Simple wiring diagram that will work with both Big Twins and Sportsters with generators.
- When using electric start it is always best to put the battery ground directly to the back of the starter, or the starter bracket. For some reason it eliminates problems with the solenoid and voltage drop. There is no reason to run a separate ground to the frame as the engine is grounded to the frame.
- Avoid using auto parts store battery cables. The ends on them are usually hollow steel garbage and most times too big and not meant for use on a motorcycle. Personally I make all my heavy battery cables out of #4 welding wire and put solid copper ends on them. Stock Harley cables are #6. The #4 welding wire is nice and heavy, but it is also quite flexible too, which is a great help for routing them.
- The positive battery cable always goes to the top lug on the back of the solenoid and the cable that runs to the starter goes on the bottom lug.
- The starter button used in this diagram is a heavy duty one (30 to 40 amps) and is always hot, meaning that if you press it the bike will always crank. However the ignition is on the key switch, so the bike will not start with out the key “on”.
- There are a couple different kinds of voltage regulators. They can be either mechanical or solid state. I usually recommend solid state. The hook ups are different depending on what regulator you have, but are also basically the same, as in there will be a battery connection, one lead going to the A terminal, or armature, and one lead going to the F terminal, or field, of the generator. There is also always a ground connection to the frame, or housing of the regulator. On mechanical regulators it is always a separate wire connection, because the regulators are mounted on rubber grommets for vibration, and these grommets isolate the frame of the regulator from the chassis of the bike. On solid state regulators the ground is through the housing of the regulator, however great care should be taken to see that the ground is a good one. I still recommend a separate wire connection just in case the regulator comes loose and you loose the ground. No ground = fried generator and possibly fried regulator. The ground is where the excess voltage goes and when there is no ground things heat up and the BBQ begins.
- The circuit breaker is where the rest of the circuits begin. Stock Harley’s use 3 or 4 normally to separate circuits, with a main 30 amp breaker in case all else fails. Here to keep things simple I just use the main breaker. It’s plenty of protection. There aren’t a lot of wires to get bunched up on the ignition switch so it works well. The 15 amp, or secondary breakers on stock Harley’s are mostly used as terminals to keep from bunching the wire connections all on the back of the switch. Obviously there is a lot more wiring on a stock harness with all the stock components.
- The ignition switch, or key switch is the next stop along the way in your wiring harness. This is the “B” terminal. This is where most everything is turned on and off including your ignition system, so you can have a way to shut the bike off. My favorite key switches are 2 position, ignition and lights. 3 position if you count “off”. This way you don’t have to run your lights in the day time and you also don’t have the lights drawing power from the battery when you are trying to start the bike.
- The ignition circuit. This is the “I” terminal on the back of the key switch. From there you can go straight to the coil, or through a kill switch, so you have a quick easy way to kill the engine just in case strange and unexpected things happen. Other stuff on the ignition circuit would be the stop light and the horn, commonly know as safety items. The horn is dual purpose, because it also serves as a great way to get the attention of a pretty girl walking down the street who just may be in need of a ride.
- The lighting circuit. This is the “L” terminal, and usually the only remaining one after the “B” and the “I”, so if you screw up here, stop immediately and seek professional help. This is the circuit that helps out a whole bunch at night when the sun isn’t so bright and it’s a lot harder to see what’s going on in front of you. It should be no surprise that the head light and tail light are on this terminal. The only tricky stuff here is that you probably want to run power to a Hi/Lo beam switch for the head light.
So that’s pretty much it for simple wiring. As you can see that start circuit is separate from everything, as too is the charging circuit. If you don’t want a component , like a kill switch, simply eliminate the kill switch and run your wire directly to the coil and so on.
When you see the ground symbol coming out of the side of a component this means the component must have a good chassis ground or it won’t work. For example should you rubber mount your head light, or you have your tail light mounted to a fiber glass fender, they wouldn’t be “grounded” and would not light up. Another good example is FXR’s and such that are rubber mounted need a ground strap from the engine to the frame, so that the engine and all the components mounted to it are grounded.