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FIRSTLY, and IMPORTANTLY, this article is NOT a substitute for Articles #14, #15, #15-A.

What this Article IS, is a special addendum to those above articles. ON PURPOSE, I have not placed lots of hyperlinks in THIS article (although a few GOOD ones ARE here), referring to sections in those articles. I WANT YOU to read those three articles!

Brief Description of the Charging System Parts:
The BMW charging system, as used on ALL airhead boxer models from the end of 1969 at the introduction of the /5 series, to the end of civilian production of these machines, in 1995.....(and with some slight modifications to the stator, rotor, and voltage regulator for some Authorities (Police) models)....are the SAME....with only some modest variations.
    The System consists of a three-phase alternator, a diode board, a voltage regulator, the GEN lamp, and the battery.   Wattage output varied by model and year.   The original /5 had a 180 watt alternator which can be upgraded to the 280 watt alternator by use of a very specific slightly later alternator which will physically fit (part of 1974-5 production year) .   Other variations over the years include adding a center-tap to the stator winding and adding a connection and three small diodes to the diode board (/6 and later), mounting the diode board on rubber (bad idea, and never done on all models), changing battery sizes, changing the windings on the rotor (different resistances), and several versions of the voltage regulator. Whilst the tendency is to simply think of the alternators as 180 OR 280 watts of output, in truth, outputs are specified at 180, 238 (R90S and Authorities), 250, 280.   The R90S rotor was smaller due to the rpm attainable, to avoid the rotor from striking the stator laminations, due to whipping. That idea was mostly unfounded. The Authorities model has the same 238 watt rating as the R90S; but, is designed to produce usable output at a lower rpm than all the others, and the 238 rating is not because of the same reason(s) as the R90S.
    The diode boards are all the same in electrical function, except that the /5 model did not have the extra three diodes for the center-tap of the stator winding, as did the /6 and later, and thus this is just one other reason the /5 had a lower output, besides the stator windings themselves. All the diode boards interchange, but use of a /6 and later board on a /6 and later, is mandatory to obtain full rated alternator output. In the early 1980's, BMW had a lot of problems with Wehrle brand diode boards. These would severely overheat at the wires from the large power diodes, due to failure of them to be bent-over before soldering. The problem is fixable.   Aftermarket high power diode boards are also available commercially. The stock board is adequate.

The earliest voltage regulators were mechanical, made by Bosch, in metal cans. Bosch also made an electronic regulator that is in a similar, if slightly shorter can. Those are very good regulators, and are adjustable internally. There are articles on this website...and the Club's .org website...on these regulators...testing, modifying, etc. The electronic regulator must be used on the 1981 and later bikes, as they have need for a smoother, less spikey (electrically) output. The mechanical regulator slowly deteriorates, and an electronic type can be substituted, whether the Bosch, or Wehrle, or even a car type. Most any VR from a car that has the same three prongs and same case mounting, will work just fine in our bikes.

Description of how it works:
When the key is turned on, some small amount of battery electricity flows from the battery, through the key switch, then through the GEN lamp, and then to the D+ terminal of the voltage regulator (blue wire). The regulator allows this small lamp limited current to travel further, via its Df terminal (Blue-black)... to the Df terminal at the brush holder for the rotor. The electricity passes into the rotor, causing a slight magnetization, and then out the rotor via the D- terminal to the engine ground, which is the same as the battery negative (-) terminal, electrically-speaking.
As the engine rotates, the rotor magnetic field is transformed into alternating current electricity in the stator windings. Six small diodes on the diode board are used to change this small amount of electricity into Direct Current, and the electricity is applied to the same input side of the voltage regulator that the GEN lamp feeds. When enough rpm is reached to have about +12 volts supplied at that point, then the alternator stator and diodes are supplying current to the rotor, and the lamp extinguishes, as it has +12 on one side, and +12 on the other side, and thus no voltage DROP across it. The small diodes...as rpm increases...produce the MUCH larger current (than the lamp could supply), through the regulator, that is needed to fully power the rotor. Thus, the system is a merry-go-round, supplying itself....once initiated by the lamp current. As the voltage in the alternator stator rises, it eventually gets high enough to have another six diodes, much larger ones (needed to pass lots of current), change the AC to DC in the same manner, and THAT output comes from the B+ terminal of the diode board, and goes directly to the battery, UNfused. If the rpm is high enough, or system need low enough, the alternator could produce too high a voltage into the battery and the rest of the bike. The voltage regulator senses the output of the mentioned six small diodes, and begins to reduce the current flow into the rotor.

The mechanical regulator does the regulating by slowly separating the contacts in the regulator...vibrating them actually...thus, they spark gently during actual voltage regulation....producing a bit of electrical noise into the wiring...as well as deteriorating the points contacts slowly. The later low ohms rotors are really a bit much for the mechanical regulator to be used for long periods of time.

Performance:
The system in any of the airheads is adequate, particularly if you keep the rpm above 3000. Those with much higher system usage...such as larger wattage headlights, many extra other lights, heated clothing, ETC., may need to make accommodation for same. Ask at the Airheads LIST.

What fails? Basic answer: anything and everything has failed at one time or the other!
Today, many rotors have failed, and many rotors are very commonly rewound....or newly made....by other companies than the original equipment Bosch. Without measurements, you may not know what rotor you have.   For the most part, the rotors are being rewound with the later lower resistance, approximately 2.8 ohms. These work better than earlier rotors.   Stator failures are VERY UNcommon. Rotor failures, diode board and/or rubber mount failures, wiring failures, voltage regulator failures....and ignition switch failures....all are seen now and then.   The MOST common failure is the battery.
    Failure to disconnect the battery before removing the outer cover of the timing chest, has caused electrical sparks and failures of the diode board.
    Failure of the voltage regulator is usually indicated by poor or no charging....or just wrong voltage.
    Failure of the diode board is usually indicated by lower charging (lower watts) capability, or lower voltage output under load of such as the headlight.     ONE open large diode will so indicate by vastly lowered charging with the headlight turned on. Problems with the small diodes, while rarer, do occur, and can be seen by poor charging.   In fact, almost anything that goes wrong with the charging system components can result in lower voltage, or lower output/charging.   Even overheated connections.
    GEN lamps that can be seen glowing dimly at night whilst cruising are usually indicating poor connections someplace, or many places.
    GEN lamps that do not light up, and thus you have no charging (or, maybe only at VERY high rpm), are usually a bad lamp, a bad lamp socket, a bad regulator, or, commonly, an open rotor or excessively worn brushes.
    From 1976, many models have a connection of the battery into the system at the starter relay under the fuel tank that can cause problems if there is corrosion at those connections. These are larger gauge red wires, and can be cut into ...NEATLY!!...and joined.

If something fails, what are the USUAL indications?, and how to determine:
1. Batteries: A battery failure can cause an enormous amount of problems and indications. It is QUITE possible for a battery to simply lose capacity, that in itself is usually not a problem, as we normally do not operate many hours of accessories with the engine off. One common failure mode is a separation....or part separation...of an interconnection in the battery. That means the battery acts as if there is a resistance inserted in series. The battery may SEEM to charge up to the correct voltage, yet be unable to properly crank the engine. The battery may not crank at all, or a simple clunk, if quite bad in this respect, yet the headlight MAY seem to operate normally. In a few instances, once the engine IS started and run, the battery MAY seem to act OK....until the next cold startup. Another common failure mode is a shorted cell, usually only one shorts. This can be partial, or complete. The battery MAY never charge up to proper voltage, and if this is seen by using an EXternal charger, the battery can be considered worthless, and must be replaced. IF partial, the battery might charge up to the correct voltage, yet after standing a half hour or hour, have its 'open terminal' voltage fall towards 12, rather than maybe 12.5.

It is possible for a poor or bad starter motor to 'pull' or 'draw' so much current from the battery, that the battery appears faulty. This is commonly seen with the later Valeo starters, in which the pole pieces (glued in place!!) come loose and lock up the starter. The Bosch starters are FAR better, and easily rebuildable anyplace...they are interchangeable, but need the end support plate, and the early versions were 8 teeth, later 9 teeth, to match a change in flywheels...and you MUST use the correct one. The best test of a battery is the old-fashioned LOAD test, done at a battery shop with the proper equipment. Few motorcycle shops have such testing apparatus. Some car/truck battery shops have this equipment...and a load test at about 80 amperes, whilst watching the battery voltage, is the best test.

2. Diode board and rubber mounts: On those models with rubber diode board mounts, the mounts will eventually deteriorate, and let the diode board fall downward and cause shorting or other problems. ALL rubber mounted diode board models should have the diode board mounts changed to all-metal. Not only is reliability improved, but grounding and output is improved too! There is some goodly evidence that the diode boards stay cooler and more reliable. Rubber mounts are a BAD IDEA! Diode board failures can cause everything from low output and charging to no output and charging. Diodes fail from opening.....or shorting.

NOTE: Once in awhile we hear of someone installing solid mounts, or, for some other reason, having the diode board out of the bike, and when replaced, there is no charging. You probably miswired at the diode board, especially at the rear of it.

NOTE: BMW has had a lot of various problems with diode boards, grounding, the rubber mounts, etc. PLEASE read the article on the diode boards. PLEASE read the other electrical articles on this website....#14, #15, #15-A, etc. Some of the problems were also caused by extra heating due to the fairings on some models, and the change in 1981 (most countries) to the square air filter...which included a change to the cooling air flow through/past the diode board....into the starter area.

NOTE: Oak Okleshen published an extensive article on testing the diode board using a transformer and lamp, in the Club publication, AIRMAIL. June 1999 was a most comprehensive article, but there are many others. Buy the Airtech Index from Oak: askoak@aol.com

Diodes are usually tested (disconnect the bike battery first!!!) by using an ohmmeter, first with the leads in one direction, and then reversing the ohmmeter leads, so that forward conducting resistance is measured (ohmmeters have batteries and pass a small current through the probes) in one direction, and hopefully no or extremely high resistance in the other (NON-conductive) direction. A test using a light bulb and a AC transformer is a VERY good test, and HIGHLY recommended, as a better test.

3. Voltage Regulator: Usually a failure here means an opening of the series pass transistor (electronic versions of the regulator) and no output/charging. Other failures have been seen. The mechanical regulators usually fail by slowly deteriorating the output voltage. A failure to charge, in which the regulator is suspected, can be PROVEN, by bypassing the regulator. This is done with a short, perhaps 4 inch, jumper wire with male spades in each end. Remove the regulator plug, and insert the jumper into the opposing (NOT BROWN WIRE) plug connections. If the system now charges, replace the VR. In a very few instances, the series transistor can short-circuit, and the indication is always a vastly too high charging voltage at high rpm.

4. Stators: Stators almost never fail unless abused. Abuse is usually nicking or otherwise injuring the wires during stator/housing removal. Due to the low resistance winding, ohmmeter tests are not usually indicative. There must be no continuity to ground, of course. There is a difference in the windings after 1990, the resistance changed slightly, but this is mostly a point of nerdy discussion. Stator failure means low output/charging. Wires to the stator (the group of three) can be in any order. Some models have a molded plug for these three.

5. Rotors: Rotors rather commonly fail, they rotate fast and thus have centripetal forces on the windings. I have theorized that some types of clutch and shifting of gears can cause very high reversing forces on the rotors, particularly susceptible will be the old non-epoxy impregnated ones. On top of which, rotors are in an area that gets hot, and thus there is heat-cycling effects. Same can be said for the diode board, which is quite susceptible over the very long term to this type of damage-effect. Wires are soldered to the slip rings. Solder joints can fail. Original rotors were sealed in a type of varnish, and are not as good as later rotors with high temperature epoxy vacuum impregnation....and thus the wires can move about from vibration on original old rotors and be damaged. Rotors usually OPEN, and a simple ohmmeter test across the slip rings will show a problem, if any. An open rotor or brush will usually not allow the GEN lamp to light up. Once in awhile a rotor will short circuit, and this may or may not be such that an ohmmeter shows a shorted few turns. Another form of rotor shorting is different: one wire passes through the rotor steel core through a small hole, a place for short circuits if the rotor is not very carefully assembled during the rewinding process. The ohmmeter WILL show up a short to the rotor steel frame. To properly test for this whilst the rotor is in place on the motorcycle, you must slide a piece of paper under both brushes. I have seen rewound rotors improperly tested before sale, that were shorted. Rotors are easy to remove, but you MUST use a hardened factory, or HARDENED home-made tool. An article on how to make this hardened tool, is on the airheads club website. http://www.airheads.org See the Technical Tips section on that website, the article is entitled "Alternator Rotor Puller".

Under NO circumstances should you use a common automotive 'legs' type of tool, nor a NON-hardened bolt!!!

DO NOT fail to use a hardened tool!!!!

6. Brushes: Brush failure is common (there is an article on the website on replacing the brushes). Brushes are a hard conductive carbon material and very slowly wear. When the brush is short enough, the snail spring that presses the brush against the rotor slip ring will bottom out on the brush holder, and thus there is no longer any...or only slight...brush pressure against the rotor. You may have strange indications on the GEN lamp...it may seem to light up properly, it may not light up properly, it may not light up at all. If the slip rings show approximately the proper resistance, yet the resistance measurement at the brushes themselves (Df to D-) is not approximately an ohm higher, the brushes are likely not contacting properly...and a physical inspection is needed. Brushes last about 80,000 miles....depending on the dust in the atmosphere, amount of wattage normally used, etc. See the brush replacement article. NOTE!....as the brushes wear to the usable limits, the snail spring starts bottoming in its slot. At just the right amount of wear, the very very slight sideways wobble of the rotor while the engine is running (at some particular rpm usually) will allow the brushes to either contact.....or not contact....causing very irregular GEN lamp and charging. Be aware of this phenomena.

NOTE! It is rare, but happens....one replaces the brushes, and now there is zero charging! You probably removed the brush holder assembly, and failed to reassemble it correctly. The D- terminal is grounded to the case, the Df is insulated from the case, even though a first glance shows them identical. You ALSO MIGHT have put the two push-on wires on the wrong terminals. BROWN is ALWAYS ground (D-, in this instance).

7. Miscellaneous items:   Key switches may develop intermittents and excessive resistance, which can confuse the voltage regulator operation. Wires can get pinched. A rare short circuit is the /7 and later bikes at the two-wire connector near the top front of the engine...the blue wire may fray and short to the frame. Another wire problem may be the blue-black wire from the Df brush holder to the voltage regulator Df terminal.
     Wires can overheat and therefore have essentially too much resistance. This is seen at two places commonly: The alternator stator terminals (the group of three)...but is not seen at the center tap of the stator windings (/6 and later). It is seen at the other end of the group of three....wires....behind the diode board. Sometimes one sees overheating at the larger gauge RED wire at the right lower side of the diode board (as you face the board from in front of the bike)....that is that battery connection.
    It is NOT at all UNcommon to find hairline cracks in the flexible printed circuit board material inside the instrument pod, which, if in the GEN lamp circuit, can cause the GEN lamp to not work, and thus NO charging.
    Fuses: Fuses are generally not used in the charging system, certainly not in the output from the diode board to the battery. There is one exception, and it is the later /5 model that has fuses in the headlight shell, and some /6 models. On these, one of the fuses opening will shut down the GEN lamp.

Here is a URL for a website page that I suggest you IMMEDIATELY print:
An excellent schematic and discussion of the basic charging system. It is so good, and so legible, that I SUGGEST you PRINT a copy and keep it in your reference material...as, who knows how long it will remain on-line.
www.buchanan1.net/charge.shtml
For another URL with just the schematic: http://www.thunderchild-design.com/images/charg1.gif

A very complete book, written by OAK, on the entire electrical system of all the models from somewhat before the /5, to the end of production, is obtainable from the Chicago Club. This contains all the schematic diagrams of the entire bike systems, as well as very detailed individual parts and systems drawings, schematics, descriptions and operations, etc. This book is really a training manual, and does contain some troubleshooting information. I consider it a must have item. My url.com section may have the latest contact information on how to obtain this book...or....it may be in the other items on the website, such as section 14, 15A.

A small book on the airhead charging system, but quite adequate, especially for troubleshooting, with illustrations and practical advice, is available from Motorrad Elektrik.
http://www.motoelekt.com (256) 442-8886

Revisions:
Final release: 01-03-2004
11-14-2005: add hyperlink for additional schematic