Motorcycle Repair: Honda xl100sb electrics problem, regulator rectifier, wiringdiagrams

Question
Hello, I have a Honda XL100SB registered 1/1/83.  It is blowing the headlight bulbs. The voltage showing at the headlight and tailight terminals is about 11 volts.  I am assuming the rectifier is the problem (appreciate possibly also earth and possibly other culprits).Parking light, tail light and stop light OK.  Appreciate your views on this also do you know if rectifiers still available or are there alternatives available.  Thanks for any help you are able to give.  As far as I can tell the battery is taking charge.  Regards David

Answer
David, you must have a fully charged 6v battery in the circuit with all wiring connections intact.

If it is generating 11volts, then the charging system output is okay, just the regulation is at fault. A dead/dying battery won't load the system properly to prevent the over-voltage condition.

There are some wiring diagrams on this site: http://www.oldmanhonda.com/MC/WiringDiagrams/MCwiring.php#class   However, their XL100 diagrams probably only cover up to 1974-78. The later XL100S design came in at 1979, here in the US. The engine design is completely different from the earlier models and the electrical system may vary as well.

Take a close look at all of the wiring connections in the headlight shell and wherever the headlight switch connections are located. Be sure that all the electrical system grounds are clean and secure.

The rectifiers just transform AC energy to DC energy to charge the battery, in most cases. The headlight feed is probably coming straight from the alternator lighting coil, rather than the battery. If the headlight only comes on when the engine is running, then the feed is from the generator directly. I would verify that the rectifier ground circuit path is intact and tightly connected to ground. Digging deeper, just now, I found http://www.cmsnl.com/honda-xl100s-83-us_model7670/partslist/F++2502.html which shows that the bike does have a regulator/rectifier (not just a rectifier, like the early bikes), so if all the wiring connections are good and the battery is capable of sustaining a load test and staying fully charged, then the reg-rectifier may be your issue, in this case. Honda part number is: 31600-KA8-600 and might be used on other models. Google the part number to see what comes up.  

Be sure that ALL lights are working, including the speedometer instrument light, to properly load the lighting system output. I don't have any access to a specific wiring diagram for your model, at this time.

Bill Silver