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).....it 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.
Poor charging: Poor charging can come about from quite a few reasons. Sometimes more than one reason
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 laminations...as that can REDUCE the maximum output of the alternator, & also can cause the
lower rpm output to be lower than normal.
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 plugs....you
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.
Poor charging can come about from quite a few reasons. Sometimes more than one reason
Description of the Charging System, Part 1:
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.
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.
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 RockyPointCycle.com or
euromotoelectrics.com. 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
Part 2: How it all works:
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.
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 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 (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:
What fails? Anything and everything has failed at one time or the other.
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.
Starter relay(the one under the fuel tank, NOT the big solenoid relay at the starter motor):
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 problems....it 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 dead...no
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%
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:
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
glued 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
2. Diode board, rubber mounts, brush-holders, etc:On those models with rubber diode
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: firstname.lastname@example.org
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
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.
>>>>>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.
RARE situation, a magnetized rotor:
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
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.
l. Front Covers: /5 covers had poor ventilation. Using a later cover will improve reliability.
A smaller book on the airhead charging system, but quite adequate, especially for
troubleshooting, with illustrations and practical advice, is available from Motorrad Elektrik.
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
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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