Diode Boards & Charging for BMW Airheads, an overview
Many are intimidated by electricity, and the amount of electrical problems with Airheads can be a sore point. Most often problems are due to lack of regular maintenance, particularly cleaning and tightening various connections, checking alternator brushes, changing the battery before a catastrophic battery failure; and general old age and mileage. This is really not much different from other motorcycles. If properly maintained, the Airheads electrical system works fine and is reliable. If, however, the bike is ridden mostly in stop and go city traffic, the charging system may well not keep up, and the battery require nightly re-charging. The Smart type of chargers are especially convenient for that purpose.
The stock charging system starts producing usable amounts of electricity at ~2000 rpm, but upwards of 4000 rpm may be required for enough output to take care of lights (including extra lighting), ignition, heated clothing, accessories, and have electricity left-over for re-charging the battery. Aftermarket alternators of higher output are available that fit inside the timing chest area.
This is not an article to discuss system quirks, problem indications, maintenance procedures, etc. There are several articles on fixing charging system problems in these Airheads.org Technical Tips articles, and many more on the author's website, http://bmwmotorcycletech.info/index.html
BMW has used the same basic charging system since the /5. In fact, a goodly portion of it was used on some earlier BMW bikes. A relatively compact generator called an alternator is affixed to the forward end of the crankshaft. Just above that is the 'Diode Board'. The diode board changes the alternating current (A.C.) of the alternator output to the Direct Current (D.C.) that is required for the electrical system and battery. Along the frame backbone under the fuel tank is located a voltage regulator. Depending on model year that Voltage Regulator is mechanical or electronic. An electronic type will substitute for the mechanical type. I have posted information in articles on this website on how to modify and/or adjust the voltage regulator for higher output voltage, if you need or want to do that, and further articles are here: