The dreaded wiring

Wiring. Don’t panic!

There’s something about wiring that strikes fear into the heart of the most competent spannermen. As we all know, electrics work on a kind of magic blue smoke. You have to keep the smoke trapped in the wires and components by connecting them up just right, because if you get it wrong there is a bang and the smoke comes out of the components, they never work again. Like my zener diode in an earlier post.

That’s nonsense of course, and vehicle electrics are actually pretty simple, especially on a classic bike. It does not have an ECU, a CDI, a reg/rec or any other modern computery stuff. It does not have any of those lamba sensors or mass air flow sensors or coolant sensors that festoon modern engines. It does not have a clutch or side stand switch, because there is no starter circuit or starter safety circuit, because there is no starter motor. There are no relays anywhere and it does not even have a fuse box.

What it has is just enough wiring to create a spark at the spark plugs at the right time so the engine runs, plus a bit more for the lights, indicators and horn. There is a battery but it’s not really needed, it will still run with a flat battery or even no battery.

The first oddity is — like other manufacturers of the era — Triumph made it a positive earth system. This means the battery positive is connected to the frame and engine, which form the return part of all the circuits. All the switches and fuses are in the negative side which effectively becomes the live feed. This feels very wrong to anyone used to modern vehicles, but actually it makes no difference to how anything works. It’s a bit like the UK driving on the left when the rest of Europe drives on the right — it works fine as long as you know about it.

Another oddity is the tiny little alternator fitted, it gives out a feeble 100W or so in a single phase alternating current (AC) whereas modern bikes will pump out 250-400W three phase AC and cars as much as 750W. But then there is not much to power, provided it can run the lights and spark plugs with a little to spare to top up the battery, job done.

The bridge rectifier. Simple but works.

The tiny alternator feeds a primitive rectifier called a bridge rectifier, which is simply four diodes connected in a circle, that converts its alternating current output (wiggly amps) into direct current (not wiggly amps). There is no regulator as such, but if the tiny alternator does manage to raise the system voltage above about 14V then the zener diode opens and basically short-circuits the excess voltage into the frame, getting nice and warm in the process.

The battery — almost an afterthought — is wired in between the rectifier and frame earth where it provides juice to run the lights and things when the engine is idling or switched off, as well as smoothing out the messy output from the crude rectifier. It even has a fuse on the live side (which is negative, remember?), the only one in the system.

There is an ignition key positioned rather unhelpfully somewhere under the rider’s right thigh. This controls a live feed for the lights (green & brown wire) and/or the ignition and accessories (white). Both head up the frame to a rat’s nest of connectors behind the headstock, where they feed the control switches on the handlebars. Cables for the rear light cluster head back the other way, while wires from the oil pressure switch, horn and front brake light switch also converge at this point.

But... why put it there?

The generic-looking switch units can be connected in different ways, offering exciting surprises when the buttons are pressed. Boringly though, I have opted for the conventional horn button on the bottom left, flasher on the top left, high/low beam on the right rocker switch and kill switch on the bottom right. 

Unfortunately this leaves only the right rocker switch for the indicators, which means you have to release the throttle to operate them, or reach across with the left hand. Worse, the switch does not move left and right, but up and down. What were they thinking? Well, I can always stick my arm out instead.

After routing through the kill switch, the ignition circuit (now white and yellow) heads back to the coils under the seat where it provides the low-voltage supply for both. The other low-voltage terminal of each coil is then connected to its respective contact breaker point hence to earth. There is also a capacitor called a condenser for each coil. The high voltage output connects of course to the spark plug on the same side.

Thanks Triumph, nice job. Maybe don’t get the tea boy to draw it next time?

It works like this... most of the time the low voltage circuit of the coil is switched on. Wrapped around this it is a second coil, with many many more turns, which is connected to the high voltage output. In other words, we have a transformer. Whatever voltage is present in the low voltage side is carried into the high voltage side by the electric field and amplified because there are more windings. 

At the (hopefully) correct moment, the contact breaker suddenly opens and disconnects the low voltage coil, collapsing the electric field. The energy from the electric field has to go somewhere, and it ends up in the high voltage coil, causes a dramatic voltage spike. If this is is high enough — 5000V or so — it will jump right across the spark plug gap like a tiny bolt of lightning, ignite the fuel/air mix, push the piston down, and away we go. And all this happens a hundred times per second.

The condenser capacitors are there to discourage the ‘lightning’ from jumping across the contact breaker points, which would rapidly bugger them up.

The rest is rather less exciting. There is a flasher unit, the old mechanical kind, fitted in the live feed to the indicator switch so that when you select and indicator it actually flashes. I don’t know if mine works yet as I don’t have any indicators. There is a headlamp switch on the lamp housing, so you can turn it on and off while riding, and that feeds the high/low beam switch on the left handlebar before returning to the headlight itself.

There is a ‘double switched’ circuit (brown) so that either the front or rear brake can turn on the brake light. Finally there are three ‘idiot lights’ in the headlamp housing indicating low oil pressure, high beam and indicators on.

So much for the theory. Expecting my wiring to be the usual sorry mess of hacked about rubbish, I ordered a complete new loom. Amazingly, you can still buy the original cloth-wrapped wiring looms from Lucas, the original manufacturer. 

Tidy wiring is happy wiring.

I start at the headlight end and with the help of the dreadful wiring diagram in the workshop manual, I label everything. Then I remove the old wiring loom, keeping careful notes of what connects where. Surprisingly, the wiring is pretty original. There are things missing of course, but not the hamfisted mess I was expecting. Still, it makes sense to replace 40-year old cables before they break / short-circuit / start a fire.

Before the new loom can go in, I have a major cleanup job to do. The crankcase breather pipe — which is supposed to vent all sorts of horrid oily/petrolly/exhausty vapours from the engine cases and safely out the back of the bike — had evidently become detached in about 1984 and ever since had been projecting said vapours into the air box and under-seat area. Man, it is foul in there. I also remove the throttle and air control cables and the tired old engine/frame breather pipes as they are rather in the way, and get busy cleaning.

I also clean and test the rectifier and coils, and they are all good. All the old terminals and connectors get a dose of contact cleaner. Finally, in goes the new loom, new horn and the lovely new battery. That’s better! I’m not quite ready to ‘close up’ yet though. I am waiting for a few parts to finish off at the headlight end before I can test everything, and I don’t want to start the engine again until it’s had a thorough service.

Finished wiring under the seat — sooo much cleaner. The round
things are the coils, the flasher unit and condenser pack are
in the middle while the rectifier is just underneath out of sight.

The relentless spending continues, as I keep on finding missing washers, stripped bolts, worn out nuts and perished seals and rubbers all over the place. None are expensive, but it all adds up frigheningly quickly. I have also ordered new tyres, and after long consideration I went for modern ones (Avon Roadrider II) rather than retro style (Avon Speedmaster/Safety Mileage). The retro ones might look the part and last longer but ultimately they are a big compromise on grip and performance. And that's not a compromise I am willing to make.

TOTALS TO DATE: Hours: 32. Cost: £5391.