Electrical

Charging System

In this section we try to de-mystify motorcycle charging systems, point out things that can go wrong, and provide a few tips

Most early charging systems were barely adequate when new. Today there is a greater load on a vintage charging system because we run with lights on all the time. We want to hop aboard a vehicle without ever thinking about turning on the fuel, setting the choke, warming up the motor or any number of things that were standard procedure in the past.

Before attempting electrical repairs, try to locate a good, color wiring diagram for your motorcycle. The examples I use are for Honda models. Honda uses green wire for ground. Other manufacturers may use different colors than shown here. Be sure to reference your wiring diagram for the actual wire colors used on your make/model. You'll also need a fully charged battery. If you have a drain on the battery with the key turned off, the battery will eventually discharge whether or not your charging system works. Keep in mind we're talking about vintage bikes. Modern motorcycles with computerized and electronic gizmos are always in standby mode. It's "normal" for them to draw power even with the key off. . Progress . . . right ?

For our purposes the charging system consists of the alternator/generator, rectifier, regulator, battery and the wiring that connects them. We include the battery here because it provides power for the field coil. Besides, the charging system would be useless without something to charge.
In this example the Generator produces AC current. The Generator is connected to the RECTIFIER which converts the AC current to DC. The rectified current then goes to the REGULATOR. The function of the regulator is to provide enough voltage to keep the battery charged but not so much that the battery overcharges and boils acid all over your nice shiny muffler. The battery is the end point of the charging system. From there it is distributed back out through the lighting and ignition systems as needed.

This shows an example of a two wire, half-wave rectifier used in the the small Honda singles. These were installed in bikes under 90cc. If you can't find a replacement 1 diode rectifier, legend has it that you can use a 4 diode bridge rectifier on these by using just one of the 2 AC connections.

This is typical of a rectifier for a mid 60's single. This looks like the one I replaced on my S90. Notice the wires are soldered to the rectifier not attached with screw terminals like on the twins. I cut my terminal wires off close to the rectifier then attached spade terminals to install a modern Radio Shack type silicon rectifier. By replacing the inefficient stock selenium rectifier the voltage output increased by about 20%. Not only was the charge rate increased, there was no detectable leak-back to ground which increased my battery life from 1 season to many years so far.

This is an example of the "Radio Shack" type of rectifier I installed on the S90. Notice that 3 of the spade connectors are pointing the same direction and one is not. The connector that is not aligned the same direction as the others is the + Battery connection. The 2 terminals on either side of the + battery connection are AC connections for the 2 AC wires coming from the alternator. The remaining terminal (farthest across from the + battery lead) is used for ground. It is connected to ground through the frame or through a green(ground) wire in the harness.

This rectifier is from a different mfg. The picture shows the rectifier connections. The Positive red wire goes to the battery. The Green negative lead goes to ground. The remaining leads go to the pink and yellow wires. Some Honda models use Brown instead of pink. Also the pink wire and the pink paint on the rectifier has a tendency to age into a brownish color.

The rectifiers on most 1960's Honda twins (C,CA,CB,CL etc.) 125/150/160/250,305/450 came with screw terminals. My CL77 has ring connectors on the wires going directly into the wiring harness. If you ruin the connectors trying to remove the rectifier, you can cut the wires off near the rectifier and replace the terminal ends. This rectifier is marked from the factory showing wire colors. .

Generators

DC generators - Some older motorcycles have a DC generator. A rectifier is not needed in this case, because DC is used to charge the battery directly. DC generators require service eventually as the commutator brushes wear. The DC generator can serve double duty as the generator and starter motor on some bikes. Once the motorcycle is running, the starter motor remains engaged and produces its own power to recharge the battery. How cool is that?

AC generators - An AC generator uses permanent magnets in place of the electromagnets used in an alternator. The disadvantage here is that permanent magnets can loose their magnetism if they are struck or dropped. They can also loose magnetism over time, so this can be a source of low output from the generator. The AC generator uses a rectifier to convert its output to DC.

Alternators - The alternator became popular in the 1970's for larger bikes. Probably no suprise that it also produces Alternating Current. Battery voltage is used to create an electromagnetic field in the stator coil. The minor disadvantage to this design is that the battery needs a charge to start charging the battery. Kind of a Catch-22. The Alternator also needs a rectifier. In automobiles the rectifier is sometimes called a diode pack and built into the alternator housing. The alternator design needs less maintenance because it uses slip rings instead of brushes.

Things that can go wrong: In general, the alternator/ generator is fairly simple and trouble free. Over years of use and cycles of overheating, the clear insulation on the wire coils may crack and deteriorate. When that happens the alternator will be less effective. It probably won't stop working right away, but it won't get better on it's own either. More often failure is accompanied by physical damage; Like when the rider falls over at speed and uses the alternator for a crash bar, or tries to remove the stator by prying it out with a screwdriver wedged against the coil. The windings fail by being shorted to ground or they lose continuity by being cut. If the windings are crushed in a crash. . . Either way this is a bad thing. You can bench test for continuity and resistance through the coils and to ground for resistance. If the bike is running, you can test unregulated voltage output. See wiring diagram for how the output wires are connected to the coils.

Rectifiers

The rectifier is that funny looking orange thing with the big cooling fin/plates. It's one of those parts that you don't usually think about until it goes bad. Unfortunately selenium diodes eventually turn to dust and no longer work. They can do this even while sitting unused on the shelf.

Rectifiers can usually be found under the seat, or side cover. Sometimes they are attached to the frame by themselves or attached to a plate with other electric devices like the turn signal flasher, regulator etc. One of the more inconvenient locations I have run across is on the Honda S90. Honda protected the electrics by mounting the rectifier and ignition coil inside the sheet metal frame. They are nearly impossible to access without dropping the motor. Fortunately these bikes are easy to work on and the motor drops down by removing a few bolts.

The rectifier converts electricity from AC to DC to charge the battery. It does this by means of one-way valves called diodes. AC current changes direction in cycles (alternates). It goes one way then reverses direction then back again many times a second. The diodes prevent current from flowing backward from the battery which would defeat the purpose of a charging system. When a diode fails, the battery can quickly discharge with the key turned off.

The simplest version of a rectifier uses a single diode and is called a Half-Wave rectifier. They only pass the current that is going the right direction in the first place and the other half goes to waste. These are used in small bikes with a small load on the electric systems. Half wave rectifiers use 2 or 3 wires.

A Full wave Bridge rectifier is the common type of rectifier found on vintage motorcycles. It uses 4 diodes configured to take advantage of all the current from the alternator. When the current is going the wrong way out one lead it starts coming out the right way on the other lead and visa versa. Bridge rectifiers have 3 or 4 wires. The 3 wire version has the ground going through the mounting bolt to the frame. The 4 wire version has the ground wire going to the wiring harness or to the frame.

A 3 phase alternator is used on the 4 cyl Hondas and other more modern bikes. Rectifiers for these systems use 6 diodes. For now though, lets just focus on the Bridge Rectifiers found on early Honda Singles and Twins.

Things that can go wrong: The oem selenium rectifier is a weak point on old motorcycles. Selenium degrades over time. Rectifiers can leak back to ground from the battery, resulting in (you guessed it) a DEAD battery. Selenium diodes have ~.65 voltage drop per diode. An upgrade to a modern silicon diode rectifier is highly recommended. Less heat is generated and you can easily increase charge voltage by .5 - 1.5v. Modern rectifiers last longer than the original design, and since they are more efficient, they run cooler with less voltage drop and are rated to run at higher voltage than oem.

Regulators

The purpose of the Regulator is to provide the correct voltage to the battery. Unregulated voltage from an alternator can easily be 70-90 volts depending on RPM.
In the later 1970's manufactures combined the Regulator and Rectifier into one unit.

No Regulator

Some small motorcycles don't have a regulator. They have a balanced load system, where the generator provides just enough current for it's needs. This works by having an extra charging coil that is turned on and off with the headlight switch. The system is designed so that Excess current is absorbed by the battery. If the battery is faulty or missing or otherwise unable to absorb voltage spikes, it is passed on to the lights which in turn will burn out the filaments in rapid succession.

Electro-Mechanical Regulator

Early regulators were mechanical. They worked with coils and points and springs. They have adjuster screws that control voltage and current output. Usually turning the screw in slightly will increase output. You can fine tune this type regulator with a screwdriver and a volt meter.

Solid State Regulator

The later solid state regulators and combination regulator/rectifiers that came out in the mid-late 1970's are designed to be tossed and replaced if they don't work. As long as they don't overheat or freeze solid and get whacked they can last a long time. Until they don't, and you're out on the desert, Ninety miles to the next town, but I digress. If you're paranoid like me, you could pack along a spare reg/rect on your next cross country trip. You probably won't need it, but they are lightweight and easy to plug in.

Things that can go wrong:

Mechanical regulators have points that can get dirty and sometimes stick. If adjusting them doesn't make a difference, you may need to pull the cover off and clean the points. You did bring an ignition points file right? Like I mentioned, the later solid state versions were not designed to be "fixed" other than by throwing them away and trading cash for a new one. Before you do that though, consider that the problem might only be corroded connections. Unplug the regulator from the wiring harness and check the brass terminals in the plug. If they don't look shiny, try to clean off all the green/white gunk before plugging it back together. Sometimes just unplugging them from the wiring harness and plugging them back in is enough to dislodge the corrosion and make it start working.