In the early days of motorcycling, Honda bikes had very basic electrical systems. However, for those of you who can fix things that you can see moving, electrical systems and components might seem like a black art where sparks and arcs are conjured up from nowhere. Let’s see if we can take some of the mystery out of Honda electrics and find a clear path of understanding where the electrons come from and go to in their various functions.
If you turn the ignition switch to ON and nothing happens (battery powered systems), then the first place to go is the battery. Lead-acid batteries require that mix of sulfuric acid and distilled water to make electricity. For bikes which have been sitting for months, years or decades, a fresh battery is the first place to investigate. In many cases, my customers bring in a bike with weak batteries only to discover that their lead-acid batteries have gone dry or at least the fluid levels are down below the tops of the lead plates. The lead plates must be fully submerged in fluid, so the first thing to do is to bring up the levels with a bit of distilled water. For batteries that have opaque cases, look for signs of the plates calcification where they have turned white along the edges. If you see that, the battery is done, so replace with a fresh one. If you don’t see the white colors on the plates, then put the battery on a slow charger, just one or two amps for a few hours and test with a voltmeter to see if it is coming back up to specifications. A 12v battery comes up to 12.6-12.8 volts when fully-charged, but they seldom maintain that level in most motorcycle batteries. A 6v battery can come up to 6.3-6.4 volts, but anything in the low sixes is fine.
If the battery seems full and gives something close to “normal” voltage, then turn the ignition key to ON and monitor the voltage with a meter. The voltage shouldn’t drop much more than a volt or two. For bikes with an electric starter, pushing the START button shouldn’t cause a huge drop in voltage. If your 12v battery plunges down to 10v with the starter cranking, then it requires replacement. If the bike has a kickstarter and the bike starts up manually, but not with the electric starter, then again the battery needs to be replaced. If the battery voltage comes up on the kickstarter function, its a good sign that the charging system is working to some extent but the battery is failing.
CAUTION: Kick-starting a bike with a failed battery and getting it to run can damage the battery and the charging system components because the battery acts as the voltage storage and regulation of the charging system. Honda didn’t use an actual voltage regulator in the early days. All you had was the permanent magnet rotor and stator, which generates AC voltage to the rectifier, whose job it is to convert the AC voltage from the stator to DC voltage to charge the battery. The fully charged battery is the source for power to the system and a reservoir for the charging system to replenish the outgoing electrons. When the battery is dead or nearly-dead, the permanent magnet charging system can generate enough voltage to fire the ignition coil and cause the engine to start up, assuming that the engine is capable of running and the fuel system is metering correctly. But the charging system no longer has the battery reservoir to store the output of the charging system and the unregulated output causes a huge spike of voltage to knock out any live light bulb that is on when the ignition switch is in the ON position. This condition can also damage the selenium rectifiers as they are not designed for huge over-voltage conditions. So, ALWAYS have a fully-charged battery in the bike when attempting to start up the engine after a long sleep. Many of the headlight bulbs for vintage Hondas are NLA and they will pop like flashbulbs if the engine is running and over-charging the system with the lights turned on.
So, now that we are starting with a known, fully-charged battery, when the ignition switch is turned ON and nothing seems to be happening, first try the horn button to see if there is power to the primary system or not. If the neutral light bulb is blown out, then you won’t get an initial indication that there is power in the system. Testing the horn, will verify if there is power available or not, assuming that the horn is functioning.
If now power is indicated, then go back to the battery box and look for a single fuse or fuse block and check the condition of the fuse. If the fuse looks blown out, then there is a short in the primary electrical system somewhere. This is where some troubleshooting comes into play.
When the ignition switch is turned ON, the power comes from the battery (red wires) to the switched hot black wires that feed the major components. Those systems include the ignition system (coil and/or kill switch), the horn circuit, the neutral light circuit, and the brake light circuit. If the replacement fuse blows immediately, then disconnect the main circuits listed above and slowly connect each one back, one at a time. Often in the process of disconnecting the circuits, you may see a pinched wire or damaged connection that is the cause of the short to ground. It is a good idea to have a box of the appropriate sized fuses to use in the testing phase, however if you only have one or two, you can just touch the inner fuse block contacts together momentarily and watch for a large spark indicating that the short circuit still exists. If the short circuit has been rectified, you will see a small arc at the fuse holder, which is normal as the initial current flow will cause a smaller arc. The difference will be pretty evident. There are automatic reset fuses and test units that will open the circuit when the short exists, indicating that the fault remains, but they don’t eat fuses in the process. Once the fault is found, then the correct fuse can be reinstalled for normal operation. DO NOT install a higher amperage fuse in place of the specified one. This can cause the wiring harness to overheat and possibly cause a fire in the harness or components. Fixing a short circuit is a pass-fail result.
A basic 12v test light, found at any auto parts store is your friend in most cases of electrical problems. The will work on a 6v system, but the bulb will not display as bright as it would on 12v. Using the probe to contact various electrical contact points, the test wire with the clip goes to ground in most cases. The bulb doesn’t care if you reverse the polarity, so you can use the test light to check ignition timing by connecting the wire clip to the point wire connector and just put the probe into a convenient screw hole to make the connection function.
Some examples where things can go wrong are:
1. A shorted ignition coil 2. a shorted out brake light switch or the wiring to the switch 3. a pinched hot wire inside the headlight shell to any of the circuits that feed the neutral light wire, horn or lighting circuits and 4. shorted out tail light socket wiring for brake and running/park light functions. 5. would be any damaged or pinched harness wires 6. damaged wiring or connectors for the lighting system components.
To prevent excessive voltage into the lighting circuits, all of the bulbs in the circuit should be functioning to balance the electrical load for the charging system. For bikes that do not have voltage regulators, the charging system needs to be balanced by a fully-charged battery and all light bulbs functioning properly when the light switch is turned ON. The charging circuits only use 2/3 of the output in the normal ON position and then supply all three legs of the output when the light switch is turned ON. The light switch turns the lights on and at the same time connects the last stator output line to help balance the increased electrical load of the light bulbs. This is done through internal contacts within the switch assembly. If the switch contacts are corroded inside, then the final leg of the charging output won’t make it to the battery, and the battery voltage will slowly drop down further and further until the engine quits.
For the 250-305 Honda twins from 1960-67, there was a time where the original charging systems were apparently putting out excessive voltage and causing the batteries to overheat or boil out battery acid from the vent lines, which often leaves damage to chrome and painted chassis parts. Honda reduced the magnetic field in the rotors and created the (L) low output charging systems. The rotors were stamped with a CB72(L) markings for Super Hawks and Scramblers, while the Dreams will show a C72 (L) markings on the rotor faces. Unless all of the components are stock and functioning normally and the battery is fully-charged, often charging system issue arise on the later low-output systems. With today’s electronic options, a solid-state voltage regulator/rectifier combination unit will solve the charging system issues, as long as the stators are fully-functioning. Using these units, the stator outputs are all connected directly to the solid-state unit, which will control the charging system output electronically keeping the battery fully charged whether the lights are on or off.
The stators for 250-305s and for early Benly models have cloth wrapping around the charging coils and are pinched into the stator housing to keep them in place. After 50+ years, the insulation gets brittle and flakes off and/or the insulation gets soaked with oil by a leaking crankshaft seal. The coils need to be tightly mounted and the insulation remains intact. For small patches where the insulation has flaked away, you can use some liquid tape products to secure the rest of the insulation in place. If the coils get loose on the mountings, they will bounce up and down on the posts and short the windings to the grounded frame, which kills that portion of the AC winding output to the rectifier. Actually, when any of the three phases of the windings are grounded, basically the whole stator become grounded and fails to deliver voltage to the rectifier. When testing a stator, you want to check for continuity between all three wires, but you don’t want any of them shorting to ground. This can be tested with an ohm meter or buzz box that checks for continuity. For removal from the bike, use a heat gun to warm up the connectors, as the female connectors can shrink and hold the connectors together so tightly that if pulled too tightly, the end of the connector will come off of the wire end.
Courtesy of AHMC training materials.
For most of the rest of the vintage Honda engines, the stators were submerged in the crankcase oil to help cool the windings and to prevent external damage. The coils are sealed with an epoxy that prevents damage to the coils and helps to make the engine more compact. When the dyno covers are removed to access the crankshaft bolts when turning the engine over by hand to adjust the valves or ignition timing, don’t be surprised when oil comes out when the cover is removed. You can minimize the oil leakage by putting a thin 1” board beneath the left side of the centerstand leg or foot to lean the bike slightly to the right. You will lose a little oil when the cover is removed, but it is minimized by a bit of bike lean from the centerstand wedge.
For bikes with stator connectors which are plastic 4-5 prong round or square plugs, always check the connections for any wire terminals that might be corroded or sometimes get pushed back inside the connector housing. When adding a solid-state reg/rectifier to the stator, ensure that the wire colors are a match for both sides of the connection. Generally, the neutral light wiring (green with red stripe) is packed in with the other three stator output connections and you don’t want to mis-wire the stator connector to the electronic unit accidentally.
There are a lot of aftermarket ignition switches, which may have different wiring in the connectors, so be sure to match up the wire colors whenever you change components like that.
The brake light switches are often connected to the brake pedals through an adjustable body. On some models, if you service the drive chain by pushing the rear axle back to tighten the chain, there is a chance that the brake light switch may not trigger properly. Likewise if you adjust the rear brake, that will affect the travel of the brake light switch plunger. Always check your brake light function after any bike service to the drive chain or brake pedal.
Honda 350-450 twins had a somewhat more sophisicated charging system, which includes a current regulator mounted on the bottom of the battery box. The function is to allow increased battery charge rate when the battery is low, then shunts off the excess battery current when it senses that the battery is fully charged.
Handlebar switches can be troublesome after 50 years of service and exposed to the elements of dirt, water and heat. When bikes have been sitting for many years, the copper contacts inside the switches can corrode and cause and open circuit when a function is selected. The switches can be taken apart for cleaning and wire repairs, if you use small tools and extreme caution so as not to lose the tiny screws, springs and ball bearings inside. Often the switch functions can be revived by simply cycling the switch on and off a number of times to help scrub off some of the corrosion buildup on the contacts.
The starter buttons on the early bikes present a challenge, especially if they are disturbed during a handlebar change or if the bike is involved in an accident where the switches are rotated on the handlebars. For bikes with inside the bar wiring, the wires can become grounded or sheared off when the switches are rotated. For the starter button function, the wire that come up the handlebars must be insulated from touching the handlebar hole edges or when the wire insulation gets pulled back and exposes bare wire just behind the starter button contact plate. The wire connection to the button plate must be insulated right down to the back side of the button contact plate. Exposed wire to the switch contact plate bracket will cause the starter to engage as soon as the ignition switch is turned ON. If that occurs you need to re-visit the starter button wiring and contact functions.
The left hand dimmer switches incorporate the two functions of selecting Hi-Low beam function and as a method to make the horn function. Again, the inside contacts can become corroded and/or shorted out to the handlebar ground for switches which have inside the handlebar wiring.
For those who have been doing a full restoration on your bikes with repainted or powdercoated chassis parts, you must be aware of re-establishing the ground paths for the rear tail light and headlights. Tail lights in particular require a ground for the bulb socket to the base plate, then the base plate grounded to the tail light bracket. The taillight bracket must be grounded to the fender and the fender grounded to the frame, otherwise the tail light functions will be puzzling and will not function normally.
I hope that I have shed some light on some of the basics of vintage Honda (and other) electrical systems. Wiring diagrams are available online for almost all models. See:
https://oldmanhonda.com/MC/MCWLister.php for the list of available diagrams.
Bill Silver
aka MrHonda
4/24
