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Troubleshooting the BMW Airhead Motorcycle Alternator/Charging Systems
Copyright, 2014, R. Fleischer
trbleshootALT.htm 
15-B



FIRSTLY, and IMPORTANTLY, this article is NOT a substitute for Articles #14, #15, #15-A.   This article is to be used in addition to those articles.    I have not placed lots of hyperlinks in this article, although a few are here, as  I WANT YOU to read those three articles!

You may also want to refer to one of the articles on the Airhead Voltage Regulators.


Troubleshooting a dead GEN lamp:

Because a common problem is a GEN lamp that does not illuminate at all, this troubleshooting section is first.
 

The problem may not be a bad bulb. The GEN lamp SELDOM burns out. More often it is the rotor that has opened, or connection where the lamp fits, occasionally the VR, and sometimes worn brushes.  You can easily test the rotor without any instruments.
As a double check: Turn on Ignition....GEN lamp should still be out.
Disconnect battery (removing all the wires to the negative post will do).
Remove front cover.
Reconnect battery.
Turn on ignition.  GEN lamp should still be out.
Pull off the wire/connector going to the brush holder male spade that is marked as Df.
Df is the place the lamp and VR both apply the initializing current (and, after the alternator produces current, where the alternator self-energizes).
Stick a bent paper clip into the wire/connector, and ground the clip to the case. The lamp should light up. If you do not have a paper clip, UNplug the associated wire, which is the D- female spade connection, and plug that Df wire into the D- female. D- is ground. If the lamp lights up when grounded, and not when to the Df terminal, then the rotor is open, or the brush(s) not making contact. If the lamp does not light up when grounded, it is the lamp, or the wiring or pod connector or the VR. 
To further test for the lamp circuit condition, and eliminate the VR as the problem:
Turn off the ignition.
Pull fuel tank.
UNplug the VR.
Turn on the ignition.  
Ground the solid BLUE female connection wire in the voltage regulator's harness socket.
If the lamp does not light up, the wiring from the VR to the pod or its large rear plug, or internal wiring or lamp or lamp connection in the pod is at fault.

Do NOT forget to disconnect the battery before replacing the front cover.


Description of the Charging System Parts:

The BMW Airhead charging system is the same; with only some modest variations, on ALL models from the end of 1969 (with the introduction of the /5 series), to the end of production in 1996.  Changes over the years included a larger stator and reduced resistance of the rotor, and one more diode board connection in 1974-1975 transition period, and two more changes to rotor resistance in later years.  The Authorities models have varied somewhat; but, in general, they begin charging at a lower engine rpm; but the TOTAL MAXIMUM output is REDUCED; but the voltage regulator is set to a higher voltage.   This is, or can be, depending on other factors, also the effect of later lower resistance rotors; with some late versions rated at ~238 watts.   These details are more for the nerdy (except the /5 information, see next paragraph), as rotors get changed (brushes and rotors are the main failure points, with occasionally a diode board or voltage regulator), and generally no one pays any attention to the rotor diameter nor resistance when replacing one.
  
The Airhead charging system consists of a three-phase alternator, a diode (rectification) board, a voltage regulator, the GEN lamp, and the battery.  Since the GEN lamp is fed by the ignition switch, we could include that switch.    As noted, the maximum wattage output varied by model and year and what is in any specific bike, since most parts interchange physically and for the most part, electrically.   The stock /5 had a 180 watt alternator, which can be upgraded to the 280 watt alternator by use of a very specific stock early /6 alternator STATOR; only one version of which will physically fit.  That is the 1974 and some early 1975 production year, but you MUST use the 105 mm stator, and some late 1974 and early 1975 models had a 107 mm stator, which will NOT fit the stock /5 timing case.  You would also need the /6 or later style of diode board in order to realize the full benefit.  Aftermarket more powerful alternators are available, and they have their own article on this website.

Variations over the years included adding the center-tap to the stator winding and adding the then needed connection and three small diodes to the diode board (/6 and later ...any alternator after the stock /5).  This was done to the 105 mm /6 alternator and later 107 mm, to make this quite clear.  The stator diameter is the measurement of the stator part that fits a small distance INTO the timing chest cavity for the stator.

BMW mounted the diode board on rubber mounts on some models which was a bad idea, and was never done on all models, see later in this article.  BMW also has used a variety of battery ampere-hour sizes (with two basic sizes of case over the years); and, has changed the windings on the rotor (resulting in 3 different resistances), and there are several versions of the voltage regulator.   

Nerdy point: There was a rotor diameter change during production, early being 73.4 mm, later 73 mm. It is unclear if this was the R90S change in rotor diameter, which was carried through to the end of production, but it is likely. 

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; and the last of the Airheads were probably 240, but it is unclear, as literature is not all that consistent.   The R90S STATOR inside diameter was changed due to the rpm attainable, to avoid the rotor from striking the stator laminations, due to crankshaft whipping. The larger diameter 73.4 rotor could be used, however.   I believe that the idea of rotor interference was mostly unfounded, perhaps in racing it was seen; or maybe some instances of tolerances accumulation. 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, the smaller rotor, as the R90S.   Almost all alternator parts are physically interchangeable (the 105/107 mm stator being the exception)....and one might find almost any combination of parts when examining an Airhead charging system on any one bike.

The /5 model diode board 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 (the /5 had a higher resistance rotor too, 6.9 ohms, often just called a 7 ohm rotor).  All the diode boards interchange, but use of a /6 or later board (with its Y connection to the center-tap on the stator) on a /6 and later, is mandatory to obtain full rated alternator output. 

The first voltage regulators (there are no current regulators used on the Airheads) were mechanically operated, in a metal can.  These had a mechanically adjustable relay, the contacts of which fed electrical current to the rotor, and the contacts began to vibrate or open, as the battery voltage (actually, alternator output) reached the adjusted-for value.  The next regulators were similar, the metal can slightly less tall, and inside was electronics for the regulation.  These regulators were adjustable also, the adjustment being sealed by a drop of paint on them, but the regulators could be modified for full adjustability.  The last of the regulators were in flat plastic boxes, fully transistorized, not made to be adjustable.  All sorts of information on these various regulators are in separate articles on this website.

In the early 1980's, BMW had a lot of problems with Wehrle brand diode boards.  These would severely overheat at the short soldered wires from the 6 large power diodes, due to failure of them to be bent-over before soldering.  This is fixable.   Aftermarket high power diode boards are also available commercially.  The stock board is certainly adequate.

The electronic regulator should be used on the 1981 and later bikes, as they have need for a smoother, less spikey (electrically) output due to the electronic ignition.  On ANY Airhead, 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 OK.  I prefer modifying the early Bosch electronic regulator in the metal can, so the adjustment is easier to do (on this website in detail); or, to use an aftermarket adjustable regulator, from such as RockyPointCycle.com or euromotoelectrics.com
 

 

Description of how it all works:
When the ignition key is turned ON (or pushed, in the case of a /5), a small amount of electricity flows from the battery, through the switch, then through the GEN lamp, and then to the D+ terminal of the voltage regulator (blue wire).  The lamp internal resistance acts to limit the current and give you an indication that the lamp is OK.  This small current then travels further, through the voltage regulator via its Df terminal (Blue-black); and then to the Df terminal at the brush holder for the rotor.  The electricity goes into the rotor and then out the rotor via the D- terminal, to the engine ground, which is the same as the battery negative (-) terminal, electrically-speaking.   This makes a complete circuit, and the small current will then produce a magnetic field in the rotor.  This is a SMALL current and SMALL magnetic field.  The GEN lamp is LIT, brightly.
 
When the engine is started and thus the rotor is rotating as it is fastened to the end of the crankshaft, the rotor magnetic field is transformed into Alternating Current electricity in the non-moving stator windings.  Six small  diodes on the diode board are used to change a small amount of produced 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, the lamp extinguishes, as it has ~+12 on one side, and ~ same +12 on the other side, and thus no voltage DROP across it.   These 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 battery-fed lamp current and enough rpm. 
 

There are six high power diodes mounted on the diode board.  They connect to the stator main output windings, the same as do the above smaller diodes.  As the rpm (and thus the voltage in the alternator stator) rises just a bit more than for lamp-extinguishing, these large six diodes will begin to pass a fair amount of current, but their connection is to the battery, and not even through the ignition switch.  These diode board 6 large diodes are 3 positive and 3 negative.  The board connections (via printed copper 'wires') have 3 diodes to the chassis (negative side of battery) and 3 diodes to the battery + terminal output of the diode board. That + output of the diode board is the larger spade terminal on the right side of the board as you face it from the front of the motorcycle.  That connection is NOT FUSED and depends on the extremely high reverse resistance of the diodes to prevent the battery from discharging into the alternator when the engine is not running (key on or off).

If the rpm is high enough, or system needs low enough, the alternator could produce too high a voltage into the battery and the rest of the bike.  The voltage regulator has an internal voltage reference, and the regulator receives the output of the mentioned six small diodes, as previously described, and compares that to the internal reference (mechanical regulator does this with spring and coil).  With the voltage rising excessively, the voltage regulator then reduces the current flow into the rotor, which reduces the stator output. 

The mechanical regulator regulates by separating two contacts in the regulator.  Separation occurs as voltage rises and increases the mechanical relay's magnetic field in its coil.  Contacts will actually open and close rather rapidly once the regulation set voltage is reached, thus they spark gently during actual voltage regulation.  This produces some electrical noise into the wiring...as well as deteriorating the regulator's points, although slowly.   The later low ohms rotors are really a bit much for the mechanical regulator, as the rotor could draw more current than the mechanical regulator points were designed for.

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; and, read the various articles on this website; particularly the one that compares the Bosch alternator output with the aftermarket alternators.   The /5 180 watt system is adequate for the later 55/60 watt headlight (original was 40/45 watts).....and maybe a few smaller lights; it can 'just' handle a 50 watt heated vest....but that is the limit for practical purposes.     Any later alternator is adequate for all of those things easily.

 

What fails?  Basic answer:  anything and everything has failed at one time or the other!  :
Rotors fail due to aging, and rotors are very commonly rewound....or newly made....by companies besides the original equipment Bosch.  Without measurements, you may not know what rotor you have.   For the most part, rotors are being rewound with the later lower resistances..  These work better than earlier 7 ohm rotors.  It is not a great idea to install a 3.4 or 2.8 ohm rotor into a mechanical voltage regulator bike, without changing to the later electronic regulator in the plastic case.     Stator failures are NOT common. 
Rarely one fails from failed insulation.   Some have failed from owners changing rotors, and nicking the stator windings in the process.  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 and/or the starter relay connections or internally.  The GEN lamp, necessary in the stock system to initiate charging, does not fail often.  There is a modification that allows the system to start up even if the lamp fails (and makes a slight improvement on rpm at which charging begins).....that is on this website....it is article #19.

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 indicated by poor or no charging or wrong voltage.  VR failure is hardly the only failure to exhibit these problems.   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, yet charging may be OK, headlight OFF.  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.   Overheated or slightly dirty or corroded connections ARE COMMON.   GEN lamps that can be seen glowing dimly at night while cruising are usually an indication of 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.  Rotors can fail at cruising and higher rpm, and be OK at lower rpm.

After the /5, the printed circuit board that the GEN lamp fit into can get microscopic cracks, and the lamp connection to the board can fail.  You can carefully repair the board....or replace it (IF you can find a replacement!).  Do NOT yank lamps out roughly.....press on the printed material and rock the lamp out carefully.


From 1976, many models have a connection of the battery into the electrical system located at the starter relay under the fuel tank.   The red wires there can cause problems if there is corrosion at those connections of the relay prongs (spade connections) and socket.  A similar problem occurs if the INSIDE of the relay box has corroded connections, from such as brake fluid or moisture.   The problem can be as slight as just a small voltage drop, or as major as a total complete lack of electrical power.    This is because the INTERNALS of the relay, even if the relay is NOT being actuated, have a 'jumper function'.  These larger gauge red wires, involved in the internal connection, can be cut into ...NEATLY!!...and joined permanently, which fixes the problem forever....unless there is an INternal relay corrosion problem.    There are NO problems caused by the external jumpering, if done neatly.  If the battery has voltage on it WHILE the lights are on (but they do not light-up), you may have a relay plug/socket problem, or inside the relay....probing with a voltmeter will tell you if power is on all red wires at the starter relay, or not.

 

If something fails, what are the USUAL indications?, and how to determine what is going on:
1. Batteries:  A battery failure can cause an enormous amount of problems and indications.   It is QUITE possible, and, frankly the norm, for a battery to simply lose capacity over time.  That in itself is usually not a problem, as we normally do not operate many hours of accessories with the engine off.  Of course, if severe enough, the starter motor will not have sufficient current to rotate the engine.   One common failure mode is a full separation....or part separation...of a connection inside the battery.  The battery will act as if there is a resistance inserted in series, and may charge up to the correct voltage at the battery terminals, yet be unable to properly crank the engine.  This is easily seen by measuring the battery terminal voltage, ignition off, then ignition on, then when attempting to crank the engine.   The battery may not crank at all, or you hear just a simple clunk if quite bad in this respect, yet the headlight MAY seem to operate normally...until the starter button is pressed.  

The headlight MIGHT dim to nearly or completely OUT, when cranking.   In a few instances, once the engine IS started and run, the battery MAY seem to act OK....until the next cold startup. It is rare, but I have even seen batteries that were temperature sensitive, regarding the opening and closing of an internal connection.    Another common failure mode is a shorted cell, usually only one fully shorts, but, it can be partial.   The battery MAY never charge up to proper voltage, and if this is seen when using an EXternal charger with a voltmeter attached, 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 to 12.7.  That battery should be discarded too.  There are dynamic battery testers at most repair shops, that can USUALLY determine if a battery has the proper cranking power.   These are usually called Battery Load Testers.  You can get your own at a quite reasonable price from Harbor Freight Company.   I test all my vehicles' batteries once or twice a year.

It is possible for bad starter motor to 'pull' or 'draw' so much current from the battery, that the battery appears faulty.  This is commonly seen with the early Valeo starters in which the pole pieces, which were glued in place, come loose and lock up the starter.  The Bosch starters are FAR better constructed than early Valeo starters, and the Bosch are easily re-buildable 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 number of teeth version.    Late model Valeo starters are OK, and draw less current than the Bosch, and Euromotoelectrics.com has them reasonably priced....see my references page.  They are a GREAT source for electrical parts for your bike.

A load test at about 90 amperes on the 28 or 30 AH batteries (and 70 on the smaller batteries), whilst watching the battery voltage, is about correct for the Airhead batteries (either size).  The load test is done by reading the voltage accurately at 15 seconds of loading time.   There is a chart available for the lower voltage limit, versus temperature....and usually the load tester comes with the chart.  

2.  Diode board and rubber mounts:   On those models with rubber diode board mounts, the mounts will eventually deteriorate, and then allow the diode board to fall downward and cause shorting or other problems.   In MY OPINION 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 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.  

The Airheads that already should have the solid metal diode board mounts (check your bike, no matter the model) are:
/5 models; /6 models; 1978-1987 R65 and R80 models.

Once in awhile I hear of someone installing solid mounts; or, for some other reason, they have had the diode board out of the bike, and when replaced, there is no charging.  They have probably mis-wired at the rear of the diode board. 

No charging, and a bright GEN lamp, can come from improperly re-assembling the rotor connections' white-colored brush connection block....the insulating washers must be properly assembled at the Df terminal!!!

NOTE:  Over the years there have been 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

Information on testing using the transformer and lamp method is also on this website, but see the next paragraph...

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 with the diode board OUT OF THE BIKE using a 6 or 12 volt brake or turn signal light bulb and a 5 to 15 volt AC transformer is a VERY good test, and HIGHLY recommended, as it IS a better test.  A series circuit is used.  Shorting the leads gives full lamp brightness.  Connecting across a diode gives roughly half-brightness.  Anything else and the diode is faulty.  Both tests, ohmmeter and lamp/transformer; are a good idea.   Many multimeters now incorporate a Diode Test function.  The function typically reads the forward voltage drop of the diode, with a small current applied.  It is roughly as good as the ohmmeter test, and the lamp test is MUCH better than either.

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.  Much more rarely is an overvoltage regulator 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 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.   Rarely the internal regulator series transistor short-circuits, and the indication is always a vastly too high charging voltage at higher rpm. 
Best sources for voltage regulators:
euromotoelectrics.com.
Rockypointcycle.com

NOTE:  a somewhat rare condition, that can drive you nuts, is when the ground wire (brown) from the voltage regulator is OPEN, either from a faulty wire or connection; OR, from corrosion or other problem with the three pin plug at the VR.   An open ground connection will generally result in a WAY high charging voltage....upwards of 20 volts is possible, depending on the condition of the battery.

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 windings, ohmmeter tests on the three phases themselves are not usually indicative. HOWEVER:  There must be no continuity to ground from any stator terminal (stator disconnected from anything else including, of course, the diode board).   There is a difference in the resistance of the windings after 1990, but this is mostly a point of nerdy discussion.    Stator failure means low output/charging.  The group of three connection wires to the one area of the stator can be in any order.   Some models have a molded plug for these three.     Typically, the failure involves a shorted turn or a grounded stator.  The shorted-turn failure can be very tricky to analyze, because while an openly visible shorted turn is easy to see (typically from those using screwdrivers to pry the stator from the engine, when changing brushes, etc....), the resistance measurement may not show any difference between phases, on common ohmmeters.
Some types of tests would be Ohmmeter tests; A.C. connection to a transformer and then read voltages or use an oscilloscope; A.C. ammeter tests, and several other types of tests can be done.
One of the better and easier to do set of tests would be the following; all can be done with the system on the bike, and are done with the phases and centertap (if present) wires disconnected from the harnesses to the stator (all simply pull off the stator spade lugs):

    a. Ohmmeter tests, between phases, and to ground.  Short the ohmmeter leads together, and subtract
        the meter and its leads indication from the readings you get from connecting the ohmmeter test leads
        to every combination of the three output spades of the stator.  The stators are typically 0.62 ohms
        per phase connection.  If you test the centertap terminal, located on the other side of the stator case,
        the values between that center tap and any of the three main phases will be ~HALF the phase to
        phase value.    This is not the best testing method, the resistances are hard to read accurately.
    b. With Df harness wire disconnected, and Df connected to battery +; measure phases with A.C.
        Voltmeter, at ~2000 rpm.  The voltage should be about the same between any connection
        type of the three terminals stator output group.
    c. Repeat, but with A.C. Ammeter....properly connected!  Few of you have an A.C. ammeter, and
        the connection must be series type, so this test is seldom done by anyone.
    d. The tests can also be run with a common headlight lamp or test lamp, for equality in brightness,
        instead of meters.  Simply put the lamp across any phase connection.
    e. FAST test for stators on /6 and later types:  disconnect the 3 phases and centertap connections. 
        GROUND the centertap.  Use a test light, at just a bit over idle rpm, on each of the three stator
        terminals, the other end of the test lamp to chassis.  All should light up the same.

        A shorted turn makes a rather gross difference in output from a phase.

5.  Rotors:    Rotors rather commonly fail, they rotate fast and can have large centripetal forces on the windings when up and down shifting as rpm change can be abrupt.  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.    Rotors are in an area that gets hot, and the current flowing into the rotor adds more heat....and thus there are heat-cycling effects.  Same can be said for the diode board, which is susceptible over the very long term to this type of heat cycling effect.      In the rotor, its 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 (hopefully by vacuum impregnation)....and thus the wires can move about from vibration and centripetal forces on original old rotors and be damaged.  Rotors usually OPEN, and a simple ohmmeter test across the slip rings will identify that.  An open rotor or brush will usually not allow the GEN lamp to light up.  A rotor can work (sort-of) at low or high rpm, and not at the other rpm.  GEN lamp indications can be erratic with rpm too, signifying an opening and closing of the rotor internal connections.
 

IMPORTANT:
Once in awhile a rotor will short circuit, and this may or may not be such that an ohmmeter can show a short.  Another form of rotor shorting is slightly 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 then 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.   


VERY!! IMPORTANT:
Rotors are easy to remove, but you MUST use a hardened factory, or HARDENED home-made tool. 
DO NOT USE A COMMON SOFT BOLT.  8.8 is the minimum specification!!   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!!!!

The BMW sold tool is 88-88-6-123-600
You can get the equivalent tool from such as euromotoelectrics.com, etc.  They are NOT expensive.  You can also make a tool from 8.8 bolts....ask on the airheads LIST.


6.  Brushes:  Brush failure is common (there is an article on this website on replacing the brushes).  Brushes are a hard conductive carbon material and wear slowly.  When the brush is short enough, the snail spring that presses the brush against the rotor slip ring will bottom out on the plastic 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.   It may be rpm sensitive.  If the slip rings show approximately the proper resistance, yet the resistance measurement at the brushes themselves (Df to D-) is not approximately 3/4 of an ohm higher, the brushes are likely not contacting properly...and a physical inspection is needed.  Brushes last ~80,000 miles....depending on the dust in the atmosphere, amount of wattage normally used, etc.   See the brush replacement article. 

As noted above, 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 or irregularity of non-concentricity of the slip rings themselves, 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).   The BROWN wire MUST be to the brush D- terminal!...if you reverse the brush terminal wires, the alternator will not work!  You do NOT have to disassemble the brush holder mounting for a brush job.

NOTE!....if you had the diode board out, and you have no charging, you probably failed to connect one wire at the rear of the diode board!
 

7.  Miscellaneous items:  
Key switches may develop intermittent and excessive resistance, which can confuse the voltage regulator operation. 
Wires can get pinched. 
A rare short circuit in some /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.  
If the POSITIVE heat sink of the diode board is grounded (the board has insulators for JUST that top section) from such as wrongly placed wire or failed insulator, etc., then the board output will be shorted to ground.  The board output connects via a heavy red wire to the battery.  Other causes for a ground at that diode board output (large spade, right side, as you face from front of the bike) are shorted diodes, maybe with a shorted stator. 
  
Wires (well, the end connections) 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 the battery direct connection.  
The female spade connectors can be carefully squeezed to fit much tighter.
There is no need for the insulating black covers over the leads to be perfect, if yours are cracked and partly missing.

It is rather common 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.   Rarely there is a problem with the pod CABLE PLUG.

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.

Front Covers:  /5 covers had poor ventilation.  Using a later cover will improve reliability.


 Addendum:

The following graphic came from: http://www.thunderchild-design.com/images/charg1.gif
I have modified the sketch with some identifications of wire colors and small diodes.

 


 www.buchanan1.net/charge.shtml:  There is information on that page, but there ARE ERRORS.  I am listing it here just because some know about it.  Please do not use....as of 4/27/2014 errors were STILL not fixed.


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.     See my URL page on this book, and there is a critique of it on this website too!

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
02/23/2009:  added some clarification to rotors and stators
05/09/2009:  recheck, fix minor typos and clarify a few details.
1
1/21/2009:  review and update entire article.
08/13/2010:  Review and minor updates, mostly for clarity
01/15/2011:  Add the Thunderchild gif, modify information about buchanan's page.
09/18/2012:  Greatly expand section on testing the stators. Add QR code.  Modify Google code. 
                     Go over entire article, and add material and update things.
05/13/2013:  Go over article.  No substantive changes, but eliminate some redundancies and add some
                    clarity here and there.
06/15/2013:  Add first section, troubleshooting the dead GEN lamp
remove language button later, due to problems on some browsers

Copyright, 2014, R. Fleischer

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