Our pals at Classic Bike Guide are a dab hand at bringing old bikes back to life – but they’ve been known to come unstuck when met with a mass of wires. To find some sense in the chaos, they’ve joined forces with Abel Motorworks to demystify wiring looms. Here’s what you need to know.
WORDS: Matt Hull / PHOTOS: Achmad Wahyudi
Electrics put the fear into many old bike enthusiasts who would be otherwise happy fixing complex parts like engines. Unnamed wires go into unnamed black boxes. But, as Mike from Abel Motorworks shows us, there’s no need to feel like that about the wiring system of your bike.
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British singles and twins have almost no wiring – just a lighting circuit and the charging system. Later bikes may have an alternator and regulator/rectifier, or even electronic ignition as an after-fit, but it is still just a few wires, and it’s often easy to spot or find problems.
The later the bike you’re looking at, the more complex the wiring loom will be. Numerous unnamed black boxes hide under the seat or shelter inside the frame, and sensors and other gubbins all have their place in a loom, making this collection of wires a thick bundle just waiting for trouble.
Imported or reimported bikes also can be an issue. Some may need wiring altering to add or take away different switches or indicators from the original market, and many bikes from hotter climates will have wiring that has been toasted in the heat, making it brittle and very unreliable.
Basics
The bikes we commonly deal with are 6-volt or 12-volt DC (direct current) systems. Except alternators, which create AC (alternating current), which is converted to DC by a rectifier. The three most common words used to describe electricity are volts, amps, and watts. Very loosely, to get an image in your head, imagine it as a running track: voltage is how fast the power is; amps are how big the power is; and watts is what happens if you run into a wall – a combination of both! So, high voltage and low amps, say 12 volts and .5 amps, you’re tall and skinny, so not going to smash the wall – there’s no power. But if you’re 6 volts and 18 amps, you’re short and fat, so you’ll smash through the wall – you have power! Other people think of it as water flowing.
Differently coloured wires are purely there to make your life easier – so get a wiring diagram for your bike, your model, and your year of manufacture. Oh, and, of course, different makes use different colours for different purposes!
Some wires are thicker than others. Wires tend to come in 0.5mm, 1mm or 2.5mm thickness, but are more often referred to in the continuous flow of amps it can safely carry without getting hot, commonly 11, 16 or 25 amps. So, for example, thin, 11-amp wire would be used for sensors, LED indicators or dashboards, while an H4 halogen headlight bulb needs 16 or 25-amp wires to deal with the 50 watts needed to power it, and charging systems would be best always with 25 amp.
Warning: Things can go wrong if you make a mistake with wiring. You do need to get a fair idea of what you’re doing here before you start tinkering or get a knowledgeable friend – you’ll pick it up quickly then. But should you wire a bike for a single headlight, but then decide to use twin headlights, you may overheat the wiring, and that could cause fire – no one wants that. If in doubt, over-spec your wire thickness; all it will do is add a small amount of weight. Or get a professional to do the job.
You got trouble
Check your battery is okay, your switches are working, and any fuses are intact. Check with a basic multimeter that you have power where you need – follow the path power should be taking. The more you narrow the problem area, the easier it is to find the culprit.
Battery terminals are often guilty, and the more electronically-controlled your bike is (i.e., modern), the more sensitive it will be to voltage. Clean up those terminals. Now check earths. Different manufacturers wire in different ways. Mike tells us that Honda has all the earth wires come together in a plastic sheath and so a small problem of corrosion could affect many parts. Ducati has them all meet at the battery terminal. Some bikes now have parts like speedos or crank sensors that are so sensitive they cannot be ground to earth; they have to earth through the ECU, the bike’s brain. But cleaning up terminals and stripping wire back until you find an area that hasn’t gone green and replacing is needed sometimes. If your earth isn’t good, the circuit cannot work properly.
Likewise, wires get old, they get brittle, and they break, so look for any wires that have been moved recently, new parts fitted, or wiring routed a different way. If in doubt, get your multimeter and set it to measure resistance along the suspect run of wire. An open circuit – a broken wire – will read zero ohms (measurement of resistance). Plugs and connectors are a constant source of problems but squirting them with WD40 or similar isn’t the answer – it doesn’t help once its washed away or evaporates. Physically cleaning the pins and sockets is the only way.
The internet is often not the answer. Just because one man in Arizona says, ‘All regulator/rectifiers will break’ doesn’t mean it’s true. Follow the path of the current methodically, check the connectors, look for broken wires, and check resistance.
Trouble always looming
You’re fed up with constant gremlins. There is a solution: you could make your own, brand-new loom. After all, we replace most parts after a certain time, why not wiring? Let’s take my trusty 1978 BMW R100, as it is close to the basic layout, but has electric start, switchgear and a modern (for the time) dash.
“Seventy per cent of making your own loom is planning,” says Mike. So, he first makes a list of what the bike has. Ignition, charging, lighting, add-ons, sensors, and so on. Then, a map is made of where the parts are. Now, this is the time when you could make changes. For example, do you want to change from twin, large ignition coils to a modern, small twin-outlet coil, saving space and weight? Perhaps you are fitting electronic ignition, or LED lights; maybe the battery could be in a better place, or you are upgrading the charging system. Make those decisions now, so you know where everything will be located.
With your map, you can now lay out a rough plan, including where any connectors will be (they need to be in a non-stress area, preferably not bent); all the joins in one section for longevity; ironing out problems; looking at the routing; and seeing if you are doubling up anywhere.
Mike uses rock-climbing wire, a genius idea, which he then sheaths to help get the shape and distance of wires.
After that, it is a case of adjustment: where to hold the loom safely to the frame, and futureproofing so that if a part has to be removed, it can with little work.
Once happy with the plan, Mike mimics his rope loom, checking with the wiring diagram and the bike itself. Building a loom on the bench is much easier than on the bike, allowing for better connections, making sure the correct thickness wire is used, easier crimping, and less error.
There are several different types of connector, depending on use or cost. There are also many different types of sheathing, for looks or for high-wearing areas. But it is possible to make your own, brand-new loom for your bike.
Starter kit
Mike loves electrics as much as motorbikes. He has a drawing of a Tesla coil (look it up – the world was torn between Eddison and Tesla – fascinating) in the workshop and used to commute on an electric bike he built himself. More recently he has come across the growing need for people who want to make a new loom for their bike, but who don’t want the cost of buying reels of wire and hundreds of connectors they may not need. So, he has now started offering a starter kit, which is becoming very popular. The basic kit includes: 54 metres of wire in 21 different colours; 17 connections; three fuses; loom tape; crimp terminals; heat shrink; and a generic wiring diagram. He’s bought other kits and made sure his wire and components are the best quality for the price. But the real beauty is you can then add to those basics to tailor it to your bike, by changing connectors for higher-quality Deutsch ones, or higher-rated cable, relays, fuse boxes… in fact, anything you need to build your bike’s perfect wiring loom. Prices start at £99, and Mike is only too happy to advise. And if that doesn’t sound like fun, you can have a loom built for you, too.
Joining wires
Ideally, never solder a wire join in a loom. Solder is hard and can fracture easily, creating a failure point. Mike suggests crimping with a crimping open barrel terminal, effectively a single connector in a block connector. Honda uses these. This is then heat-shrinked, but care is taken when planning to have this in a straight run of the loom.
Did you know?
Points are widely preferred to electronic ignition on older bikes because they can run with a lower voltage, as low as 7/8 volts, through the charging system failing or lights having been left on, whereas electronic ignition needs at least 11.5 volts (on a 12-volt system). But points are less efficient, so use more power, while the electronic system uses very little – truth is, you would probably get about the same distance home on both!
And 6-volt systems use a lot more amps than 12-volt bikes, meaning the wire needs to be larger. Converting to 12 volts uses less amps and allows brighter lightbulbs.
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