’s roads. This article explains:

  • Legal requirements for rear lights.
  • Installing a rear brake switch.
  • Installing a front brake switch.
  • Changing the bulb and LED options.

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Improving your headlights

The headlights on classic bikes may not be as bright as those on modern vehicles, but there are some simple things you can do to help including:

  • Checking the quality of the wiring.
  • Installing a headlamp relay.
  • The most appropriate type of bulb.
  • Selecting the optimum bulb wattage.

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