Many are intimidated by electricity, and the amount of electrical problems with the airheads is a sore point, MOST OFTEN due to lack of regular maintenance, particularly cleaning and tightening various connections, checking alternator brushes, changing the battery before a catastrophic battery failure.....and really not all that different from many other motorcycles. If properly maintained, the system works just 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 system starts producing usable amounts of electricity at APPROXIMATELY 2000 rpm, but upwards of 4000 may be required for enough output to take care of lights, ignition, heated clothing, accessories, and have any electricity left-over for re-charging the battery.   Aftermarket alternators of somewhat higher output are available, that fit inside the timing chest area.

This is not an article to talk about all the system quirks, problem indications, maintenance procedures, etc. There are several articles on fixing charging system problems on the Airheads.org Tech Pages, and many on the author's website, http://bmwmotorcycletech.info

BMW has used the same basic charging system since the /5. 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 backbone
(under the fuel tank) is located a Voltage Regulator. Depending on the year or what you have installed, that Voltage Regulator is mechanical or electronic. An electronic type will substitute for the mechanical, and vice versa for short periods of time. I have posted information in THREE articles on this airheads.org website on how to modify the voltage regulator for higher output voltage, IF you need or want to do that.  There is further information in similar articles on my website:  http://bmwmotorcycletech.info

BMW has used a number of different alternator rotors over the years. All look pretty much the same and fit interchangeably (for the most part) but there have been changes that should be noted. The /5 rotors had a higher electrical resistance (about 7 ohms). In the /6 the resistance was reduced by roughly half, and reduced again slightly in the 1990's. The /5 alternator output was rated at 180 watts, the /6 at 280 watts, the very slightly smaller diameter rotor of the fast-spinning R90S was rated at 238 watts, and the later models rated at 250 or 280 watts, depending on year. If a rotor fails, you can purchase new ones, or rebuilt ones, and MANY if not all of the rebuilt ones use the later lower resistance windings, and the performance of the alternator is somewhat
enhanced by having more output at somewhat lower rpm. The stator housing usually has a tag on it describing the maximum ampere output, 20 for the 280 watt unit. A /5 can have a 280 watt alternator installed; but, only certain 1974-5 (and MAYBE a FEW early 1976 calendar year) 280 watt alternators will fit the /5 timing case.

The stator winding in all the alternators have three wires, one from each 'phase' (the alternator is called a 3 phase alternator) for the electrical output that is applied to the diode board. In the /6 and later stator, a connection to the 'center' of the three windings was additionally made, and that became another connection on the stator housing to the diode board, and the diode board was modified with additional small diodes that slightly increased the electrical output. All the diode boards physically interchange, a /5 board will work in a /6 and later system, but the output will be slightly reduced as there is no place on the /5 board for the /6 stator center-tap connection. A /6 and later board will work just fine in a /5 system, one simply makes no connection to the
additional terminal on the board (unless one has upgraded the /5 alternator, as mentioned). The 6 large size solid-state silicon diodes, press-fitted into the aluminum angles used as heat sinks/mounts, are of two types, although they look the same.  The 6 large diodes work hard, and produce a fair amount of heat. That heat must be removed, or the diodes will fail. It is NOT
uncommon for the temperature of the diode board parts to be hot enough to boil water...and then some. The heat in the early airhead models was reduced by air passing over the board, from inlets at the bottom of the aluminum front cover. This was helped along by the way the aircleaner (the 'snail' housing area) 'sucked' air. In 1981 when BMW went to the electronic ignition, they also went to a snorkel type of rectangular air cleaner housing, and air flow was reduced. SOME air still passes that way, and out the louvers of the rear of the cover over your starter motor. This flow is small at low speeds.  The front cover was modified by BMW, which helped some.   On the faired models, such as the RT, the front center lower cover was initially a solid piece, but that was found to restrict air flow to the front aluminum cover, so BMW louvered it.  They interchange, or, you can modify your older solid outer cover with some sort of decorative hole design.

The reduction in air flow for the new style air cleaner, and the front outer cover, ETC....all caused some diode boards to fail, principally on the hotter running RS and RT models due to the fairing. The overwhelming percentage of failing boards were of the Wherle brand, due to improper manufacturing.   The major problems really started occurring in the 1981 era, where BMW was seeing a lot of melted solder joints/failed diode boards, almost all of them the Wherle-made boards as noted above.   BMW mistakenly thought that vibration was the problem, and installed rubber mounts. They recognized the diode board problem eventually, and began warranty replacements with an updated board that had the proper bent-over leads. The Bosch-made boards never really had the, or much, of a problem. Rubber mounts fail from age, are easy to damage when old,
probably cause a bit of additional heat to remain the boards, require extra and troublesome wires for special grounding, and are just plain AWFUL, and should ALL be changed to solid metal mounts, available from at least two sources in the Aftermarket, Thunderchild, and Motorrad Elektrik. They are a PIA to install, but never need to be removed, and HELP the alternator performance AND reliability.   There is a LOT more to all this, see the Author's website, especially in article #18.    http://bmwmotorcycletech.info

The early Wherle boards were improperly made.  The wire leads from the 6 power diodes were not left long enough, and being very short, were not bent-over and THEN soldered to the printed circuit board. The bending over of the longer lead and soldering, like Bosch did on its boards,  spreads the heat arising from the connection current to a larger area of the printed copper medium. So, the Wherle boards large diodes solder joints overheated, and visibly so. In order to see this fully...AND fix it... you must chemically remove the paint covering the board. OAK posted a fix for the Wherle boards problem in AIRMAIL. This is NOT an easy fix for most folks, as it involves drilling a tiny hole next to each of the 6 diodes, and adding a wire running through the board, wrapped and soldered to the diodes' leads, some of which are VERY difficult to get to with conventional soldering irons. I have experimented with a modification that uses a higher temperature solder, 50-50 plumber's solder, with added rosin flux, and so far have had good luck.

In the instruments group is a GEN lamp bulb. That lamp provides the Initializing Current to the alternator rotor; and, after the rotor is spinning, the alternator self-energizes.   That is, a small amount of total alternator stator output is rectified by a set of smaller diodes, which apply that current to the voltage regulator input, from which the rotor is supplied.   A modification article for this lamp circuit is on this website, and the Author's, to eliminate non-charging if the lamp should burn out (rare).

The rotating part of the Alternator is called the Rotor, the stationary part is called the Stator. The rotor is affixed to the male taper-nose of the crankshaft by a bolt and mating female-taper of the rotor. The tapers must be dead-clean to mate properly, and a SPECIAL and HARDENED bolt is used to remove the rotor, an article on this website describes how to make one if you do not wish
to buy one from BMW. The rotor is a combination of magnetic steel material, copper wire windings, and two copper alloy rings, called Slip Rings, upon which two carbon brushes ride, which wear slowly. An article on this website describes the procedure on changing brushes.

The 1981+ electronic ignition needed an electrical system with a smooth (called 'clean') electrical supply, so the arcing points type of mechanical voltage regulator was replaced by an electronic type. Late models of electronic regulator can pass more current; required by the low resistance later rotors.  That is a slightly moot point, due to the regulators configurations internally, with built-in protective resistors, but that is beyond the needed information for this article.

When the ignition key is first turned on, and the engine not rotating; or, rotating at near idle rpm, the battery supplies a tiny bit of electricity through the GEN lamp, through the regulator, and thence to the rotor, which slightly magnetizes that rotor. The rotor, when turning fast enough, causes a large amount magnetic field to be developed in the stator into electrical current.   Thus, mechanical forces are ultimately changed into electricity.

Rotors fail, especially early ones that used a type of lacquer to hold the windings in place. I have speculated that this is exacerbated by sudden large rpm changes during POOR shifting. The most typical failure mode is to have an open internal rotor connection, and often this is annoyingly intermittent.   Rotors do not fail in any high percentage amounts, but enough do, to be commonplace. Rotors can be rewound by such as Motorrad Elektrik. It is rare for a stator to fail; and when they do it is from MIShandling; or, otherwise a shorted winding.  The mishandling is often a clumsy slip with a tool.

Diode boards can fail in strange ways and cause strange symptoms. BMW also has had problems with the diode board upper heat sink (due to the rubber mounts) not being in solid electrical contact with the engine case, and BMW added some grounding wires, and that and two other updates made for a total of three updates! You can read all about it on this .org website, or my website, which has more details:  http://bmwmotorcycletech.info

The installation of the aftermarket solid diode mounts, which bolt to the engine, eliminate most if not all of the grounding problems that required the factory bulletins. The solid mounts also
conduct some modest amount of diode board heat to the engine casting and to the air flow, and I believe THAT ALONE, besides the rest, improves the reliability of the diode board considerably.

Another modification that BMW made, as noted earlier in this article, was to put large louvers in the fiberglass fairing cover, allowing, hopefully, more air to flow into and through the alternator/diode board area.

Removing and installing the aftermarket solid metal mounts is a PIA. BUT, once installed, these solid mounts never need replacing. Around 1992 or so, BMW went back to the solid cast-in mounts that it used many years previously.   When replacing a diode board, an 8 mm tubular wrench tool (or modified socket) is best.   I suggest such a tool be obtained, modified for a thinner wall at the working end, and kept in the on-bike tool kit.  Trying to remove the diode board using a common open end or box wrench is VERY frustrating.

***NEVER, EVER, remove the front aluminum cover of the engine without FIRST disconnecting ALL wires at the battery negative terminal.   IF...and only IF...you have ONLY the ONE large battery cable connection at the battery negative, THEN it is safe to remove the connection at the speedometer and breather bolt.

snowbum #1843
http://bmwmotorcycletech.info
12-03-2007