Many are intimidated by electricity, and the amount of electrical problems with Airheads reflects …to a considerable extent…a lack of owner-maintenance, such as a lack of regular inspection, particularly cleaning and tightening various connections, checking alternator brushes, testing the battery and changing the battery before a catastrophic failure; and just not paying attention to something that seems not right.  If properly maintained, the Airheads electrical system works fine and is reliable.  There are numerous articles on identifying problems, doing maintenance, and dealing with charging system problems in these Airheads.org Technical Tips articles, and many more on the author’s website,  https://bmwmotorcycletech.info/index.html.

If the motorcycle is ridden mostly in stop and go city traffic, the charging system may not keep up, and the battery might require 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.

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 the Voltage Regulator is mechanical or electronic. An electronic type will substitute for the mechanical type. I have posted information in articles on this website, and more extensively on my own website, link just below, 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:
https://bmwmotorcycletech.info/index.html

BMW has used a number of different alternator rotors over the years. All look nearly the same and most physically fit interchangeably but there have been changes that should be noted. The /5 rotors had a higher electrical resistance (about 6.9 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 R90S was rated at 238 watts, and the later models rated at 240/238/250/280 watts, depending on year and model and if a Police model or not. Stator differences accounted for some of the variances besides the rotor differences; and the /6 and later had a more complicated diode board that increased the alternator output.  If a rotor fails, you can purchase new ones or rebuilt ones, and many (if not all) of the rebuilt ones use later lower resistance windings, and the performance of the alternator is somewhat enhanced by having more output at somewhat lower rpm. On some the maximum output is decreased a bit, which is generally of less importance than the lower rpm improvement, particularly for stop and go city riders. 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 a VERY FEW early 1976 calendar year) 280 watt alternators will fit the /5 timing case. The /5 timing case requires a stator of 105 mm diameter.  That stator size was only used in the 1974-1976 era, and NOT all in 1975/1976 were 105 mm.  All later stators, that is, most in/from 1976, were 107 mm in diameter.

The stator (stationary, non-rotating) 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 a new connection located on the stator housing, and the diode board was modified with additional small diodes that 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 fine in a /5 system, one simply makes no connection to the additional terminal on the board (unless one has upgraded the /5 alternator to a /6 and later type, as mentioned previously).

On all diode boards there are six large silicon diodes press-fitted into aluminum angles used as both heat sinks and mounts.  These large diodes are of two types, three of each, 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 common for the temperature of the diode board parts to be quite hot during running, from engine heat and diode electric current flow. 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 snorkels type of rectangular air cleaner housing, and air flow was, IMO, reduced. SOME air still passes that way, and out the louvers of the rear of the cover over the starter motor. This cooling flow is small at low speeds, but the aluminum front cover was modified by BMW, which helped some with air flow.   On the faired models, such as the RS and RT, the front center lower fairing 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 (IMO) with the new style rectangular 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 problem was a failure to bend the leads of the six large diodes sideways, increasing current capability, before soldering, and the soldering was not on a large enough copper area.  Generally, problems 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.   BMW mistakenly thought (IMO) that vibration was the problem, and changed the inner timing chest casting so that they could install 4 rubber mounts for the diode board. They recognized the diode board problem eventually, and began warranty replacements with an updated board that had the proper bent-over leads.

Explained further:  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 soldered to the printed circuit board). The bending over of the longer lead and soldering, like Bosch did on most of its boards, spreads the heat arising from the connection current to a larger area of the thin printed copper medium.  The Wherle boards large diodes solder joints overheated, and visibly so, right through the protective coating sprayed onto the boards.  It is probable that normal heating, cooling, and expansion & contraction of the board helped to break solder joints.  In order to see this fully…and fix it… you must chemically and physically remove the paint covering the board at the six solder points for the big diodes. OAK posted a fix for the Wherle boards problem in AIRMAIL that is NOT an easy fix, 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.  This is withOUT drilling holes and adding wires.  Most folks never try repairs to the diode board, as new ones or aftermarket versions are not too expensive.

The Bosch boards never really had much of a problem, even though some were made with non-folded-over large diode leads. I do not have a solid answer for you on this. I suspect that Bosch and Wherle traded back and forth!

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 (additional grounds were added by Bulletins later), and are just plain AWFUL.  ALL rubber diode board mounts should be changed to solid metal mounts, available from Thunderchild, Motorrad Elektrik, and Euromotoelectrics. They are a PIA to install, but never need to be removed, and they do help alternator performance, reliability, …and can also improve voltage regulation.  There NEVER WAS a vibration problem, IMO.  There is a LOT more to all this, see the Author’s website, especially in article #18:    https://bmwmotorcycletech.info/diodebds&grdgwires.htm

In the instrument pod is a GEN lamp bulb. That lamp provides the initializing current to the alternator rotor; and, when the rotor is spinning moderately, the alternator self-energizes.   That is, once rpm is up a bit, perhaps over 1500, some alternator stator output is rectified by a set of smaller diodes, the output of them is applied to the voltage regulator input, from which the rotor is supplied its energizing current.   A modification article for the lamp circuit is on this website, and the Author’s (in more detail), 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 actual holding is by the tapers, the bolt is simply there to draw the tapers into full contact with each other, and ensure they do not separate.  Thus, the bolt need NOT be very tight, and best it is NOT over-tightened. I DISLIKE the practice some folks have of rotating the engine by an allen wrench in that bolt, for such as valve adjustment procedures, and I think the factory torque specification on the bolt is too high.  The tapers must be absolutely clean to mate properly, and a SPECIAL and HARDENED bolt is used to remove the rotor.  Use of a hardened bolt or tool is an absolutely, positively, no question necessity, when removing the rotor!!!!   BEWARE of not using a hardened tool going into the rotor!!!

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.  The brushes wear slowly.  The slip rings can look black, dirty, and worn after huge mileages and yet still be OK.  An article on this website, and expanded in the author’s website, describes the specifications on brushes, snail springs and 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 regulators can pass more current; required by the low resistance last series of rotors.   It is possible to replace the current-pass transistor in any of the voltage regulators, if they should fail, most just purchase a new regulator, available at a quite modest price from aftermarket suppliers, such as Euromotoelectrics and RockyPointCycle.

When the ignition key is first turned on and the engine not rotating; or, rotating at idle rpm, the battery supplies a small amount of electricity through the GEN lamp, which then is applied to and through the voltage regulator, and thence goes to the rotor.  This current slightly magnetizes the rotor. The rotor, when turning fast enough, causes enough of a magnetic field to be developed in the stator which is turned into alternating current, some of which is rectified to D.C. by small diodes on the diode board, and re-applied to the voltage regulator, bypassing the GEN lamp, and the current available is now much larger, so the magnetic field in the rotor…and the stator, are vastly increased, and the alternator output can, upon need, be very large.

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 considered relatively  commonplace. You can purchase aftermarket rotors from Motorrad Elektrik at http://www.motoelekt.com and also from  http://www.motoeuroelectrics.com.
You can also get metal replacements for the troublesome rubber diode board mounts.  Makers were noted, above.  See also: http://www.thunderchild-design.com; who also makes an aftermarket diode board, which I am not fully convinced is better than the Bosch type….but, may be.

It is rare for a stator to fail; and when they do it is often from mishandling use of screwdrivers or other tools …shorting or otherwise ruining the lacquer insulation of the windings. Much rarer seen is a shorted winding from, perhaps, repetitive heat/cool cycles or excessively tight winding or both.  The common problem of mishandling is a clumsy slip with a tool, typically when removing the stator and case assembly. I use a small piece of hardwood.

Diode boards can fail in strange ways and cause strange symptoms. BMW 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, better, my website, which has more details:  https://bmwmotorcycletech.info

Installation of aftermarket metal diode mounts (which bolt to the engine inner timing chest wall), eliminate most of the grounding problems that required the factory bulletins, although I still like two grounding wires to be installed. The solid mounts also conduct some diode board heat to the engine casting and to the air flow.  Note that all years of Airheads did NOT have rubber mounts, this is explained in more depth in the author’s website.

Other modifications that BMW made, as noted earlier in this article, was to put large louvers in the fiberglass fairing cover, allowing more air to flow into and through the alternator/diode board area; and, BMW also modified the outer aluminum cover, so it had different air inlets and passageways for air flow.

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 (NOT ALL) years previously.

When replacing a diode board, an 8 mm tubular wrench tool (or modified 1/4th square drive size 8 mm socket) is best.   I suggest such a tubular wrench tool be obtained, modified for a thinner wall at the working end, and kept in the on-bike tool kit, as, otherwise, trying to remove the diode board using a common open end or box wrench is 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 ONE large battery cable connection at the battery negative, THEN it is safe to just remove the connection there, or at the speedometer and breather bolt.  It is OK to modify that lug at the speedometer cable fastening HOLLOW BOLT.  Cut just enough of the lug to allow it to push over the hollow bolt.  You MUST have 1 or 2 washers there, to avoid opening (expanding) the lug (if cut), when tightening the hollow bolt.  That bolt is a breather for the transmission, and you should NOT overtighten it ….and its hole must be clear.

https://bmwmotorcycletech.info/index.html
02/10/2021