Mechanic Repairing Motorcycle
Woman Fixing a Motorcycle


The ALTERNATOR and BATTERY on your bike provide the necessary voltages/currents that your bike needs for starting, lighting, and running. The alternator turns the mechanical rotational energy of your bike into electrical energy. The electrical energy is then used to power most importantly the coil(s), which supply the high voltage necessary to generate sparks at the tips of the spark plugs. The alternator is responsible for providing the juice necessary for the lights, horn(s), and other electrical accessories. It also has the very important responsibility of keeping your battery charged so that it can start up after the bike has been turned off. In order to be sure that it can provide the energy necessary to run the bike, as well as start the battery, it will output a voltage slightly higher (usually about 10-20%) than your bike's 12 volt system.

Key Electrical Components

The Battery

The battery is what gives your bike the "juice" to fire up when you hit the starter button. Most ALL batteries on bikes are 12 volts negative ground. The negative ground means that the negative terminal of the battery is connected to the metal frame of the bike. This simplifies wiring for electrical components on the motorcycle. The best way to test a battery is to perform a "load test." This test is performed by essentially partially "shorting" out the battery with a high current resistor and checking the battery's ability to still provide power. Now this doesn't mean that you should place a screwdriver across the terminals and read the voltage - a load test should only be performed by a "load tester" (mechanics like to keep names simple, don't they!). Is your battery charged fully? I hope you didn't leave it in the cold garage all winter long (you should remove it and store it inside for the winter.


Yes, you can jump start the bike with a car battery (most all bikes are 12V, just like cars. BUT - as always, be SURE not to reverse the polarity (positive goes to positive and negative goes to a grounding point like the axle bolt on the bike).  If you're jumping it, be careful not to let the positive (usually the red cable) clamp touch any metal on your bike - you are likely to see a small arc and you may damage your charging system if you're not careful.

The Rotor and Stator

The rotor and stator are essentially an AC motor working in reverse and are called the alternator.  Energy is provided to the shaft of the "motor" (rotor) by the rotating engine.  So the rotor rotates inside of the alternator body and it's an electromagnet. On it is the "field" coil, which is a series of wound wire that is wrapped around a bunch of metal fingers, or a metal core (metal conducts magnetism better than air).  When an electrical current runs through the wire conductor (coil), each of the metal fingers (located on opposite sides of the wire) picks up the polarity of that "wire pole". So, as you look at the rotor and field coil assembly and turn it, the poles will be arranged North-South-North-South-N-S-N-S-N-S, etc. When the engine drives the rotor, the polarity that passes a given point that is stationary (separately on the outside edge), that point sees an "alternating" pole passing by. Current is provided to the spinning "field coil" (the coils on the rotor that form the electromagnet) by means of nice and smooth conducting "slip rings". The magnetic current generated is then "passed" on, or induced to the windings on the stator.

The stator around the rotor is stationary and is usually a set of 3 coils wound up and spaced evenly around a ring of iron (again, to transmit the magnetic current better) giving us 3 separate coil wire/metal cores with the iron ring acting as the common core for all of the windings. Each of the wires is wound into a number of coils that are spaced so that winding #1 is followed by #2, which is followed by #3, which is followed by #1, etc. This is what is called 3 phase (or 3F) winding. As the rotor (the electromagnet) turns, it passes by the series of coils that are part of the stator. As the poles of the rotor come close to one of the coils in the stator, there is a current that is induced and increases as that "pole" gets closer. It then decreases again as that pole passes by. After the "N" pole passes a particular coil, the "S" pole then approaches. This pole creates the same increasing then decreasing current, but in the opposite direction. "Alternating Current," get it? This procedure happens for each coil one right after another. The end result is 3 sets of peaks and valleys of alternating current (3 phases, remember?)

AC To DC Conversion

On most all bikes, one end of each stator coil winding is grounded to the bike frame. The other ends of each of the windings go to the diodes (or diode assembly). A diode is essentially a "one-way street" for electrical current. Because the alternator provides "alternating magnetic fields" and hence alternating electrical current, and our batteries and electrical systems are Direct Current (DC), the resulting current needs to be "rectified", or chopped from AC to DC. The diodes do this job. Each diode allows the current from its stator coil to only pass in one direction (cutting out half of the current supplied). Current comes from the stator coil (one at a time as the rotor windings pass by) and is allowed to go one way through each of the diodes. Essentially what we now have is a pulsating current (that increases and decreases as each piece of the rotor passes a winding on the stator (one half of the electricity is being blocked by the diodes - that isn't precisely correct, but good enough for here). The pulses are smoothed out and the precise voltage is held "in check" by the regulator. The result is a slightly "lumpy" 12-14 Volts DC that pass on to the battery and accessories.


Your Bike Starts, But Dies (Electrically) Soon Thereafter

So your bike won't hold a charge… I would say the most common problem is usually the simplest. The battery. Just because a battery reads 12 VDC when you stick the volt meter on it, doesn't mean that is functioning correctly. So…be sure the electrolyte (the water/acid mix) in your battery is between the lines or marks on the battery in all of the cells. If it's low, pop off the cap(s) and fill it towards the top of the area with DEIONIZED or DISTILLED water (most tap water contains minerals that will reduce the effectiveness of the battery). Now that you've topped it off, try recharging the battery overnight with a trickle charger. You'll want to charge it in the garage (or outside) and be sure to put a board or something else under it, in case some of the (very) acidic electrolyte boils over during charging.

If you have the luxury of owning or having access to a "load tester", you can see if the battery holds its voltage level with a load applied.

Ok, so your recharging didn't work. See if it'll let you jump-start it (if you choose not to read my explanation on jump starting - please BE CAREFUL when jumping to do it correctly!!!). If your problem is just the alternator or the battery, jumping should work (as long as it hasn't damaged any other part of the electrical system.)  Click HERE to read about how to safely jump-start a motorcycle.

Your Bike Doesn't Start At All

First - when you put the key in and turn it to the ON position, do the lights come on?  If not, there definitely is a problem with either: the ignition switch or switch wiring, the battery, the battery connection (either end of the wires / terminals), your main fuse or fuse wiring.  Trouble-shooting this problem is just a pain in the rear.  Always start simple: unplug everything electrical, clean it all off, then plug it all back in.  I would then put a known working battery in, and hook one end of a volt meter up to the ground of the bike and take the other end and trace the positive (+) line until I found a bad connection.  This would be one of those "buy the manual" issues.

Checking The Starter "Relay"

The starter relay's function is to take the small current switch of your starter button on the handlebar, and close another (larger) switch that lets a large amount current go the the starter motor.  It (usually) has 4 terminals on it. Two of them are the smaller, low current (or low "tension"), and the other two are high current (high tension). Many motorcycles have other safety devices that may prevent the bike from starting.  I can't address all of the variations here, so I will assume that they are working/you have tested them.

I'm assuming that when you turn the key to "ON" the lights come on quickly and nice and bright (that's a poor man's way).

Assuming the kill switch is on RUN (you did check that, right?), when you press the START button, do you hear the relay "CLICK?" If you don't, the problem may either be with the wiring leading to and from the relay, or it may be a bad relay. Here's how you test the relay after you've determined you didn't hear any click (assuming your battery is charged): · Disconnect the thick wire that goes from the relay to the starter motor (it's usually bolted to the relay). Hook up each of your multimeter probes to the two high current leads on the relay to test for continuity (Ohms) · Turn the key to the "ON" position and the kill switch to the "RUN" position and be sure the bike is in NEUTRAL. · Press the starter button. ·If the relay "clicks" and the meter reads 0 (zero) Ohms, the relay should be ok.  If you heard no click or the meter doesn't read real close to 0 Ohms, let go of the start switch. Disconnect all wires from the relay (mark them so you know where they go back on). · Measure the resistance between the two low current leads (where the small wires went) on the relay, if it's infinity, the relay is probably bad. If it's zero (literally) then the relay is probably bad. Check your service manual to find out what the resistance is supposed to be if you are still curious. · Ok, so your relay is probably ok. Now what? GET THE SERVICE MANUAL! Due to the different ways relays can be designed, I would have a difficult time steering you from here.

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