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Bike Wiring Diagrams - What is all that coloured spaghetti. 2016-08-16

Understanding Bike wiring diagrams.

  1. cjvfr
    When you first look at a motorcycle wiring diagram it can be daunting. Lots of lines running every which way. Like all navigation finding the landmarks is the way to start and from there a wealth of information can be gleaned.
  1. #2 cjvfr, Aug 16, 2016
    Last edited: Sep 3, 2016
    Below is an example of the wiring diagram for a Suzuki GS500. When you first look at a motorcycle wiring diagram it can be a bit daunting. Like all forms of navigation the way to start is to look for the landmarks that all bikes will have. Start with the battery, in the words of "Julie Andrews" a very good place to start. Without the battery the bike wont start.


    1. Battery
    2. Alternator
    3. Regulator/Rectifier
    4. ECU/ICU
    5. Starter motor
    6. Starter Solenoid
    7. Handlebar switches.
    8. Blinker system
    9. Rear lights
    10. Front Lights


    To make a bad pun because I can't resist we start with the battery. The battery as most people know converts energy stored in chemical form into electricity. It provides the power to crank your engine over until the petrol demons can take over. It has another role, it provides a source of power to run the electronics and electrical equipment of the bike when the load is higher than the alternator provides. When you are travelling slowly "yes I know heresy" or the load on the bike is higher than usual, electric gloves,. extra lights, electric coffee warmer, heated vest etc then the battery has to take some of that slack.

    When you first start the bike the battery is low and needs to be recharged, slow speeds, electric heated grips etc all make the situation worse in the first part of your ride. If you are a short commuter the battery may not recover before you reach your destination. This is bad for batteries and you will generally end up with a dead battery. So if pulled over by the police at speed you can honestly say "I was charging my battery officer" :whistle::sneaky:

    More Battery info can be found in my Resource Winter and my Battery and the resource Battery Maintenance Guide


    As briefly touched on on the battery section the alternator is to create the power you use as you go along, charge the battery and hopefully keep all your electric goodies functioning. Not least of which are the bits that make the bike run, electric fuel pumps, Engine Management system, Fuel injectors, lights etc.

    In its simplest form, from high school science class you wave a wire around in front of a magnet and electricity will be produced. Two things are important, the magnetic field and the movement of the wire. A still wire gathers no electricity. The alternator has two sections, the rotor, because it rotates and the stator because, of course everyone remembers their Latin from "One who Stands". The bit that doesn't move.

    Just as waving a wire in a magnetic field produces electricity, the reverse is true passing electricity though a wire produces magnetism, this is known as an Electromagnet. The rotor part of an alternator in a car (yes boo hiss, I promise we will get back to motorcycles) is an electromagnet. In motorcycles most commonly this is a permanent magnet. Lightness and simplicity are the design watchwords in motorcycles so this method is a way of making a powerful alternator in a small package. The alternator produces alternating current. The battery and the rest of the bike uses Direct current. In alternating current the voltage goes up and down as the magnet spins close to the wire and then further away


    But in an alternator there are generally 3 windings in the stator which are 120 degrees apart around the circle of the case. So it produces 3 phase electricity.


    This is more efficient and you get more usable power for the same physical size. But of course the bike can't use alternating current it requires a fixed Direct current. This leads to the next device the Regulator/Rectifier.


    The RegRec has two jobs, the rectifier part converts the Alternating current into a steady Direct current that the bike and battery can use. The Regulator part keeps the voltage within voltage limits. If the voltage is allowed to go too high the battery cooks and the light globes generally blow. If the voltage is too low the battery won't charge. Faulty rectifier/regulators can exhibit faults of both types overcharging and undercharging. Undercharging faults are more common however.

    The rectifier part is fairly simple and is made up of a group of diodes. Diodes are a special form of conductor that will allow electricity to flow one way but not back the other. There are six of them in a standard 3 phase rectifier although commonly they are in the same package so you don't see them as individual devices.


    The resulting green line represents the Direct current you end up with. slightly bumpy but more than adequate. The battery will take a lot of those lumps out to give you a relatively smooth direct current supply.

    The second part of the RegRec is the regulator. Although we generally refer to a bike electrical system as 12 volt. (Some older bikes may be 6 volt) this is just a nominal value. The voltage on your bike may drop as low as 10 volts when you are cranking the motor to start and when healthily charging your battery should be around 14.5 volts.It is the job of the regulator to do this. Let's personify the regulator as Rupert Regulator.

    Rupert is a busy fellow because you can't resist changing the bikes speed and therefore how quickly the alternator is turning. Unlike cars motorcycle regulators are of a type called shunt regulators. That means they either provide extra resistance to the current from the alternator and rectifier to drop the output voltage down or decrease the resistance to allow the voltage to rise. It may seem a strange way to do it because all that extra energy dropped by the regulator has to go somewhere, and indeed it does it becomes heat. Designers try and shed that heat by a number of means, bolting it to something cool like the frame of a motorcycle is one method, heat conduction. Putting cooling fins on them to allow air to take heat away, heat convection is another.

    Below is an image of the RegRec for a GS500

    There is another technology becoming more common in RegRecs and that is MOSFET type. They have some advantages, more efficiency so less waste heat and generally tougher. Rather than Rupert having a resistance dial he dials up and down, in a MOSFET regulator he just has a big switch, On or Off. He bounces up and down on that switch to alternately short the supply or allow it to be open. He is very fast and does that many many times a second. For the purists this is known as Pulse Width Modulation (PWM).

    That's about it for regulator rectifiers. I will write another resource on basic fault finding once I get to the end of this introduction.


    Next is the brains of the motorcycle known by a few names. They range from the very simple as in our example carburettor version GS500 and commonly known as an ICU or Ignition Control unit up to fuel injected bikes where they are more commonly known as Engine Management Units (EMU) or Engine Control Units (ECU).

    In the example bike above the ICU's job is to work out where the pistons are and fire the spark to ignite the fuel at the right point. All bikes will have a method of determining where the pistons are so they can do this. For many years this has been done by a sensor that detects the position. Sometimes that is separate as it is in the example bike, the pulse generator coils. Sometimes t is an extra winding built into the alternator that gives this information. In more complex bikes it is generally done by a Crank Position sensor.

    At the top end of the range is the Engine Management Unit of Engine Control unit. This has the job of firing the fuel injectors at the correct point and has a number of sensors to make sure it is burning the fuel efficiently. Usually air temperature and possibly pressure, burnt gases in the exhaust using the O2 sensor, throttle position using the Throttle Position Sensor etc. It also may have built in an engine immobiliser system that requires a specific code sequence from the bike key to start. It may also control the fuel pump, any variable valves in the exhaust and air intake path, in fact whatever the wizened minds of bike designers can think of.

    These had a bad reputation as an expensive point of failure on bikes but for many years they have been designed to be very robust and it is rare to get an ECU failure now days.

    Next the Starter system Starter Motor and Solenoid

    NOTE: This is a work in progress, please check back as I write more.
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  2. Yes very funny and informative, cant wait for the rest.
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