Mk3 Roadster Lucas Main Wiring Harness Question

Several observations here. The Lucas company finished in the mid 1980s. The name is used by the Wassell empire to remanufacture parts, mostly these parts are adequate, but if you have any problem with the technial side of things then you should contact Wassell, but of course you won't get any technical understanding.

Any red wires in this loom will be considered EARTH. So connect it to the bike and 'go for it' or connect a continuity tester on any earth and 'chase' them all around.

The four red wires you have circled, are they joined together? what terminals do they have? Are  the Zener diodes earthed by paired red wires? (two red wires crimped to one ring terminal?)  Loom makers use the crimps to join the various reds together   A photo of the loom would be helpful. Disconnect everything. then connect one test lead from your multi meter to battery earth (positive) use the other test lead and check each red wire terminal for continuity. You should see all the reds are a whole (connected together)    

This exact same harness.  The four reds go to a suitable earthing point.  This harness exactly matched the previous old harness, unlike another one I bought!

There is also an important modification required to this wiring diagram. It's on Grant Tiller's web site:

Grant Tiller | Bad Earth

I would have thought it better to delete the unreliable (highly stressed by movement) wire to the cylinder head and keep the loom earth to battery positive wire on the Mk3. Just my opinion.

Regards, Al.

Also search “Mk3 Commando final wiring” in the box at the top of this page for more info.

The wires you are referring to wouldn't be above the oil tank? as I had a similar problem with an Andover Mk3 loom, after contacting Andover I was told that my loom had been supplied without the correct connector and are linked to a common connector. Posted are before and after photos. 

I would have thought it better to delete the unreliable (highly stressed by movement) wire to the cylinder head and keep the loom earth to battery positive wire on the Mk3. Just my opinion.

Yes BUT, what you are forgetting is that the ignition -spark plug/coil has to have a route from the spark plug back to the battery/earth. Bearing in mind that the Commmando engine is wobbling in 'rubber'. But again you have the starter current large cable to the crank cases so perhpas this takes the strain?

Yes but, I wasn’t forgetting the spark plugs at all, we are talking Mk3 so starter earth lead is all that’s needed.

Regards, Al.

The wiring loom mentioned above is NOT for a MKIII Commando. The original MKIII with its Hi Output alternator had 2 zenor diodes fitted on the AC side of the rectifier (on the two zed plates). In some respects this issue is accademic as anyone rebuilding a MKIII would not have access to new zenerdiodes-there isn't any new ones since the mid 80s You could have the old ones checked (I can do this). OR you would probably change to a modern regulator/rectifier for battery charging. In which case the loom shown could be used, but there is also the potential issue of Electronic igniton-another few issue to over come/understand within the wiring supplied.

You might also notice wires going to the 'starter motor' not so this would be the starter 'solenoid'.