Understanding the design and function of the electrical charging system of your vintage Honda two cylinder motorcycle is a key system to keeping your bike running reliably. The two cylinder motorcycles produced by Honda from 1965 to 1977 all use a permanent magnet single phase charging system to keep the battery charged when the bike is running.
What vintage Honda motorcycle models does this apply to?
This article specifically refers to the permanent magnet single phase charging systems found on these families of motorcycles:
CB175 / CL175 / SL175
CB200 / CL200
CB350 / CL350 / SL350
CB360 / CL360 / CJ360
CB450 / CL450 / CB500T
So what is a Permanent Magnet Single Phase charging system?
The design of a permanent magnet single phase charging system is primitive by modern standards. In essence, It is a type of generator that produces alternating current electricity (AC) via a mechanical input. In simplest terms, there is a permanent magnet (a common refrigerator magnet is a type of permanent magnet vs. an electro-magnet) that is physically moved past a coil of wires wrapped around a steel core. Every time the permanent magnet passes by the coil of wires it generates a quick pulse of electric current in the coil winding. Pass the permanent magnet past the coil of wires fast enough over and over again, and those pulses stack up to become a steady flow of electrons able to power electrical parts (light bulbs, horns, etc) or recharge a battery. Each pulse of the electric current is a phase; in this example, it is one pulse at a time, thus a single phase.
The Common Design of Honda twin cylinder Charging Systems
The design of the charging system on the twin cylinder models utilizes several permanent magnets evenly spaced apart in the rotor which is bolted to the end of the crankshaft. The rotor is located under the round "Honda" cover on the left side of the engine and has the markings for ignition timing and valve adjustments. The permanent magnets are integrated into the body of the rotor and cannot be removed.
Surrounding the rotor are six windings, each wrapped around a common steel body, which we refer to as the stator coil. Every other set of windings of the stator coil are joined together in series, thus we have two sets of three windings, with each group of three are on their own circuit with one common wire to join the two sets of windings together. As the engine runs, the permanent magnets in the rotor pass by one group of the windings creating an electrical pulse, and then past the second group creating a second electrical pulse, and then back to the first group. This back and forth of the pulses between the two different groups of winding creates an alternating or AC current. The rectifier in the charging system converts this AC electrical signal to a DC (Direct Current), which is what the motorcycle uses to power all the electrical parts on the bike.
It is important to note that the charging system on the Honda twin cylinder motorcycles is drastically under engineered and is just barely adequate to keep the battery of the motorcycle charged when working properly. Even a properly working charging system on a bike will have difficulty recharging a battery that has less than 12 volts initial charge. Unlike alternators in a car that are able to recharge a battery that has low voltage, do not anticipate the output of the charging system of the Honda twin cylinder motorcycle to recharge a dead or low voltage battery.
Charging System Woes: Before troubleshooting the stator, what else should be addressed first?
The build quality of the stator coil of these Honda motorcycles is nearly bulletproof; they are not often the common cause of charging problems with a bike. Before your attempt to troubleshoot the stator coil as the cause for charging problems, make sure these more common culprits listed below are addressed first. It could be a single part or a combination of any of them that are the real issue.
The usual culprits to charging systems are:
- The voltage regulator or rectifier - The stock Honda voltage regulator and rectifier are notorious for being the source of charging system problems. 75% of the time it is a voltage regulator failure and 25% of the time it is the rectifier. Do note that older bikes prior to 1970 typically are not equipped with a voltage regulator, only a rectifier.
Regardless of your bike's configuration, we recommend upgrading to a modern regulator/rectifier combo unit. This is the #1 electrical upgrade that is needed on the vintage Honda twin cylinder motorcycles. The newer design makes the charging system run more stable and slightly more efficient than factory components.
- Learn to install a modern voltage regulator / rectifier on your motorcycle right here: Installing Your Regulator Rectifier Combo Unit
- The Battery - Batteries are a wear part of any motorcycle, vintage or not, and can cause a range of strange issues on a vintage Honda motorcycle. A few tips will help you better understand and troubleshoot your battery:
- The battery is not a power generator and is best thought of as a power pool. Once the battery drains of its electrons, it needs to be topped back up. The two best ways to do this are by either riding the motorcycle or using a battery charger to keep the battery at a full charge. The battery can start to drain after only a few days of sitting so if you aren't riding daily its best to keep the motorcycle on a trickle charger while you are off the saddle. In addition, the charging systems on these motorcycles are noticeably anemic and it can take at least 30 minutes or longer to begin to significantly recharge the battery by riding.
- The best way to check your battery voltage is right after a 15-30 minute ride, if you end the ride with anything higher than 12v then your charging system is working. These vintage Honda motorcycles range anywhere from 12v-15v, but if you are dipping below into the 11v range or lower something is not functioning properly.
- If you purchase a flooded lead-acid battery, take the time to properly activate it by charging it for 24 hours before the first use. The life and full capacity of the battery are set by how much power is pumped into it during its activation period. We see a lot of these batteries stop working a month or two after purchasing them because they were not properly activated. If you are looking for a rock-solid and ready-to-go battery, we highly recommend our Motobatt AGM battery that you can learn more about right here.
- The white/yellow wire mod - Read up on what this modification is and how to do it right here:
How to test the resistance & continuity of your stator coil
The first and easiest test to do on your stator is testing the resistance between the wires, as well as the continuity to ground. One of the failure points of your stator coils is the coating protecting the windings can wear away, either causing the circuit to skip certain windings or arc straight to ground.
- Testing for grounding: Set your multimeter to a low resistance (ohms) range and test between each wire and a known good frame ground. If you find continuity or presence of anything than an open circuit between the wire and ground, then the stator windings are somehow touching the metal core and need to be replaced.
- Testing the resistance of the coils: Set your multimeter to a low resistance (ohms) range and stick the probes into each wire to get a reading. Each combination should read at the approximate values below. If you are close then you probably do not have an issue here.
- Pink → Yellow - .8-1.2 ohms
- Pink → White - .2-.6 ohms
- White → Yellow - 1 - 1.8 ohms
How to test the Voltage Alternating Current (VAC) output of your stator
The second test, which is our preferred way to test the stator at CMC, is to test the actual AC voltage output of the stator. Testing the voltage with the motorcycle running will give the real-world output that the regulator & rectifier sees revealing the full picture of its health. The stator could have the proper resistance but still have an issue with the wiring causing a weak output and poor charging.
- Set the mulitmeter to VAC. Insert one of the probes into the Yellow wire and one into the Pink wire immediately after exiting the stator. With the motorcycle idling, you should see around 20 VAC or greater on the multimeter.
- Repeat the same test but use the white and pink wires to test the other circuit of the stator.
If both sides of your stator are outputting around 12-20 VAC or greater, your stator and rotor are working properly. If either of the values are lower than 10 VAC then there's an issue with the stator and you need to look for a good used unit to replace it with. If both sides are low, check the magnetic health of your rotor before ruling the issue as solely the stator.
Magnetic health of the rotor
In a few rare cases, we have noticed the permanent magnets in the rotors begin to lose a significant portion of their magnetic strength (flux), thus leading to low voltage output from the stator coils. The best way to test the rotor's magnets would be to examine the "sticking" power of each of the magnets to something heavy made of steel. We recommend using a regular hammer as your test object. Touch one magnet of the rotor to the head of the hammer and pull the hammer apart from the magnet and note the amount of force needed to separate the two. Or you can attempt to pick up the hammer with the rotor assuming it has been removed from the engine. Repeat the test for all the magnets on the rotor. They all should have the same feel in terms of how much force is needed to pull away the hammer. If one is weaker than the others it will be noticeable. If such is the case, then the rotor will have to be replaced with one that has good magnets that test OK.
My stator coil wires are faded / not the correct color?
It is common for the wires coming out of the stator coil to have either faded beyond recognition of the color or have been repaired with wire with all the same color. It is possible to identify them by testing the resistance of each combination of wires. See our simplified graphic of how the stator is wired below:
We draw the stator like this because it helps illustrate the different resistance values you will see when testing each combination of wires. It is important to note that even though each of the two circuits, yellow (primary) and white (secondary) of the stator each have three sets of windings in series, they are not equal in the total number of windings between the two circuits of the stator coil. The yellow to pink side has more overall windings on its circuit versus the white to pink side of the coil (as illustrated above). From the factory, the yellow wire side of the stator coil was always active when the engine is running. The white wire side of the stator was activated when the headlight is turned on. Thus the yellow wire circuit of the stator coil has significantly more windings, and therefore greater resistance value when measured. When testing the resistance between each wire, here is what to expect:
- White → Pink - Being the smallest number of windings in the coil you should get the smallest ohm reading, usually in the .2-.6 ohms range
- Yellow → Pink - Being the larger half of the stator coil, you can expect a higher Ohm reading at around .8-1.2 ohms.
- White → Yellow - This is why we find this illustration so helpful. When reading the resistance of the white to yellow connection you are reading the full resistance of both sides of the coil in series. The reading will not be exactly the mathematical addition of both sets of coils individually but will be the highest resistance value of the three different combinations.
Wiring Notes CB450 K0 models:
The CB450 K0 has the same setup as the later bikes but the wiring colors are different from the stator coil. These bikes did not have a voltage regulator.
- Brown from stator = Pink on later bikes
- Yellow = Yellow
- Pink from stator = White on later bikes
Should I run a lithium-ion battery on my motorcycle?
The short answer is: No, you should not... let's explain why...
Lithium-ion batteries are a great advancement in battery technology and provide many motorcycles with a battery that is longer-lasting, weight-saving, and allows a deep depth of discharge, but just like any battery, there are downsides to them as well. The biggest issue is lithium-ion batteries need to be charged at very precise voltages (think sniper level precision). However, the stock single-phase charging system on the vintage Honda twins is has a much larger voltage output range; it could too little or too much for a lithium-ion depending on the charging system efficiency on each individual bike. The output charging voltage range tends to be over a much wider range of values (shotgun approach), which is not an issue for the original style lead-acid batteries. Modern gel or AGM (absorbed glass mat) batteries are more resilient to the sloppiness of the original Honda charging system design. Additionally, they hold their charge for longer terms of inactivity and lower temperatures compared to lithium-ion batteries. These features offset the weaknesses of the stock charging system to give you the most reliable combination out there.
My bike has a kick start, that means it should start without a battery?
This is a frequent misconception about the Honda twins. The bikes need a charged, healthy battery to run as the ignition system is powered by the battery. While many small engines, such as lawnmowers, outboard motors, and even some smaller dirt bikes will run without a battery, they utilize what is known as a magneto ignition system. The common assumption is that because the kick starter is present on all the Honda twins, the engine will start and run via kick starter without the battery, which is not the case.