Modern motorcycle electronics are, thankfully, a great deal more user-friendly than they used to be. Older riders will have painful memories of setting ignition points with feeler gauges and test lights, juggling the contradictory requirements of performance and easy starting.
The consequences of getting it wrong were invariably dramatic. Either the bike had no power at all (timing retarded) or would overheat and seize (timing advanced), or funniest of all, would kick back and then start running in reverse. The starting line havoc when this happened was hysterical.
These days motorcycle electronic issues are rare. But things sometimes go wrong, so here is a brief description of your electrical system. We will assume that the hypothetical bike we are looking at has a starter motor, battery and headlight. At the end we will do some trouble shooting.
This is where it all starts. On the left hand side of the motor, under the ignition cover, lives the flywheel. The flywheel is a magnetised disc connected to the crankshaft. As it turns, it spins past a number of fixed coils called stators, mounted in close proximity to the flywheel. Each time a pole of the magnet sweeps past a stator coil, a bit of electricity is generated. All magnets have a north and south pole. As each pole alternately passes the coil, the bit of electricity will go in alternate directions. This is called an Alternating Current (AC), and the whole system is called the alternator. This AC will be modified in different ways to provide power to all the motorcycle electronics.
The job of the battery is to provide power to
the starter motor when the happy button is pressed. Batteries require, and
deliver, Direct Current (DC).
DC does not swop polarity with every cycle, so the AC is going to have to be modified to provide DC. Furthermore, if a battery receives too much power it will overheat and fail almost immediately. So, the AC has to be rectified to provide DC, and regulated to limit the power.
This is done by a single unit called, funny enough, a regulator/rectifier or reg-rec. This is generally a finned aluminum device a couple of inches square. The output current from the reg-rec will be direct current at a maximum of about 14.5 volts.
The headlight also requires a regulated current. Otherwise, as engines revs rise and more power is produced by the alternator, the headlight globe will get hotter and hotter until it pops. But the headlight can run equally happily on AC or DC. As there is no requirement to rectify the AC, manufacturers will generally tap headlight power off the alternator before the reg-rec. Regulating the power is still done by the reg-rec as both the headlight circus and the reg-rec are earthed to the frame of the bike.
Someone will ask why the headlight is not simply powered by the battery. The answer is that dirt bike batteries are a bit marginal due to size and weight considerations. So it is better to run the headlight from the alternator and leave the battery to its task of spinning the starter motor and sometimes driving a radiator fan.
On the flywheel is a raised section of metal about 15-20mm long. As the flywheel spins, the raised section passes a sensor called the ignition pick-up. The pick-up generates a signal and sends it to the Capacitor Discharge Ignition unit (CDI). The CDI takes that signal and amplifies it using power from the alternator. The amplified power is sent to an ignition coil which, through a bit of electro-trickery, creates a high voltage, short duration pulse which arcs across the spark plug and creates the spark.
A differentiation should be made here between electronic ignition and digital ignition. Essentially, digital ignition has a little computer that can take inputs from various sensors and change the ignition timing to suit the conditions of the moment. Electronic ignition is an analogue device, simpler, smaller, lighter and more reliable. Most off-road bikes have electronic ignition with a limited number of digital inputs, such as Yamaha with their Throttle Position Sensor. It changes the ignition timing according to RPM and throttle position. At present, fully digital ignition is confined to fuel injection bikes.
If your bike has a starter motor, you will have noticed that the wire that goes to the starter motor is a big thick thing, whereas the wires to the starter switch are skinny. Obviously there is some mechanism in your motorcycle electronics to change the light current going to the switch, to the heavy current going to the starter motor. The mechanism is called the solenoid. When you press the starter switch, a light current goes to the solenoid. This current activates an electro-magnet which closes a much bigger switch capable of dealing with the heavy current going to the starter motor.
When your battery is a little flat you will often hear a click from the middle of the bike, followed by nothing from the starter motor. The click is the solenoid switch closing, the nothing is from the lack of power in the battery to turn the starter motor.
Many manufactures will install interrupter switches in the starter circuit. Typically there may be a switch in the clutch perch which will prevent the bike from being started without the clutch being pulled in. A less common device is a side stand switch which stops the bike from starting if the side stand is down.
The following are the most common causes for electrical failures:
I hope that answered most your questions about motorcycle electronics. If you have a specific problem or an interesting topic that you would like to read about on DIRT-BIKE-SECRETS let me know and I will see what I can do.
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders