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Troubleshooting the BMW Airhead Motorcycle Alternator/Charging Systems
Copyright, 2014, R. Fleischer

This article is NOT a substitute for Articles 14, 14A, 15A, 15B, 15E, 17.  This article is to be
used in addition to those articles.   
I WANT YOU to read ALL OF those three articles.

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

Troubleshooting a dead GEN lamp:

The problem may not be a bad lamp. The GEN lamp SELDOM burns out. More often the rotor has
opened electrically.  Sometimes there is a crack in the socket area where the lamp fits.  A bad
connection for the lamp can include at the lamp base, the pod plug connection, or inside the
external pod plug itself. Other places for problems are occasionally the Voltage Regulator located
under the fuel tank, on the right side of the frame backbone.  Worn brushes frequently cause problems,
but usually the lamp is intermittant, especially with rotor RPM. 
You can easily test the rotor 'reasonably well' without any instruments.
Assuming that the lamp does not illuminate when the ignition key is ON; recheck that situation first:

Turn on Ignition....GEN lamp should still NOT be lighted.
For a /6 or later motorcycle, wiggle, a bit, the umbilical cord that goes into the back of the instrument
pod.  If the GEN lamp lights up, the problem is in the plug, or, the mating male parts in the pod.
Assuming the lamp does NOT light up:  Turn off the ignition. Disconnect battery (removing all the
wires to the negative post will do). Remove front-of-engine metal cover. Reconnect battery.
Turn on ignition.  GEN lamp should be the same as before: NOT be lighted.

Connect a jumper wire from the brush holder Df terminal to the case.  You can also use a screwdriver
to do the jumpering.  If the lamp DOES light now, remove the jumper or screwdriver, and, with the lamp
now not lit, short the Df and D- connections.  You should get the SAME results, lamp lights up.
Conclusion:  Bad rotor, bad brush(es), or bad brush wire(s). To try to determine which/what, use your
jumper wire, or, a screwdriver, and short one rotor slip ring to the other slip ring.  Do not let the
screwdriver touch the case structure, as then you get a false indication.   If shorting the two slip rings
together turns on the GEN lamp, then you need a new or rebuilt rotor.  Otherwise, you need to fix
the brush problem.

Assuming the lamp does NOT light up, from any of those above things you tried: 
You have a bad voltage regulator, or bad wiring or connections in the lamp circuit.
Note that the following test should probably be the last test for the lamp circuit, as you want to be
sure what is going on with the rotor, brushes and Df wire at the alternator first.

How to determine if the voltage regulator may be bad:

Turn off the ignition.
Remove the fuel tank.
Be sure the D- and Df connections at the brushes assembly IS REconnected.
UNplug the voltage regulator.  It has a three female connection plug. You may have to press at a
side-clip to allow the harness plug to be removed.
Turn on the ignition.  
Ground the solid BLUE female connection wire in the voltage regulator's harness socket.  To 'ground'
means to connect a wire from the socket having the blue wire, to any shiny metal place...the cylinder
fins, or other place.
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.

Assuming the lamp DOES light up: 
Remove the grounding wire you just added.
Jumper the OPPOSING female connections in the plug.  Usually this is blue/black and blue. DO NOT
jumper to the brown wire.
If the light now shines, you have a bad regulator. 

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

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 (& makes a slight improvement on
rpm at which charging begins) is on this website....iarticle #19.


Troubleshooting a constantly on (usually) GEN lamp:

If, when your ignition key is ON & the GEN lamp goes ON brightly, but it usually never
goes out or only fades a bit when the engine is revv'd up, it usually indicates NO charging,
or very little charging.  A number of things can cause this, and there are various
approaches to figuring out the problem. While you can use a test lamp, if you know how,
to analyze things, use of a multimeter is probably easier, and you need to know how to use
one.  Read my article on them:

That article covers both multimeters AND test lamps usage.

You can obtain a multimeter for FREE from Harbor Freight Company...they often give out
quite adequate and accurate ones for FREE, upon ANY sale.  Otherwise, they sell them for
a variety of prices, depending on a lot of factors, almost always the price is LOW.

1.  The brushes may be well-worn.  Usually when the brushes are quite worn, one of the brushes will
      be seen to wear faster than the other.  The snail spring on the brush will be seen to be bottoming on
      the white plastic brush holder.  You can put a tiny piece of paper or other insulating item under the tip
      of the spring, which will work until you can get around to installing new brushe(s).  Brushes do not fail
      suddenly (unless its wire breaks...which is rather rare). Rather, brushes get short enough that they
      make INTERMITTENT contact with the rotor slip rings....usually this is RPM sensitive.

2.  Something in the diode board has failed.  Bad connection, bad diode(s).  This is not all that common,
      but happens.  SOMETIMES the problem is caused by the owner removing the front cover, before
      FIRST disconnecting the battery!....and thereby shorts the diode board in the cover removal operation. 
      SOMETIMES the motorcycle is one with the diode board RUBBER MOUNTS, with a mount failure
      and shorting. Rubber mounts are abominable for SEVERAL reasons: replace them with METAL mounts.
3.  The voltage regulator has failed.  To test for that, either install another one; or, UNPLUG the VR, & in
      the PLUG, connect a paper clip or other item, between the opposing female connections.  DO NOT
      connect to the solid BROWN wire!    If the charging is now good, and voltage keeps rising with
      increased RPM (do NOT let it go over ~14.6 or so), then the VR is almost for sure bad.

4.  The rotor has OPENED (open connection internally in the rotor).  This is VERY common.  Here are
      two additional tests (see earlier) to test for a bad rotor, and to eliminate that the brushes might be the
      problem.  Put a piece of paper under both brushes. Use an ohmmeter between the slip rings, should
      be very low resistance, a few ohms.  As a second check, use the ohmmeter between either brush
      and the chassis.  If you get a low resistance reading, the rotor is shorted to its steel structure.
5.  You replaced brushes;  now you get constantly on GEN lamp, was OK before the brush replacement.  
     You have probably took the brush holder apart, misplaced the correct order and installation of the
      insulating washers in the Df brushholder section.  Testing with an ohmmeter, D- and Df wires OFF,
      and paper under the brushes so they do not contact the slip rings of the rotor, are a quick method of
      determining things.  ONLY the D- connection may be grounded.
6.  You have mixed up the two wires going to D- and Df of the brushholder.  The BROWN wire goes to the
      D- spade.  Also see #5, above.
7.  It is very rare for a stator to be bad.  Usually that happens, if rarely, when someone removes the
      stator, IMPROPERLY using metal tools, and damages some of the wound-wires. 
8.  The stator magnetic metal is made of thin special steel laminations.  Do NOT put scrape marks across
      those that can REDUCE the maximum output of the alternator, & also can cause the
      lower rpm output to be lower than normal.

Poor charging:

Poor charging can come about from quite a few reasons.  Sometimes more than one reason
at the same time, which makes finding the problem(s) more involved. 

One of the first things to do is to make sure that connections are clean, tight, no strands of
wire are broken.  When you have the front cover of the engine removed (ALWAYS disconnect the
battery negative leads before trying to remove the cover), you can eyeball the stator connections,
those push-on connections need to fit tightly.  You can check at the RIGHT larger spade terminal
of the diode board (as you face the alternator from in front of the engine) to be sure the fairly large
red wire and its female connector are tight, not burned, and that the stranded wires into that
connector are intact and properly secuely crimped into the connector...wiggle to make sure.  
It is important that the battery be good, and the best test is by an instrument called a load tester. 
You could also just charge the battery fully, then try to crank the engine for 10 seconds or so,
without starting the engine, and monitoring the battery terminals (NOT the connections) with a
digital meter.  This is not as good as a formal load testing, but is usually adequate.  With the
float bowls empty, and gas off (or, ignition PRIMARY! connections DISconnected), the engine
will not start, which is what you want for a load test.  Do not remove the spark
want engine compression to make the starter motor work reasonably hard for this test.
Be sure the connections to the starter solenoid, located on the body of the starter motor, are
TIGHT.  If they are even a bit loose, that can limit proper charging.  There are a lot of other small
details areas too, but the above should suffice for a beginning checkout.

Description of the Charging System, Part 1:

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 1995/1996.  Changes
over the years included a larger stator and several changes to the rotor electrical resistance;  an added
diode board connection in the 1974-1975 transition period, and some stator winding resistance changes,
and assorted types of voltage regulators.

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.   The
last Airheads did NOT have 280 watt alternators, but the charging began earlier, just like the Authorities
models, and note also that charging on later standard and Authorities models can be poor if the associated
ROTOR is not the correct model. 
Some testing seems to show that the late low ohm rotors do slightly better in all models for
lower rpm charging.  

The Airhead charging system consists of a three-phase alternator, a diode (rectification)
board, a voltage regulator, the GEN lamp, & the battery, and Ignition switch.  The voltage
regulator (that controls alternator output) is NOT part of the alternator brush assembly,
as it is in most cars.  The GEN lamp is, before charging begins, fed by current coming
through the ignition switch.   Models after the /5 had a KILL SWITCH located on the right
side controls assembly on the handlebars.  That switch is also in the circuit sending
current to the GEN lamp.  SOME riders never use that kill switch, so if you are having some
problems, I suggest you move the kill switch back and forth half a dozen times and then
see if your GEN lamp is working more properly.  When you do testing on the system, you
need the KILL switch centered ("ON" or "RUN" position).

The maximum wattage output varied by model & year.... and what is in any particular
bike, since many parts interchange physically and electrically (fully or partially).  
NOTE: substituting for the last version rotor in a system with an earlier stator, is NOT
always a good idea, power output might suffer.

The stock /5 had a 180 watt alternator, which can be upgraded to the 280 watt alternator
by use of a very specific version of early /6 alternator STATOR.  That is the 1974 & some
early 1975 production year.   You MUST use that 105 mm stator.  Some late 1974 & early
1975 models did have the 107 mm stator, which will NOT fit the stock /5 timing case. 
Any & ALL  107 mm stators will NOT FIT the /5.  BMW apparently phased-in the changed
timing chest casting & incorporated the larger stator at irregular times/models, perhaps
until the stock of early cases was used up?   ALL 107 mm stators have an extra terminal
and that is a center-tap on the stator windings.  It enables some additional power output,
and thus the full 280 watts is obtained.  You can use a /5 diode board with the 105 mm or
107 mm alternators, but it takes a /6 or later diode board with the extra diodes and the
extra single wire connection to the stator, to obtain the full wattage output.

The stator diameter (105 or 107 mm) is the measurement of the stator portion that fits a
short distance INTO the timing chest cavity.

BMW mounted the diode board on rubber mounts on some models. This was a bad idea,
& was never done on all models, see later in this article.  The following models come with
cast-in metal mounts, and do NOT need to have 'solid diode board mounts' installed: 
/5; /6; 1978-1987 R65 and R80.

BMW also has used a variety of battery ampere-hour sizes (with two basic sizes of battery
case over the years); and, as noted well above, 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), 240, 250, 280.  The last of
the Airheads had 240 watt alternators, very low ohm rotors, and charging began a bit
lower in rpm than earlier models.  That was the tradeoff.  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 rotor was, I think, the smaller one,
see the above paragraph.  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 slightly lower rpm than all the others.   Almost all alternator parts are physically
interchangeable (the 105/107 mm stators being the exception)....
and one might find
almost any combination of parts when examining an Airhead charging system on any
bike.  NOT all combinations are wise or will work properly.  In general, the only low-
charging problems occur with the last rotors (2.8 ohms) if installed in earlier bikes.

As I earlier described, the /5 model diode board did not have the extra diodes for the
center-tap of the stator winding (there was no such), as did the /6 & later.  This is one
of several reasons the /5 had a lower output, besides the stator windings themselves.
The /5 had a higher resistance rotor, 6.9 ohms, often just called a 7 ohm rotor.  Due to
that higher resistance (which was done to limit wear rates on the MECHANICAL
Voltage Regulator that was used then, that rotor can not be energized by the battery,
nor by the alternator after charging begins, to as high a current as later models with
lower resistance rotors.  Thus, the magnetic field in the /5 rotor can never be as high
as either of the two later rotors.  It is actually a bit more complicated than that, as there
is limited room in the rotor for the windings, and a compromise was made.  Since the
battery voltage is more or less fixed, to gain an increased magnetic field in the rotor
one must have either more turns of wire and somehow have the same maximum
current flow (increased turns of same gauge wire means increased resistance to
current flow), or, change the structure to allow the same turns but of lower resistance
larger gauge wire.

There are NO current;  NO amperes regulators; used on the Airheads.  Alternators are
self-limiting for current output. The first voltage regulators on the Airheads were
electro-mechanically operated, and located in a metal can.  These had a mechanically
adjustable relay, the contacts of which fed electrical current to the rotor.  These contacts
began to vibrate/oscillate open, as the battery voltage (actually, alternator output) reached
the adjusted-for value.  There are articles on all the regulators on this website....including
how to service/adjust the mechanical ones (the Electronics regulators will fit OK).  The
next regulators were similar, the metal can less tall, and inside were electronics for the
regulation.  These electronic regulators were also adjustable, the adjustment being sealed
by a drop of paint on them, but the regulators could be rather easily modified for full
adjustability by removing the paint or replacing the tiny control.  The last of the regulators
were all in flat plastic boxes, fully transistorized, not made to be adjustable (although
possible).  These came in several varieties, the very last versions being slightly upgraded
to better handle the possible increased rotor current of the 2.8 ohm rotors. All sorts of
information on these various regulators are in separate articles on this website.

In the early 1980's, BMW had many problems with what was always said to be Wehrle brand
diode boards.  The truth is that some with Bosch names on them had the same problems.
All the problems were that of overheating at the short-length soldered wires at the 6 large
power diodes, due to failure of those wires to be left long enough to be bent-over and
soldered over a wide area.  This is sort-of fixable.  It is a nasty job to fix them by adding a
tiny hole in the board and wrapping/soldering an extra piece of wire to the diode wire UNDER
the board, and then to the top of the board.  LONG ago, Oak published that method in AIRMAIL.
It is NOT practical for most to even attempt it.  Replacing 1 or more large diodes is possible, 
but probably too expensive to seriously consider, particularly when considering the labor to
remove and replace the diodes. HOWEVER, it can be done, and you may be able to find the
large size diodes at a good price; and can get a lot of satisfaction about DYI.  It is also
possible to replace any of the small diodes...that is typically a lot easier. The information
on diode part numbers, etc., is on this website, see articles # 14A and 15.  

What has been needed is a simple, easy (and cheap) method of repairing the common
diode board overheated solder joints at one or more of the 6 large diodes places. 
I have done lasting repairs by using a higher temperature solder on cleaned-up copper pads
on the stock boards, no drilling needed.  That is what I would recommend, if your problem
is one or more of those solder joints.  You can VISUALLY see the problem, if present. 
Plumbers used to use a type of solder that was 50-50 mixture of tin and lead.  You probably
can get a few inches of large round gauge size from a plumber.  Plumbers also have used
this solder in bars form, which could be used by you (tiny piece at a time).  The fix on the
diode boards is to use a sharp Xacto or other knife blade, and on QUITE AN ANGLE, scrap
away the outer colored coating all around the diode solder joint.  You must be thorough,
using the tip of the knife to get right to the edge of the wire.  Do not scrape away the copper.
If you want to try chemical means, use a strong gel-type paint remover.
You need to use common soldering flux (rosin type), as the 50-50, AFAIK, never comes
with a core of flux, as does electronics 60/40 and other electronics solders.  Enlarge the
shiny solderable copper pad area some to get a larger solder joint.  The diode wire MUST
be soldered all-around, cleanly and neatly.  Preparation is your friend here.  A common
soldering iron can be used, I suggest one with a fairly substantial mass to its tip.

You could always, at considerably more expense, purchase a later board, or also an
aftermarket high power diode board.  The stock board is certainly adequate... contrary
to a lot of false ideas & some misleading advertising. 

The electronic regulator should be used on the 1981 & later bikes, as they have need for a
smoother, less spikey electrical noise due to the electronic ignition.  The older mechanical
regulator slowly deteriorates, & an electronic type can be substituted, whether the Bosch,
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 or  
They have them quite reasonably priced.  Also OK are the higher
voltage non-adjustable types, usually 14.2 or 14.4 volts.

There is some indication that the last version of the BMW installed voltage regulators will
handle the increased rotor current drawn by the later lower ohm rotors more reliably.  
Yes, if anal enough, you can modify earlier regulators to handle more current.  That
involves changing the power transistor to a higher rated type, and possibly increasing
the heat dissipation method.   Interestingly, it is difficult to find the current rating of
any of the VR's.

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).  It is on this website as
article #19.

Part 2:  How it all works:
(This is where ALSO reading article 15A may particularly help in understanding)

When the ignition key is ON (start/run position), a small amount of electricity flows
from the battery, through the ignition switch, then through the GEN lamp, and then to the
D+ terminal of the voltage regulator (blue wire).  In a motorcycle with a KILL switch, the
electricity also flows through it.   The lamp internal resistance acts to limit the current
& also gives you an indication by illuminating, that the lamp is OK. This small current
must travel to electrical ground in order to illuminate.  It does that by passing 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 through one carbon brush into the rotor
via the rotor slip ring....& then out the rotor via the other slip ring, through the other
brush, to the brush D- terminal, to the engine ground (which is the same as the battery
negative (-) terminal, electrically-speaking).  The connection to ground is made doubly,
by the mechanical connection of the D- terminal (it does 'look' insulated), & the brown
wire going to it, which goes to the voltage regulator.

This makes a "complete circuit" in electrical-speak, & the small current now produces
a SMALL magnetic field in the rotor.   The GEN lamp is LIT, brightly.
When the engine is started (the rotor is rotating as it is fastened to the end of the crankshaft),
the rotor's moving magnetic field produces a magnetic field in the non-moving STATOR
windings.  Thus, rotor magnetic field is transformed into Alternating Current electricity
in the non-moving stator windings.  Certain small diodes on the diode board change the  A.C.
electricity into Direct Current, which is applied to the same input side of the voltage regulator
that the GEN lamp feeds.   When enough rpm is reached to have approximately +12 volts at
that point, the lamp extinguishes, as it has approximately +12 volts on one side of the lamp,
& approximately same +12 volts on the other side of the lamp, & thus no voltage difference
across the lamp.   These small rpm increases...produce the MUCH larger current
than the lamp could supply, to the regulator, that is needed to fully power the rotor.  Thus,
the system can be though of as a merry-go-round, supplying itself....once initiated by the
battery-fed lamp current & enough rpm.   If you think about it for a few moments, you will see
that once charging begins, the system actually converts mechanical movement into usable

There are six large 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 (thus
current flow 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 output connection is
to the battery.
  These 6 large diodes consist of 3 positive & 3 negative as far as the internal
layout of the diodes are concerned.  More on those 6 diodes in the next paragraph.

The diode board connections (via printed copper 'wires') have 3 of those large diodes
connected to the chassis (negative side of battery).  The other 3 large diodes connect
to the battery + terminal output of the diode board. The + battery charging 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.  The normally extremely high
reverse voltage resistance of the diodes prevents the battery from discharging back into
the alternator when the engine is not running (key on or off).   The same system is used
in most cars & trucks.  There actually is a TEENSY bit of leakage current (usually under
a milliampere or so).  IF a diode fails such as 'leaking' lots more current, the battery CAN
discharge, over time.  Leakage current can be measured at that same + output terminal
of the board, by putting a current-reading meter in SERIES between the terminal
and the red battery wire that you pulled off it.  You could also have put such a meter into
the battery negative wire circuit at the battery, and then checked the meter with the +
wire ONTO that diode board terminal, or, pulled off of it....and compared readings.
Either testing method can be used.

If the rpm is high enough, or system needs low enough, the alternator could produce
too high a voltage at the battery & the rest of the bike.  The voltage regulator has an
internal voltage reference called a Zener diode, and the regulator receives the output
of the mentioned six SMALL diodes as previously described, and compares that to the
internal reference zener diode (mechanical regulator does this with spring and coil
which then operate the points).  When the voltage rises higher than the design voltage,
the voltage regulator reduces the current flow into the rotor, which reduces the stator

The mechanical regulator regulates by separating two contacts in the regulator to
reduce the charging.  Separation of those contacts occurs as voltage rises enough &
thus increases the mechanical relay's magnetic field in its coil enough to cause the
contacts to open, which turns the rotor current OFF. The contacts close again at just
the regulation set voltage.  This on/off of the contacts tends to happen fairly fast.  The
points spark gently during actual voltage regulation.  This produces some electrical
noise into the well as deteriorating the regulator's points, although slowly.  
The later low ohms rotors are really a bit much for the mechanical regulator (which
was not used after the 1970's), as the rotor could draw more current than the mechanical
regulator points were designed for.

Performance and failures:

The electrical system in any of the Airheads is adequate and reliable, particularly adequate
if you keep the rpm above 3000.  Those with much higher system needs...such as having such
as larger wattage headlights, driving lights, heated clothing, heated grips...ETC., may need
to make allowances for those things.  You may OR MAY NOT need a larger output alternator.
Ask on the Airheads LIST; and, read the various articles on this website; particularly the
article 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)
& maybe a few smaller lights...AND a 50 watt heated vest....but this is the limit for practical
purposes.  It is quite marginal with extra electical needs if you do considerable in-city stop
and go traffic.    

What fails?  Anything and everything has failed at one time or the other.

Gen lamp failures are rare.  The lamp is necessary in the stock system to initiate charging. 
There is a modification that allows the system to start up even if the lamp fails (also makes a
slight improvement on rpm at which charging begins) is on this website as article #19.

Rotor failures are typically due to aging of various types.  Rotors are either rewound or
discarded for new or rebuilt ones.  Aftermarket rotors are also available.  Rotors that were
unsealed, or sealed with lacquers or shellac, tend to fail due to stresses on the windings
during larger RPM changes, typically when downshifting into first gear.  Sometimes a rotor
will short a winding, or short it to the metal magnetic pole pieces material.
Now and then a rotor solder joint fails.    ETC.
I have seen TWO 'out of the box' rotors that were bad.  One was a rebuilt, improperly tested,
because there was continuity between slip rings and the magnetic metal, amounting to a
direct short of Df to ground.

Without measurements, you may not know what rotor you have.  You may need to know what
you should have.   For the most part, rotors are being rewound with the later lower resistances. 
These work better than earlier 7 ohm rotors.  BUT 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; but, it will work for awhile with the mechanical VR.   I suggest you avoid
using a 2.8 ohm rotor with early stators....the charging voltage might be inadequate.   This can
be a puzzling thing, in trying to figure out what the problem is.

Stator failures are NOT all that common; rarely one fails from age failed insulation.   Some have
failed from owners changing rotors and nicking the stator windings in the process. NOTE that
the stator windings are done on multiple thin special sheets of steel (generally called laminations),
& those thin sheets are coated with a varnish insulation. If you scratch across these sheets
accidentally by using metal tools, you can have a stator that seems to work fine, but has somewhat
REDUCED output.

Rotor failures, diode board and/or rubber mount failures, wiring failures, voltage regulator
failures....and ignition switch and kill switch failures....all are seen now and then.   Often
found are loose connections at the battery, or starter motor solenoid, or battery ground lead
at the speedometer cable hollow bolt (DO NOT over-tighten that one!)

Over and over we technicians find that the MOST common failures are a bad battery and/or
the starter relay connections or internally in that starter relay. The reason for the small starter
relay causing problems (total lack of system power is common complaint) is that all the power
in the motorcycle (except for the starter power itself) travels through an INternal jumper
connection INSIDE that relay (from 1976).  While the rare internal connection has failed, often
annoyingly an intermittent failure because of a loose internal rivet or some such, MORE often
the problem is one or two poor connections AT the SPADE CONNECTORS at that relay and
its socket connectors.  When the jumpering or corroded (even slightly) connections at this
relay are at fault, you may have a completely dead (electrically) motorcycle. UNPLUG and re-plug
in the relay, wiping the connections.  I will get deeper into this starter relay situation, and a
permanent fix, a few paragraphs downwards.

Failure to disconnect the battery before removing the outer cover of the timing chest, has
caused electric sparks & 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.  Occasionally a failed VR will fail in the Full Maximum Output
condition; and system voltage will be very excessive.    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.  I usually see such a glowing lamp at night
and my first assumption is that electrical wiring has never been cleaned or checked for
tightness of connections, etc.

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 & higher rpm, yet be OK at lower rpm,
and the reverse, too!  GEN lamps can do crazy things at different RPM.

After the /5 (which had no printed circuit board because it had no instrument POD), 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! ones NLA from BMW).  Do NOT yank lamps out
on the printed material with a fingertip, rock the lamp out carefully.

Starter relay (the one under the fuel tank, NOT the big solenoid relay at the starter motor):
You are probably asking yourself: HUH?...why talk about the starter relay?...this is an article
about troubleshooting problems with the alternator (and charging system)!  READ ON!
Usually the problem with the starter relay causes a complaint of an electrically dead
motorcycle....yet the battery is charged.  Not always that complaint.   READ ON!

If the system seems to be working (Gen light is ON with ignition on, engine off) & there
is some charging, perhaps not enough, & you have checked out the rotor, brushes, diode
board, etc. :   Check the Starter Relay connections...unplug that relay, plug it back in...which
wipes the contact male & female connections a bit...if charging voltage improves, consider
doing a careful cleaning job on the contacts....or a modification.  The whole story is right

From 1976, most Airheads 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 the connections of the relay prongs (spade connections) & socket. 
Early models had screw connections type of relay, and those sometimes had problems
with corrosion INside the relay.  I have seen that happen on those relays because the
terminals are on top, as is the crimping, so moisture can get inside,....even seen brake
fluid get on top and cause attracts water...and does NOT totally evaporate,
the 'mix' gets inside. 
You can often carefully UNcrimp the edges and fix the internals.
I recommend you SEAL the top after re-crimping it.

RARELY the later plug-in type of relay had problems INside the relay, sometimes
it was a loose rivet....fixable.  The more common problem is with the PLUG-IN later version,
from corroded male/female relay/plug connections.  TYPICALLY the bike is
lights, no starter function.  Battery tests good...battery cables are good too.  SO...???

There have been instances of a slightly loose rivet causing problems.   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 for the starter motor, it has a 'jumper function'.
  If the battery has
reasonable voltage on it (over 12.3) WHILE the lights are turned-on (but they do not
light), you may have a relay plug/socket problem, or problem inside the relay.
Probing with a voltmeter will tell you if power is at all red wires at the starter relay, or
not.   This same symptom  can occur (of course!) if the large + and - battery wires,
which bolt to the battery (and the - one to the speedometer cable hollow bolt) are
faulty...perhaps eaten away by battery acid (happens; at + connection), etc.
Back to discussing the starter relay:

The starter relay has RED wires going to it.  These wires pass 100% of the bike's
electrical power (except the much larger wire at the + battery terminal that goes
directly to the starter motor).   These larger red wires at the starter relay go to
different starter relay connections.   A temporary & QUICK fix, if the external spade
connections are the problem, is to simply UNplug the relay, and plug it back in. 
That almost always works, might even last.  It is helpful to use tiny tools and try
to clean the female connection in the plug, and to lightly abrade the relay's male
spades, before reinstalling.  You can use silicone dielectric grease or even Vaseline!
...on these during re-plugging-in; it helps protect against environmental things that
caused the original corrosion.  If the is very visible, and nasty, you will have to install
new female connections. There is a modification that can help.  Used with relatively
brightly cleaned male and female spade connections, this will likely be a 100%
permanent FIX!
Firstly, on all three RED wires, determine a place near the plug socket where you can
CAREFULLY remove SOME of the red insulation from the wires adjacent to each other.
1/2 inch of removal or as much as 1 inch, is desirable.  I suggest you use a very sharp
thin blade on a very flat angle. I use a hobbyists Xacto knife.  Try to NOT nick/cut any
copper strands.
  JUMPER all the bared-wire places to each other, using common 60-40
rosin-core electronics solder. You will need to wrap some shiny copper wire around
the soldering area so that the connection is solid and of considerable mass, essentially
you are continuing the large wire gauge size already in use, but joining them. Solder
using a quite large mass type of soldering tip on your soldering iron.  A high power
soldering gun also works nicely.  I've even used a propane torch soldering tip...BUT,
you must carefully avoid side-flame damage.  Do a clean & neat job.  INSULATE the
result with electrical tape or self-sealing tape.

There are NO problems caused by this external jumpering, if done neatly.   Be sure:  
jumper all the red wires at that starter relay, to do it slowly, do NOT break any wire
strands, join the bared wires NEATLY, soldering with a hot large tip well-tinned iron,
plenty of rosin flux; insulate your joint carefully.  I usually use soldering braid or
de-soldering braid to join the wires, but a wrap of common lamp cord (insulation
removed! and the wires look shiny, not corroded from age) will do. 

Charging system failures; a further discussion, in depth:
If something fails, what are the USUAL indications?  How to determine
what is going on:

1. Batteries:  A battery failure can cause an enormous amount of problems & 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 severely enough discharged, the starter
motor will not have sufficient current to rotate the engine properly.   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, & may even 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, &
re-measure the battery terminal voltage 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.  Usually the
voltage drops off a huge amount.  You can also use a formal Load Tester on the battery
for confirmation, after trying to fully charge the battery. 

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 somewhat rare, but I have seen batteries that were temperature sensitive,
regarding the opening & closing of an internal connection.    Another common failure mode
is a shorted cell, usually only one fully shorts, but, it can be partial for one or more cells.  
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.  Falling from 12.6 or so, to closer to 12, is also seen when the headlight
is turned-on, perhaps quickly after what seems like a full charging.  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...and LIFE left.   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, and, yes, with a Harbor Freight Company Load tester. I suggest you purchase
the slightly more expensive version (they sell TWO types); which has TWO METERS. It is
much more versatile and works great.  It is on sale now and then, and you may be lucky &
combine it with a 20% off coupon!    Really worth the $50-$80 you will pay.
A load test at about 90 amperes on the 28 or 30 AH batteries (and 70 on the smaller
batteries), while watching the battery voltage, is about correct for the Airhead batteries. 
The load test is done by reading the voltage accurately at ~15 seconds of loading time.  
That is done AUTOMATICALLY by the HF 2-meter model.  That HF unit is ADJUSTABLE
to match battery capacity size!  There is a chart available for the lower voltage limit,
versus temperature....and the load tester comes with the chart.  

It is possible for bad starter motor to 'pull' or 'draw' so much current from the battery, that
the battery appears faulty.  While any well-used/worn starter motor can exhibit this problem
from use, it was especially egregious on early Valeo starters (which were installed by BMW
instead of the Bosch starters, in the later eighties) in which the pole pieces, which were
in place, came loose and mechanically locked up the starter.  The Bosch starters are
better designed & constructed; the Bosch are easily re-buildable anyplace...they are nearly
100% interchangeable, but the Bosch needs the forward end support plate, and the early
versions were 8 teeth, later versions 9 teeth, to match a change in flywheels...and you MUST
use the correct number of teeth version (for ANY starter motor).    Late model Valeo starters
are OK, and draw less current than the Bosch, provide MORE cranking power too....and has them reasonably priced....
see my references page:
There are also other sources for starters. is my recommendation.

2.  Diode board, rubber mounts, brush-holders, etc:   On those models with rubber diode
board mounts, the mounts will eventually deteriorate, allowing the diode board to fall
downward & 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, output, & voltage regulation is improved too!   There is some
evidence that the diode boards stay a bit cooler and are more reliable.  Rubber mounts are
   Diode board failures can cause everything from low to NO charging.  

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

NOTE that the RS and RT models are particularly prone to diode board problems...PLEASE
do yourself a favor; install these cheap diode board solid mounts.   Euromotoelectrics;
Motorrad Electrik; Thunderchild.  It is a good idea to add openings/louvres in the front
lower plastic cover that is in front of the engine, on models using it.  Early ones had no
louvres; later ones did; that improved cooling air to the outer timing chest (where the
alternator is located).

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:

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, & 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 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 multi-meters 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 (properly interpreted).

The AC transformer and lamp test is MUCH better than either; but a 100% job INCLUDES
using the ohmmeter to measure reverse leakage.
  You CAN use an ohmmeter on the
diode board fully installed and connected...but, you MUST disconnect the battery first.

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, & when replaced, there is no charging.  They
have probably miss-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.  Read the article on the diode boards and grounding wires:

Read the other electrical articles on this website....#14, #15, #15-A, 17, etc. 

Some problems were caused by extra heating due to fairings on RS/RT models; 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.

3.  Voltage Regulator:  Usually a failure here means an opening of the series pass transistor
(electronic versions of the regulator) and no output/charging.  Rarely the internal regulator
series pass transistor short-circuits, or there is some other electronics failure...and the
indication is vastly too high charging voltage at higher rpm. 

The mechanical regulators usually fail by slowly deteriorating the output voltage.   A failure
to charge, in which the regulator is suspected, can be PROVEN, on BOTH the mechanical
and electronics regulators, by bypassing the regulator.  This is done with a jumper wire with
male spades in each end.  Remove the regulator plug; insert the jumper into the opposing
(NOT BROWN WIRE!!!) plug connections.  If the system now charges, replace the VR.  
Good sources for voltage regulators (get the adjustable type!!):

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 seldom fail unless abused.   Abuse is usually nicking or otherwise injuring
the wires during stator/housing removal. Nicking the steel laminations can reduce output.   
Due to the low resistance windings, common non-lab versions of ohmmeters tests on the
three phases themselves are not usually or always 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, unless you are trying to use an
earlier higher resistance rotor with it, or the reverse.....both of which are where charging can
be poor in voltage output.    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 wires.     Typically, a large 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 can usually be seen, 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. For MOST,
these sophisticated tests are not likely to be possible; nor, understood.  I will list some
tests, just below, that mostly can be done by YOU:

All these tests, below, can be done with the system on the bike, & are done with the phases
& centertap (none on stock /5) wires disconnected from the harnesses to the stator
(simply pull off the stator spade lugs):

    a. Ohmmeter tests, between phases, & to ground.  Short the ohmmeter leads together
         solidly, subtract the meter indication from the readings you get from connecting the
         ohmmeter test leads solidly 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, the values between that center tap & any of the three main phases will be
         ~HALF the phase to phase value.  The resistances are hard to read accurately, &
         hence only gross changes/differences may mean anything.
    b. With Df harness wire disconnected & D- grounded & Df terminal connected to battery +;
         measure phases with an A.C. Voltmeter, at ~2000 rpm.  The voltage should be about the
         same between any connection type of the three terminals stator output group at the
         SAME rpm.
    c. Repeat, but with A.C. Ammeter....properly connected!  Few of you have an A.C. ammeter,
         so this test is seldom done by anyone.
    d. The tests can also be run with a common headlight lamp, for equality in brightness,
         instead of using meters.  Simply use jumper wires and connect the lamp across any
         two phase connections. Don't raise the rpm too high. Test all combinations of phase
         connections.  For /6 and later types:  disconnect the 3 phases & centertap connections
         (pull off the connectors). GROUND the centertap.  Use a test light, somewhat MORE
         than idle rpm, on each of the three stator terminals, the other end of the test lamp to
         chassis.  All should light up the same.    For the /5 alternator stator which has no
         centertap terminal, you ground nothing, just check with the lamp between all
         combinations of the three stator outputs.   You do not have to ground the /6 centertap
         connection as described, but do it as I described for the /5 (no ground for the meter);
         but the /6 centertap test (to round with that) is a slight bit more sophisticated.
The lamp tests are really quite good ones.

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

        Testing the stator under quite high output & loading can be done by reconnecting the wires
         to the phases, but NOT reconnecting the brush Df wire, and following step b.  The phase
         voltages between phases, and voltages from phases to ground will give some ideas about
         where problems are...even tends to identify bad diodes on the diode board.  I won't go into
         this any deeper here.

5.  Rotors:    Rotors rather commonly fail, they rotate fast & 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 plus the transformation of the magnetic field
adds more heat....and thus there are heat-cycling effects.  Same can be said for the diode board,
which is susceptible over the 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 with a type of
varnish, and are not as good as later rotors that might have been sealed and protected with high
temperature epoxy (hopefully by vacuum impregnation).  Non-impregnated wires can move about
from vibration and centripetal forces and be damaged.  Rotors usually OPEN circuit, 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 (or sort-of) at low or high rpm, and then not at
another rpm.  GEN lamp indications can be erratic with rpm too, signifying an opening and
closing of the rotor internal connections. It is possible to use an ohmmeter on a rotor with the
engine running over a range of rpm, with the rotor D- and Df connections disconnected, but
there can be meter complications, so I suggest you NOT do that.

Once in a while 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. The ohmmeter WILL then show 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.
SOME home-made tools are NOT adequately safe to use.  I suggest purchase of the factory tool
OR an aftermarket ONE PIECE tool.  If you want to make the 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 remover tool, nor a NON-hardened bolt!!!

>>>>>DO NOT fail to use a hardened tool. I VASTLY prefer the ONE PIECE tool.

The BMW-sold tool is 88-88-6-123-600.
You can get the equivalent tool from aftermarket suppliers.
They are NOT expensive. 
You can make a tool from 8.8 bolts....ask on the Airheads LIST. I am NOT in favor of
this multiple piece tool method....due to the possibility of wrong use, INCLUDING
canting of the inner part
, causing you HUGE problems.

If you do NOT use a hardened tool & in some instances the home-made 2-piece type
as shown in the photo..... may BEND inside of the rotor, & now you are in DEEP trouble. 
Besides the stock Bosch rotor, where I recommend you do not use two piece tools
as shown...ALSO DO NOT use any such two piece tools in aftermarket rotors, such as
the EnDuraLast Alternator or the Omega Alternator.


Aftermarket rotors have been seen
with different threads than the stock Bosch
rotors have.  For those aftermarket rotors,
be sure to use the maker/suppliers tools
for removal, which MAY LOOK SOMETHING










RARE situation, a magnetized rotor:
This can drive you crazy!

If you get no output from the alternator:
You have checked the diode board, checked for proper & functional connections, checked the function of the GEN lamp, checked the stator, checked the rotor resistance (& for resistance to ground from either slip ring), could have an instance of a pre-magnetized, pre-polarized rotor.  This very rare situation, typically happening with a replacement rotor.   NOTE that the D- terminal is the same as engine case ground, and that the Df terminal is insulated from the case, and that the D- terminal is the one that should have the SOLID BROWN WIRE.
The D- terminal must be grounded. Wrongly assembled brush assembly insulating washers, and then reversing the two wires connecting to the brush assembly spades from the engine harness means YOU could possibly have caused the problem. OTHERWISE....
Just how the rotor might become magnetized in the first place in the wrong direction, is a matter for speculation. Since the D- brush connection is always grounded (one hopes), it is unlikely to have a rotor wrongly magnetized from reversing the two connections, to D- and Df. Wrong magnetization could come from rotor TESTING, if DC current was used, or, wrongly used AC current. It is possible for a rotor to have a default during winding, which could cause the problem.....any sort of grounding would especially be suspect. It is possible that some rotors are more susceptible, due to the structure and materials.

The fix:   Unplug the connectors going to the brush holder D- & Df terminals.    Do an ohmmeter check of the D- terminal to the case, it should be ~zero (after subtracting the meter leads if they are shorted together).  If not, then plug in the D- wire and recheck.  If then near zero, I suggest either fixing the insulating washers situation at the brush holder, or, just as good and less work, fashion a very short wire to go from the D- terminal to the nearest case screw.

Read the resistance of the rotor AT the slip should be as expected, under 5 ohms.  Read the resistance between D- and Df. It should be the same as the slip rings resistance PLUS a modest fraction of one ohm, to account for the two carbon brushes.  As a final measurement, slide a piece of paper under the brushes so the brushes do not contact the slip rings. Measure AT either slip ring TO THE CHASSIS/ should be in the millions of ohms, if anything is shown.  NO SHORTS or near shorts.

Remove those pieces of paper.  Recheck that D- to Df shows under 5 ohms.

With the Df connector re-CONNECTED
, connect a piece of wire (any jumper wire from 14 to 22 gauge will work OK) from battery +, momentarily to the Df brush holder terminal.  DO NOT touch the D- terminal, nor ground, with this jumper wire.  The battery must be connected to the frame and engine, so if you disconnected the battery before removing the front aluminum cover, reconnect it for this.  Connect the wire for only a second.  That will properly de-polarize, re-polarize, and magnetize the rotor very faintly. 

The stock Bosch rotor used on any of the Airheads, is not designed, necessarily, and certainly not specifically, to work as a residual magnet. By that, I mean that while it certainly is an electro-magnet, its design was not specifically to keep some sort of residual magnetism in its pole-pieces. Saying this differently, the rotor structure is not like a permanent magnet you use to post notes on your steel refrigerator.  It may well be that a very tiny residual magnetism is left in the rotor structure, most, I think, do have that, and it is THE reason that if the GEN lamp fails, you can sometimes get the alternator to start producing current, typically only by using a very high rpm.

If it was possible, at reasonable cost, to have the rotor structure be a strong permanent magnet, which could be enhanced during rotation by the stock method (described later herein), Bosch might have done that, ....I do not know. High performance magnetic materials HAVE been available for some time now, at considerable cost (China is purchasing as many rare earth mines as it can, rare earths are needed), but the original rotor was never re-designed FOR THAT as there was no specific need. As a side-note, I have thought that such an upgraded structure might well produce usable current from the otherwise-stock alternator system at a much lower RPM than the stock one does. THAT would be nice for commuters, that is, city riding, stop and go folks. NO proof, and to my knowledge, no one has produced such a rotor for the stock Bosch system.

Certainly, 100% permanent magnet type rotors (NO rotor windings) for alternators have been used for many decades. There is an aftermarket permanent magnet alternator for the Airheads, the original EnDuraLast; it produces a fair amount of current at lower than stock rpm. A description of its performance, and for the original Omega, versus the stock Bosch, is in an article on this website.

I do NOT know any specifics on the stock Bosch rotor steel, nor for any aftermarket-made rotors. It could be that the magnetic properties has been different on them. For this reason, rotors MIGHT vary in residual magnetism. I have never done formal testing to find out, as I was not curious enough.

Back to the stock Bosch:
The alternator requires a rotating magnetic field for charging/output to begin. In the stock system, the INITIALIZING magnetizing current is supplied by the battery, through the ignition switch, through the GEN lamp, and through the VR, then through the rotor. This initializing current is small, with the main current limiting item being the GEN LAMP. The GEN lamp allows a quite small amount of current to flow through the rotor, which magnetizes the rotor faintly. The rotor has a very low resistance (amongst other characteristics nor conducible to being magnetized much) compared to the lamp, particularly when the lamp is lit (lamp resistance rises when lit), so the magnetic field really is miniscule.

When the rotor rpm rises, its rotating magnetic field induces a modest amount of current in the STATOR windings, first very tiny, then a bit more as RPM rises, and if the rpm is enough, and the system needs the electricity, several amperes is produced. That AC current is rectified to DC by the SMALL diodes on the diode board. This DC current is then applied to the Rotor (via the Voltage Regulator), to GREATLY ENHANCE the rotor current that existed (just from the lamp current).

If you are getting the idea that the alternator is SELF-energizing above a modest RPM, you'd be absolutely correct. The rpm for this varies somewhat with system condition and with the model of rotor and stator, but typically is evident at a few hundred rpm above idle. The point of transition is approximately when the GEN lamp EXtinguishes.

IF the rotor happens to have its quite low residual magnetism in the reversed direction, then the lamp current must overcome that, to cause the proper direction magnetic field. If the lamp current can not overcome it, then there is no output, and the lamp would likely just stay lit. The rotor needs to be magnetized in the correct direction. THAT is what the re-magnetization procedure I developed, above, does.

6.  Brushes:  Brush failure is common; there is an article on this website about
replacing the brushes.

Brushes are a medium-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 as for when the GEN lamp lights.  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. 

At just the right amount of wear, the very very slight sideways wobble of the rotor or very
slight 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.
SEE article #17.

NOTE!   It is rare, but happens.  You 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).  If you reverse the brush terminal
wires, the alternator will not work! Possibly you can injure the VR. You do NOT have to
disassemble the brush holder mounting for a brush replacement job.

The brushes rarely have had the flexible wire they connect to the holder terminals, break,
sometimes invisibly.
   You can pull a SLIGHT bit on the wire, as if to pull the brush out of the
holder, slightly, to see if the wire is intact. 

7.  Miscellaneous items:  
    a.  If you had the diode board out, & you have no charging, you probably failed to connect
         one wire at the rear of the diode board!
    b.  Key switches may develop intermittent & excessive resistance, which can confuse the
         voltage regulator operation. 
    c.  Kill switch intermittents...or corrosion effects that don't seem to show as an intermittent,
         can cause a variety of strange problems...including lousy throttle response, & actually
         failure for RPM to rise properly with increasing throttle!  That one has confused experts!
    d.  Wires can get pinched. 
    e.  A rare short circuit in some /7 & later bikes... at the two-wire connector near the top front
         of the engine...the blue wire.... may fray and short to the frame. 
    f.   Another wire problem may be the blue-black wire from the Df brush holder to the voltage
         regulator Df terminal.  
    g.  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. 
    h.  The diode board output connects via a heavy red wire that fastens to an oversize spade
          lug on the right side of the diode board (as you face it from the front) to the battery!!  It is
          common to see these overheated due to insufficient grip...fix that with a pliers with the
          wire connector removed, then reinstall.
     i.   Other wire connectors, beside the one at h. can overheat & therefore have too much
          resistance.  This is commonly seen at several places:  The alternator stator terminals
          connectors (the group of three)...AND, the other end of those three wires, behind the diode
          board. It is usually NOT seen at the center tap of the stator windings (/6 and later).
          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.   Female
         spade connectors can be carefully squeezed to fit much tighter. 
There is no need for the
         hard plastic insulating black covers over the leads to be perfect, if yours are cracked &
         partly missing, which is commonly seen, you need not repair that.
 j.  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, or the pod side of that.  There is an
          article on this website about modifying the lamp circuit so even if a lamp fails, charging
          will occur.
     k.  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 failing will shut
          down the GEN lamp.
Front Covers:  /5 covers had poor ventilation.  Using a later cover will improve reliability.


There is information on that page, but there ARE ERRORS.  Please do not use.   As of
12/26/2015, my most recent check, errors were STILL not fixed, after MANY YEARS of him
knowing about them.  The sketch, very similar to the below one, is NOT CORRECT.

The following sketch came from:
This sketch is similar, but NOT the same as Buchanan's.  Long ago I had a corrected
version of Buchanan's here via link...I removed it, gave up trying to get him to fix his.
I have modified Thunderchild's sketch, just below, with identifications of wire
colors, small diodes, and /5 system information.   The only item I have not
put into this sketch is the Kill switch.  


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
Airheads 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 smaller book on the airhead charging system, but quite adequate, especially for
troubleshooting, with illustrations and practical advice, is available from Motorrad Elektrik.    (256) 442-8886

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.
11/21/2009:  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 with some browsers
09/08/2014:  Clarify a few details, clean up for narrower screens.
11/05/2014:  Go over entire article.  Simplify some testing for ease in understanding and speed.  Edit
                    entire article as needed for clarity.  Update sources, links, etc.
01/18/2015:  Add section on rotor magnetization; which came from a response I did on the
                    Airheads LIST.
07/07/2015:  Add new section on gen lamp always on, clean up some of the rest of the article
                    (more should be done, when I get to it....).
12/27/2015:  Go through entire article.  Clarify many details and add many.   Narrow the article, including
                    adding more left edge justification(s) and increasing font size.  Update meta-codes. Update   
                    the alternator sketch so /5 era version is more clearly shown as to details on Y and diodes.

Copyright, 2014, R. Fleischer

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