Motorcycle wiring for dummies: Keep it simple!

By Dave Abrahams Time of article published Mar 22, 2017

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Cape Town - It sounds familiar, because it’s happened to all of us. Your motorcycle doesn’t have a specific problem, it just won’t crank over hard enough to start and if you push-start it, the dashboard lights go dim and flicker when it’s idling.

OK, let’s keep it simple. The headline above is accurate, because I’m a dummy when it comes to electrics. All my training is in mechanical engineering, and my knowledge of electronics is limited to first-year Principles of Electricity – so I know how little I know.

Nevertheless, the principles of electricity haven’t changed in more than 200 years, so most of these straightforward checks apply equally well to the latest superbike as to a 1980s musclebike or a 1960s classic.

None of which has stopped me from making all the usual bone-headed mistakes that nearly all riders make when it comes to electrics. And the biggest one of all is “rewiring the bike”. A frightening number of riders, when confronted with an electrical problem on a motorcycle, particularly an older one, will insist on ripping out all the bike’s wiring (because it’s “old”) as well as all the small electrical components – and then they have absolutely no idea where to start to get it running again.

Any bike technician will tell you that when you strip the wiring of a motorcycle, you have about a five percent chance of ever getting it running again, no matter how good condition it is in mechanically - and if you do it is almost certainly going to cost you more than the bike is worth to get it back on the road.

First, do no harm

The first thing to do is to check the battery - and for that, all you need is a big cross-head screwdriver and a clothes-peg. Unscrew the negative terminal first – that’s important – then the positive, and then lift the battery out just far enough so you can unplug the clear tube (the battery breather pipe) that runs down the side and disappears into the guts of the bike - and clip the clothes-peg onto it so it doesn’t slide away out of sight and get lost.

Then put the battery on a flat surface, high enough off the ground that you can get down alongside it, and check the level of the acid in each cell. They’re separate, all six have to be checked individually. If one or more of the cells is low, top them up with distilled water (sometimes simply labelled battery water – you can get it at any supermarket) - not acid - and put the caps back on the cells.

Then put the battery down on a hard surface outside (preferably concrete), boil the kettle and pour a cup or so of boiling water over each terminal - that will instantly dissolve any furring that may have built up on the terminals.

It’s also a good idea to clean the lugs that bolt onto the battery with a clean rag and hot water, for the same reason – just remember to ditch the rag afterwards, it’s likely to have some battery acid on it and you don’t want that to get anywhere near anything you want to keep.

But yours is a late model superbike with a sealed battery, so none of this applies, right?

Yes and no. If your bike has a sealed battery, it won’t have a breather, so you can ditch the clothes-peg. It also has a black plastic casing and no caps, so you can’t check the cell level (and you don’t need to) but the terminals are just as prone to furring, which can cause voltage drop, so the boiling water trick is still just as important, as is cleaning the contact surfaces of its connections.

The best way to envisage voltage drop is to think of electrical circuitry in the same way that you think of your own blood circulation system - if it’s partially blocked the blood (or electricity!) will have trouble getting through to where it’s needed.

Which is why making sure the terminals are spotlessly clean before you reconnect them is the best thing you can do for your battery – and don’t forget to stop just before the battery drops all the way into its box, grab hold of the clothes-peg, remove it and plug the breather pipe (if your bike’s battery has one) back onto its fitting. Then you reconnect first the positive and then the negative – that way if one of your tools touches the frame and the terminal at the same time, you won’t get any arcing.

Like the blood in your arteries and veins, the electricity in your bike’s electrical system flows in a circle, from the battery to wherever it’s needed and back again.

Earth strap

But instead of having a complete set of duplicate negative wires, the negative side of every component on the bike – including the starter motor - is connected to the frame, and the frame is connected by a single, short, heavy wire (the earth strap) to the negative terminal of the battery, which completes every circuit in the bike, with half the number of wires than if every component had its own negative connection to the battery.

Every single volt of electricity that flows through even the most state-of-the-art superbike has to go back to the battery through that earth strap – and both the strap and the frame are prone to corrosion at the point where they make contact.

So, follow the earth strap from the negative terminal of the battery to where it’s bolted onto the frame - it won’t be far. Unbolt the strap from the frame and clean the area where the lug touches the frame with clean, bone-dry steel wool from the kitchen – and clean both sides of the lug on the end of the strap the same way.

Then bolt it back on and protect the joint from corroding again by covering all the exposed metal with a layer of grease – or if you haven’t got any grease, you can give it a quick squirt of chain wax, or just use Vaseline from the bathroom.

Braided wires 

On a lot of cars, and some late-model bikes, the earth strap is just that - a flat strap of braided steel or aluminium wires, which is cheap and easy to make, since it doesn’t need to be insulated from anything, and works very well when it’s new and clean. Over time, however, corrosion gets into and between the fine wires, and the electricity has a very hard time getting through.

This not only affects your bike’s starter motor, but also the electronic ignition control unit, and can make it very reluctant to start. If the earth strap on your bike looks suspicious, measure its length by running a piece of string alongside it it, cutting it to length and pulling the string out.

Then buy a piece of heavy-duty welding cable the same length (25 square millimetre thickness is about right) – and of course two new lugs – from an electrical supply shop and make up a new earth strap, by peeling the insulation of the cable back for about 12mm, slipping a lug over the bare copper wire and flattening it to fix it in place - in a vice, with a hammer, or even with a half-brick if you have to.

Clean up both lugs with the same piece of steel wool and assemble the earth strap just like the old one, using the same routing. You do that by running a piece of string or household wire along the earth strap from one end to the other before you take it off - that way you’ll know exactly how it was routed when it comes time to connect up the new one.

Raid the beer money

The difference can be astonishing – I once cured a high-speed misfire on a 500cc sportsbike by simply replacing the earth strap, without touching the bike’s ignition system.

If all this hasn’t helped, you may have to raid the beer money to get a new battery for the bike - but that’s an expensive way to go, so try the simple stuff first. Then take the bike to a car battery fitment centre that doesn’t sell bike batteries, and ask them to test it.

Why a car place? If they can’t sell you a new battery, they have nothing to gain by telling you that your bike needs one, when the old battery may in fact be perfectly all right.

I had the battery of the bike with the high-speed misfire checked at a bike shop; they told me the problem was a dead cell in the bike’s battery and sold me a very expensive new battery. Of course, that didn’t help at all - but the old battery is still going fine in another bike, so it wasn’t altogether a dead loss.

IOL Motoring

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