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Wiring Harness Diagram - / - Wiring Wizardry - / - Has Anyone Zener? - / - Lucas Service Information

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Electrical Wiring Harness

by Bob Cox

Wiring Wizardry

Some tips for wiring vintage bikes.

1) Use 16 awg, stranded copper wire. Make sure that the insulation is thick, to protect from abrasion.

2) Avoid the cheapie connectors marketed at after market auto shops. The AMP company makes 1/4" Faston spade connectors. Theses are used in many automotive, electrical / electronic & industrial applications, and they can be purchased from various suppliers to those industries. An especially nice style is the one that has the connector totally enclosed in plastic, so when the connector is mated, the male and female ends interlock to keep out dirt.

3) The plastic harness wiring covers are good to keep the wires together in a bundle and out of harms way. They also add an extra layer of abrasion protection. Tape the cover at regular intervals to keep the wires inside & from bulging open around bends.

4) To ensure having the electric’s working all the time, use an independent ground wire to connect your headlight, instruments, taillight, and other bits directly to the battery ground.

5) To add extra protection,

(a) slather them with dielectric grease to protect from corrosion

Edited from the Internet

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Has Anyone Zener?

Hello ?! "Has anyone seen my zener." Or in the German, "Has anyone seen mine zener?"

What the hell is this thing with a name that beings with the last letter of the alphabet and why should we care?

The Zener diode's job is really to act as an automatic valve that dumps excess electricity as required. To be technically correct, it is a voltage-sensitive semiconductor that only begins to conduct once the applied voltage reaches a certain "zener" threshold. At that point, the Zener begins to conduct more and more current, with very small incremental increases in its applied voltage. Since the Zener is effectively in parallel with the alternator via the rectifier, the applied voltage is the alternator's rectifed output: the Zener senses and dumps any excess electricity from the alternator by converting it to heat.

So, if your alternator is low on output, or other things use up the available power (like headlamps being on all the time in Norway), it's pretty clear that the Zener won't have much work to do: there isn't any excess electricity to deal with, because it: 1) isn't being generated in the first place, or 2) something like a high power headlamp is using it instead.

A higher voltage Zener can only help by letting the system output rise further before the Zener starts dumping electricity. A Zener can't compensate for a weak alternator, or excessive loading on the system. So on the flipside, in a properly operating system, removal of the Zener, and running with lights off can be disasterous because there's nothing there to shed the excess generated power.

Lucas's use of a Zener is pretty unsophisticated, since Zeners require a series resistance in the system to protect them from self destructing as the input voltage increases. (It's output voltage is not temperature

compensated either, the way lead-acid batteries like it to be). Lucas simply relies on the internal impedance of their alternator to rapidly choke down the available output voltage as the Zener comes on.

Reprinted from the Internet

Zenor Diode Comments
By Robert Baucom

Concerning Lucas electrics and Zenor Diodes ALWAYS make sure the Battery has a GOOD GROUND connection. At both the battery post and the frame connection. No corrosion allowed. (first wash with solvent, then Bicarb of soda), scrape, sand, bolt up and cover with grease (Silicon grease if you have it). There are now available, some neat, Hi-Temp greases associated  with auto disk brakes.

The reason this is so important: When there is a bad ground, you have resistance. When you have resistance, to obtain proper amperage (current flow), there is an increase in voltage (pressure). With this excessive increase in voltage ( the zenor opens at 13.5 volts if I recall) the zenor is dumping to ground constantly, creating a lot of heat. This destructive temperature causes that expensive little goodie to fail.

When working for Triumph, in 1970, we held a Dealers Mechanics school in Okl. City. Dennis Manning (BUB Pipes) of Bonneville Streamliner fame, was the instructor. Okl. City was on interstate 40 and was the the main, summer time, conduit from West coast to East coast (or the marijuana filled creek beds of Kansas). All the Hippy Dippies and other pond scum, came rattling down the pike on their jury rigged, death trap, Choppers with their dirty leg gals on the back. Being cool, they were cruising back & forth from coast to coast, getting sunburned, playing "Easy Rider" and avoiding their creditors.

One of my OKC dealers was close to said interstate. They got a lot of chopper trash, traffic with guys wanting to trade "The services of their scaggy passenger, aka. Giving everyone in the service dept a "Hot Water Skull Job", in exchange for a new rear tire or chain. His main mechanic, "Leeroy", was a slightly tattooed, down home, nose picking, crotch scratching, Harley rider with a slight speech and pronunciation defect ... He spoke out, "Why do these here Zen-E-R Die-odes cost so much money ($23 at that time)? The customers think I'm uh try'n to rob'um when I tell 'um they needs uh new Zen-are!" There was a group murmur, validating his query.  Dennis went into a technical dissertation, explaining the Zenor was really an electrical valve: manufactured to complete a circuit at a predetermined value of EMF. When the EMF exceeded this predetermined voltage, the molecular construction of it's internal components changed: thereby causing the "valve" to establish continuity to the outer body and/or ground.

Leeroy's (that really was his name) eyes glazed over for a moment. His eyebrows worked up and down a few times, as he mentally tried to digest all these "Physics Class", unfamiliar, technical terms. He closed his slackened jaw, stood up and asked, "Have yew ever tried to Ex-plain, Mo -lek-uh-lar Con-stuck-shun to a 'Hells Angel'? ... Everyone in the room, including Dennis, exploded with laughter. Leeroy stood there, beaming proudly, over having made a "Funny".

Dennis would start to simplify his explanation and then self distruct with laughter. After three of four tries, he wiped his eyes and said, "Tell the dumb son of a bitch, It's a top secret. If you told him, then you would have to kill him!" A still grinning Leeroy, sarcastically said, "Even kidding around, I'll be damned if I'm goin'' to tell a Hells Angel that, I was a-gonna kill 'em. 'Spes-lee with three or four of his buddies standing around."

Robert Baucom

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Lucas Service Information
Location and Remedy of Faults

Although every precaution is taken to ensure trouble-free running, electrical faults may sometimes arise and the following test procedure is recommended.

When checking the continuity of circuits, a flashlamp battery and bulb should be used. Do not "flash" the end of a live cable to earth, as this may cause heavy currents which damage the equipment. If a vehicle battery is to be used, a low wattage bulb of similar voltage must be connected in series with the circuit to be checked. Never check the continuity of alternator stator windings when the rotor is in position. The rotor should be removed to avoid demagnetisation and reduced output.

Battery Charging Systems

Engine will not start on IGN position:

  1. Turn the switch to EMG position if provided. If the engine now starts, the battery is probably discharged. If there is no EMG position, check the condition of the battery and recharge, if necessary.
  2. Remove the H.T. cable from the sparking plug and hold the cable end about 1/8" from a metal part of the engine while the fatter is turned over. If sparks occur regularly, the ignition system is functioning correctly. Check for engine defects after examining the sparking plug.
  3. If sparks do not occur in test (2), check for a fault in the low tension circuit. Check the wiring from battery to fuse, switch, coil and contact breaker. If the circuit is continuous, examine the contact breaker and if necessary clean and adjust the contacts. Also, check the engine timing.

Engine will not start on EMG position:

Carry out previous tests (2) and (3), check that the alternator rotor is the correct way round on the engine shaft (the name "Lucas" should face away from the engine).

Engine misfires:

  1. Examine the contact breaker. If necessary, clean the contacts and adjust the gap.
  2. Remove the sparking plug (or each plug in turn), rest it on the cylinder head and observe if a spark occurs at the plug points when the engine is turned. Irregular sparking may be due to dirty plugs (which may be cleaned and adjusted) or to defective high tension cables. If the insulation of any cable shows signs of deterioration or cracking it should be renewed.
  3. If sparking is regular at each plug when tested as described in (2), the trouble is probably due to engine defects, and the carburetter, petrol supply, etc., must be examined.
  4. If misfiring occurs after the engine has been running for some time, check that the ignition switch is in the normal IGN position. If run continuously in the EMG position, the rising voltage of the battery may eventually cause misfiring to occur.

A.C. Ignition


  1. Keep the contact breaker clean and ensure the maximum opening is set at 0.014'- 00l6'.
  2. Keep the sparking plug electrodes clean and correctly set.
  3. Keep to the manufacturer's timing instructions.

Engine will not start or misfires:

  1. Remove the H.T. cable from the sparking plug and hold the cable end about 18' (3 mm) from the cylinder block. Sparks should jump this gap regularly when the engine is turned at kick-start speed.
  2. If sparks are obtained, check the sparking plug, reset and clean, or renew as necessary.
  3. If no sparks are obtained, inspect the H.T. cable and renew, as necessary. Check contact breaker gap setting.
  4. If the sparking plug, H.T. cable and contact breaker gap setting are satisfactory, check for engine defects, faulty fuel supply, etc.

Charging Equipment Battery in low state of charge:

  1. This will be indicated by poor or no lights when the engine is stationary, and varying light intensity when the engine is running.
  2. Check the condition of the battery, and recharge if necessary.
  3. Check the wiring from the battery to switch, rectifier, and alternator. Ensure all connections are clean and tight.

Excessive circuit voltage:

  1. This will be indicated by burnt out or blackened bulbs and possibly burned ignition contacts.
  2. Examine the wiring for loose or broken connections.
  3. Check the earth connections to the battery, rectifier and Zener diode.
  4. Examine the battery, checking the electrolyte level and removing any corrosion.

Lighting Equipment Failure of lights:

  1. If one bulb fails to light, replace with a new bulb.
  2. If all lamps fail to light, check the fuse (if fitted) and check the battery condition, recharging if necessary.
  3. Examine the circuitry and replace broken or loose connections.

Lights fade when switched on:

  • Check the battery condition, and recharge if necessary.

Brilliance varies with engine speed:

  • Check the battery condition, and recharge if necessary.

Lights flicker:

  • Examine the wiring for loose connections or damaged cable insulation. Check the battery condition.

Headlamp illumination insufficient:

  1. If the bulb is discoloured or filaments have sagged as a result of long service, replace with a new bulb of the same type.
  2. Check the setting of the lamp and the condition of the reflector.
Lucas Service Information
Joseph Lucas (Sales & Service) LTD.

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Disclaimer: The following information has been collected from various sources on the Internet and publications for the expressed purpose of providing NTNOA members with useful information for the enjoyment, maintenance & preservation of old British & European motorcycles. While the information is intended to be as accurate as possible it can not be guaranteed to be 100% correct, therefore the reader should use good common sense and safety before implementing any of the suggestions and ask questions if in doubt.

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Revised: January 05, 2018 .