wiring harness - start run logic control

An Express Guide to the Motorcycle Wiring Harness

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Looms and the Motorcycle wiring harness

We spruce up our wheels, add aftermarket kits, get more horsepower, polish until the paint comes off but the wiring harness is the most neglected part of the motorcycle. What to do, what to do.

Schematics  and wiring diagrams

wiring harness - start run logic control
Start Run Logic – Wiki Commons credit IngerAlHaosului

Motorcycles and automobiles share one thing in common – an almost unreadable schematic designed for another type or model. Motorcycles are often shipped complete from the factory for a specific country, certain models to Canada and the USA, some to the UK, some to Australia, South Africa or maybe Cambodia. These models often don’t differ that much but it’s amazing how many times you may think you have the exact manual for your bike only to find that the wiring diagram differs.  When ordering a manual for your bike remember that they differ, the vintage is important, the country destination is important and any suffixes or prefixes to the model are important. Haynes manuals usually cover more than one model and can be country specific. The original workshop manual is obviously the best but they are expensive.

So you don’t have a manual, you are stuck in the sticks with a dead system. Or like myself you find that the battery has run down – riding all day with the headlight on and not an inkling that the charge circuit has failed. Yet the charge light was off.

All roads lead to Rome or ground or the headlight.



If you don’t have a manual:

First of all get a manual for any motorcycle and study the schematic or Google it, look for a small bike and not a Goldwing (which may appear to be that of a Boeing 747).  Multi-cylinders and single-cylinders are virtually the same except for the mono or multi-spark/ignition system.

Most electrical problems in a motorcycle are caused by the following:

  • loose wiring – intermittent
  • chafed wiring – watch that loom! Most motorcycles have the loom terminated in the headlight bucket with individual connections feeding switches and rockers.
  • bad earth – dim lighting, unswitched lights come on with a switched circuit

Not all motorcycles follow the same charging circuit design as that of a motor-car e.g. alternator, rectifier (ac-dc), regulator, battery. In some cases they are downright primitive. Although simplicity is key, car systems are very reliable – not all motorcycle charging circuits are.  Over simplifying means cost cutting and unreliability. The older GS range of Suzukis were prone to frying the charge circuit. The alternator can put out out tens of Amperes and will burn thin gauge wiring very easily.

So where to start?

How accessible is the electrics on your bike. The saddle comes off, the side panels come off, the tank comes off, the fairing comes off. If you are fortunate and have an old bike this should taken under half an hour. Modern bikes just look better because they have a million and one parts. Usually covering the electrical system. Sometimes, not always.

Must Haves

A 10W lightbulb with wire connectors and crocodile clips is indispensable.  Some times more so than a multi-meter. Why?

The multi-meter, especially the digital units have a very high input impedance – designed not to load sensitive electronic circuits. A multimeter may show a potential of 12V from a live cable to ground but a light bulb may not illuminate or just be dim – if there is a poor connection (high resistance) the meter will not reflect the poor connection, the light bulb will glow dimly or not at all. Where there is a poor ground, for instance in an indicator circuit, the supply will look for a ground to complete the circuit, often through the earthing of other circuits e.g. another light-bulb. You may see this in flashing tail-lamps when the indicators are on, brake and tail lamps coming on simultaneously (both dim) or a dim headlamp bulb.

Dim lamps, if caused by a poor connection means local heating around the poor connection. If your headlamp is 60W and measures only 2V across it, you have 10V across the poor connection, assuming the battery voltage is 100%. This may mean power is dissipated at the poor connection, limited only by the headlamp resistance, which can cause a fire.




Check your earth strap! Check your battery pole connections!

Assuming only a 1 Ohm drop in circuit wiring, which is substantial, will mean nearly a 1/3 drop in voltage across the head light-bulb. (this is an estimation). Enough to warrant a serious investigation into the cause of a dim headlamp.

In many single cylinder bikes, spark is derived from a magneto arrangement or pulse from a coil fired by the spinning of the crank.  In multi-cylinder bikes this may be through two pulses or four or more from the spinning of the crank. These pulses feed the CDI or capacitive discharge ignition system which feeds the high voltage ignition coil.  The CDI system is very reliable. Wiring from the magnetic pick up circuit may not be. Check for frayed connections. The CDI system can also run at a very much lower voltage than nominal battery voltage to enable the engine to start while cranking – CDIs also eliminate contact bounce as in older ‘points’ systems.  They can run effectively on a low battery voltage so the rider assumes all is good. As sophisticated as they may be they do not like reverse polarity – always make sure your battery is put in the right way round. This may sound a very stupid thing to do but this occurs more frequently than one may think. Owners will never disclose this however. Under dim lighting conditons red and black look pretty much the same. So be warned.


A typical charging circuit is a regulator positioned between the alternator field coil and ground, with one output to the battery. (i.e. the field coil sits between ground and regulator output). The alternator output may be a single coil unit feeding one diode for rectification (ac to dc) or multiple coils, multi or three-phase, as in most motor cars. From half wave rectification to three phase full wave, they all serve one purpose – supplying direct current to charge the battery. Three phase is more efficient. The regulator serves to charge the battery and power all electrical circuits from the alternator without an over-voltage situation. The regulator also has one connecting wire going to the charge lamp on the console or idiot panel.

Regulation and the GS Suzuki Series

The regulator is essentially a device which controls the current through the alternator field coil. The output voltage of the alternator is sensed, fed back to a zener (silicon diode voltage regulator) and output transistor which controls the current through the field. This is a very reliable circuit and has been used for many decades. The Suzuki GS however did not have a reliable charging circuit – what made them veer off a tried and trusted design we’ll never know.  If you do have a GS series Suzuki it’s best to do the modifications. I looked for this on the GS Resources website but they are in the process of updating and making changes so could not find anything there. The forum may carry it otherwise go to the kendoll website.  Some owners report of getting an excess of 30V from the charging circuit when regulation was lost – this fries the battery, bulbs and of course alternator windings.

GS Series Regulator
GS Series Regulator

An old trick:  Park the bike with headlamp shining against a wall. Gently throttle the machine to about 3 000 rpm, the headlamp should grow brighter but not excessively. Build your own regulator here (will not work with a permanent magnet field e.g. magneto)

What does the wiring harness connect to? Some simple circuits to whet your appetite.

Indicator circuits are simple, usually with a mechanical ‘blinker’ relay or electronic with switching semiconductors. Indicator lamps are usually two wire devices, one side been grounded through a wire (common) in the loom. Loose or flaky wiring systems usually cause more than just the two switched lights (L or R) to flash.

Brake lights are usually switched via means of a mechanical or pressure switch.

Some switches on the handlebars are grounded – clean these connections if you can. Be careful not to lose balls and springs.

The starter or cranking circuit is nearly always done via means of a push button switch, relay (starter relay or solenoid) and starter motor. The low current side may or may not go through the gear shift selector and/or clutch making the engine impossible to crank whilst in gear or when the clutch is engaged and in gear. Because our trusty motorcycle no longer has a kick starter this can become a bit of a bind if the battery is flat and you stall at an intersection.

Modern motorcycles are released with some very interesting gizmos in their electronics.  Usually totally unnecessary for the engine to kick over and run. When it comes to wiring always look at the basics first. Power to the CDI and the engine cranks. Always handy to have a 3 ft piece of 2.5mm wire with croc clips in your tool kit.

If you do come across wiring which has become chafed, replace it at your earliest convenience – never just insulate. Always find out why the chafing took place. Never replace fuses with a higher rating. Never use galvanised bolts in place of glass fuses, neither jumpers for flat fuses (blade).

When replacing bullet or spade terminals the general rule of thumb is to solder and crimp. When in the field this may not be possible – keep connection blocks handy in your tool kit. Make sure you have the right sized screw driver in your kit. Along with spare headlamp bulbs.

Motorcycle batteries are killed deader than dead and very quickly if left uncharged for any length of time. They are designed to be used. In fact they last longer if used.

If you do have a jumper with a bulb and a multimeter the correct way for diagnosing dead electrics is to ground the one terminal (jumper or negative probe), go from battery plus, to ignition switch, to starter relay coil (+) – then to battery (+) on solenoid. If all is good, push starter switch and check solenoid main contact out to starter. If this relay just clicks without the motor turning over the solenoid has burnt contacts. You can open a solenoid and burnish the contacts but you will need to unsolder the coil – this can be tricky. Solenoids should not be expensive to replace.

Most wiring can be checked fairly easily – if you are at home and have the tools. Wiring is something you do need to check regularly on older machines, bike wiring is especially susceptible to the elements and any bike older than ten years (even some newer bikes) will have wiring problems. The most common is in the charging circuit and intermittent running. The electronics is very reliable, unfortunately the connections not always.  Sea air is a big killer. Some scientists even ride their off road bikes into the sea as a test of durability. Whilst after a six pack or two this is a novel idea, it’s not a long term solution to the longevity of the bike.  Upcountry and a thousand miles from the sea purchases are always better than from a coastal town.

Rewiring the wiring harness

If you do decide to rewire your bike there’s one very important fact that you need to get right first time – cable length. If you have a loom you can work off at least then you have a workable ‘template’. If not, run the wiring as you would from point to point, no longer or shorter than is necessary. Longer means you can trim, never shorter. Do not kink the wiring and use the correct wiring gauge to carry the current. Remember that when running a common black wire (-) from a central point to ground then the wire to chassis must be capable of carrying the total current of all circuits in operation. Your charging circuit ground is usually hard wired to chassis already (sometimes internally) or use a thick black cable, as short as possible. Likewise battery negative and positive poles must have the shortest possible length and thick gauge wire, from negative to chassis and positive to starter solenoid – solenoid to starter motor. Main ignition (+) also must be the same gauge as the common grounding black wire (not battery negative to chassis, these are very thick). Always fuse the main positive ignition line, preferably at the battery. I like motogadget’s mUnit-v2, this will make your build more professional and safer.

Do not use twin core flex (I have for a temp fix) – they are not heat rated and are very prone to brittleness and damage.  Wire used in auto electric work is expensive, rightly so. Use proper connectors and not the lightweight terminals from the DIY store. No parts of the wire or connector, positive or negative ground should ever be bare. On older machines the stator windings of the alternator may have aged to the point where heat has taken it’s toll – usually seen in colour change, brittleness etc. Have this rewound professionally or look for aftermarket kits. Modern stock is more reliable and capable of higher current ratings than vintage models. Switches may still work but become unreliable or show excess mechanical wear. Replace with aftermarket kits, or replace the entire lever assembly with switches.

The headlamp bucket is a great place to bunch up wiring which can cause connectors to pull loose. If the wiring is not dressed properly terminals will pull loose when the front wheel is moved hard over to either direction. Wiring must remain slack at any angle of the wheel. This can be a nightmare – be patient and make sure wires are not intertwined.

Ensure that the loom does not hang anywhere, the loom must fit snugly against the chassis, preferably on top. Loose wires hanging from the now tightly wrapped or shrink wrapped loom is also a big no-no. If the bike falls over the wiring should not be damaged. If the bike is involved in an accident wiring should not be exposed and if it does, the main fuse should rupture if there is a short.

Wiring your old bike, possibly even making up a new loom should be a fun exercise. If done properly it will last another ten years.



Reading matter:

The Wiring Harness Manufacturer’s Association and IPC/WHMA-A-620Acceptability of Electronic Wire Harnesses and Cables







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