Troubleshooting the
BMW Airhead Motorcycle Alternator/Charging Systems/Problems
©Copyright,
2011, R. Fleischer
trbleshootALT.htm
15-B
FIRSTLY, and IMPORTANTLY, this article is NOT a substitute for Articles #14, #15, #15-A. This article is to be used in addition to those articles. I have not placed lots of hyperlinks in this article, although a few are here, as I WANT YOU to read those three articles!
You may also want to refer to one of the articles on the Airhead Voltage Regulators.
*********************************************
Brief Description of the Charging
System Parts:
The BMW charging system is the same; with only some modest
variations; as used on ALL airhead boxer models from the end of
1969 with the introduction of the /5 series, to the end of civilian production
in 1995. The Authorities (Police) models have some modifications to the
stator, rotor, and voltage regulator. The Authorities
models have varied somewhat; but, in general, they begin charging
at a lower engine rpm; but the TOTAL MAXIMUM output is REDUCED.
The Airhead charging system consists of a three-phase alternator,
a diode (rectification) board, a voltage regulator, the GEN lamp,
and the battery. Wattage output varied by model and
year. The stock /5 had a 180 watt alternator,
which can be upgraded to the 280 watt alternator by use of a very
specific early /6 alternator STATOR only one version of which will physically fit. This
is the 1974 and some early 1975 production year, but you MUST use
the 105 mm stator, and some late 1974 and early 1975 models had a
107 mm stator, which will NOT fit the stock /5 timing case.
You would also need the /6 or later style of diode board in order
to realize the full benefit.
Variations over the years included adding the center-tap to the
stator winding and adding the needed connection and three small diodes to the diode board
(/6 and later ...any alternator after the /5). This was
done to the 105 mm
/6 alternator and later, to make this quite clear.
BMW mounted the diode board on rubber on some
models
which was a bad idea, and
was never done on all models, see later in this article. BMW
also has used a variety of battery ampere-hour sizes (with two
basic sizes of case over the years); and, has changed the windings on the
rotor (resulting in 3 different resistances), and there are
several versions of the voltage regulator.
There was a rotor diameter change during production, early being
73.4 mm, later 73 mm. It is unclear if this was the R90S change
in rotor diameter, which was carried through to the end of
production, but it is likely. Whilst the tendency is to simply
think of the alternators as 180 or 280 watts of output, in truth, outputs are
specified at 180, 238 (R90S and Authorities), 250, 280. The R90S
STATOR inside diameter was
changed due to the rpm attainable, to avoid the rotor from striking the stator
laminations, due to crankshaft whipping. The larger diameter 73.4 rotor could be used,
however. I believe that the idea of rotor
interference was mostly unfounded. The Authorities model has the
same 238 watt rating as the R90S; but, is designed to produce
usable output at a lower rpm than all the others, and the 238
rating is not because of the same reason, the smaller rotor, as the R90S. Almost
all alternator parts are physically interchangeable (the 105/107
mm stator being the exception)....and one might find almost any
combination of parts when examining an Airhead charging system on
any one bike.
The /5 model diode board did not have the extra three diodes for
the center-tap of the stator winding, as did the /6 and later,
and thus this is just one other reason the /5 had a lower output,
besides the stator windings themselves (and the /5 had a higher
resistance rotor too). All the diode boards interchange,
but use of a /6 or later board (with its Y connection to the
center-tap on the stator) on a /6 and
later, is mandatory to obtain full rated alternator output.
In the early
1980's, BMW had a lot of problems with Wehrle brand diode boards. These
would severely overheat at the wires from the large power diodes, due to failure
of them to be bent-over before soldering. This is fixable. Aftermarket high power diode boards are also available
commercially. The stock board is certainly adequate.
The earliest voltage regulators were mechanical,
most were made by Bosch,
and all were in metal cans. Bosch later made an electronic regulator that is in a
similar, but shorter can. Those metal can electronic regulators are very
good and are
adjustable internally. There are articles on this website...and the
Club's .org website...on these regulators...testing, modifying, etc. The
electronic regulator really should be used on the 1981 and later bikes, as they have need
for a smoother, less spikey (electrically) output due to the
electronic ignition. The mechanical
regulator slowly deteriorates, and an electronic type can be substituted,
whether the Bosch, or Wehrle, or even a car type. Most any VR from a car
that has the same three prongs and same case mounting, will work just fine in
Airheads. 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, use an aftermarket adjustable
regulator, from such as RockyPointCycle.com
Description of how
it all works:
When the ignition key is turned ON, a small amount of
electricity flows from the battery, through that key switch, then through
the GEN lamp, and then to the D+ terminal of the voltage regulator (blue
wire). The lamp internal resistance acts to limit the
current and give you an indication that the lamp is OK.
This small current then travels further, through the voltage
regulator via its Df terminal (Blue-black); and then to the Df
terminal at the brush holder for the rotor. The electricity
goes into the rotor and then out the rotor via the D- terminal, to the engine ground,
which is the same as the battery negative (-) terminal,
electrically-speaking. This makes a complete circuit,
and the small current will then produce a small magnetic field in the
rotor. This is a SMALL current and SMALL magnetic field.
When the engine is started and thus the rotor is rotating as it
is fastened to the end of the crankshaft, the rotor magnetic field is
transformed into Alternating Current electricity in the
non-moving stator windings.
Six small diodes on the diode
board are used to change a small amount of electricity into Direct Current, and the
electricity is applied to the same input side of the voltage regulator that the
GEN lamp feeds. When enough rpm is reached to have about +12 volts
supplied at that point, then the alternator stator and diodes are supplying
current to the rotor,
and the lamp extinguishes, as it has ~+12 on one side, and ~+12 on the other side,
and thus no voltage DROP across it. These small diodes...as rpm
increases...produce the MUCH larger current (than the lamp could supply), through the regulator, that is
needed to fully power the rotor. Thus, the system is a merry-go-round,
supplying itself....once initiated by the lamp current and enough
rpm.
There are six high power diodes mounted on the
diode board. They connect to the stator main output
windings, the same as do the above smaller diodes. As the
voltage in the alternator stator rises just a bit more from a bit
more rpm than in the lamp-extinguishing mode, these large six diodes
will begin to pass a
fair amount of current, but the connection is directly to the
battery. The diode board 6 large diodes are 3 positive and
3 negative. The connection has 3 diodes to the chassis
(negative side of battery) and 3 diodes to the battery +
terminal. The + output of the diode board is the large spade
terminal on the right side of the board as you face it from the
front of the motorcycle. The connection is NOT FUSED and
depends on the extremely high reverse resistance of the diodes to
prevent the battery from discharging into the alternator when the
engine is not running (key on or off).
If the rpm is high
enough, or system need low enough, the alternator could produce too high a
voltage into the battery and the rest of the bike. The voltage regulator
has an internal voltage reference, and the regulator receives the output of the mentioned six small diodes,
as previously described, and compares that to the internal
reference (mechanical regulator does this with spring and coil).
With the voltage rising excessively, the voltage regulator then
reduces the current flow into the rotor, which reduces the stator
output.
The mechanical regulator regulates by separating
two contacts in the regulator. They will actually open and
close rather rapidly once the regulation set voltage on the battery is
reached, thus they
spark gently during actual voltage regulation. This
produces some electrical
noise into the wiring...as well as deteriorating this mechanical
regulators points, although slowly. The later low ohms rotors are really a bit much for the
mechanical regulator to be used for long periods of time with
them, as the rotor could draw more current than the mechanical
regulator points were designed for.
Performance:
The system in any of the airheads is adequate,
particularly if you keep the rpm above 3000. Those with
much higher system usage...such as larger wattage headlights,
many extra other lights, heated clothing, ETC., may need to make
accommodation for same. Ask at the Airheads LIST;
and, read the various articles on this website; particularly the
one that compares the Bosch alternator output with the
aftermarket alternators. The /5 180 watt system is
quite adequate for even a larger headlight (original was 40/45
watts) of the standard 55/60 watts.....and maybe a few smaller
lights. The /5 180 watt alternator, with not over a 55/60
watt headlight, can handle a 50 watt heated vest....but that is
about the limit for practical purposes.
What fails? Basic
answer: anything and everything has failed at one time or the other!
Rotors fail due to aging, and rotors are very commonly
rewound....or newly made....by companies besides the original equipment
Bosch. Without measurements, you may not know what rotor you
have. For the most part, rotors are being rewound with the later
lower resistance, approximately 2.8 ohms. These work better than earlier
rotors. It is not a great idea to install a 3.4 or 2.8 ohm rotor into a
mechanical voltage regulator bike, without changing to the later electronic regulator in
the plastic case. Stator failures are
NOT common. Some have failed from owners changing rotors,
and nicking the stator windings. Rotor failures,
diode board and/or rubber mount failures, wiring failures, voltage regulator
failures....and ignition switch failures....all are seen now and
then. The MOST common failure is the battery.
The GEN lamp, necessary in the stock system to initiate charging,
does not fail often. There is a modification that allows
the system to start up even if the lamp fails (and makes a slight
improvement on rpm at which charging begins).....that is on this
website....it is article #19.
Failure to disconnect the battery before removing the outer
cover of the timing chest, has caused electrical sparks and failures of the
diode board. Failure of the voltage regulator is indicated by poor
or no charging or wrong voltage. VR failure is hardly the
only failure to exhibit these problems. Failure of the diode board is usually indicated by lower
charging (lower watts) capability, or lower voltage output under load of such as
the headlight. ONE "open" large diode will so indicate by
vastly lowered charging with the headlight turned on, yet 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. Even overheated or slightly dirty or
corroded connections. GEN lamps that can be seen glowing dimly at night whilst
cruising are usually an indication of poor connections someplace, or many
places. GEN lamps that do not light up, and thus you have no charging
(or, maybe only at VERY high rpm), are usually a bad lamp, a bad lamp socket, a bad regulator, or, commonly, an open rotor or excessively worn brushes.
After the /5, the printed circuit board that the GEN lamp fit into can
get microscopic cracks, and the lamp connection to the board
can fail. You can carefully repair the board....or replace
it. Do NOT yank lamps out roughly.
From 1976, many models have a connection of the battery into
the electrical system located at the starter relay under the fuel tank.
These red wires can cause problems if
there is corrosion at those connections of the relay prongs
(spade connections) and socket. The problem can be as
slight as just a small voltage drop, or as major as a total
complete lack
of electrical power. This is
because the
INTERNALS of the relay, even if the relay is NOT being actuated, have a 'jumper
function'. These are larger gauge red wires,
and can be cut into ...NEATLY!!...and joined permanently, which fixes the problem forever.
If something fails, what are the
USUAL indications?, and how to determine what is going on:
1. Batteries:
A battery failure can cause an enormous amount of problems and
indications. It is QUITE possible, and, frankly the
norm, for a battery to simply lose
capacity over time. That in itself is usually not a problem, as we normally do not operate
many hours of accessories with the engine off. Of course,
if severe enough, the starter motor will not have sufficient
current to rotate the engine. One common failure
mode is a full separation....or part separation...of an interconnection inside the
battery. The battery will act as if there is a resistance
inserted in series, and may charge up to the correct voltage at
the battery terminals, yet
be unable to properly crank the engine. This is easily seen
by measuring the battery terminal voltage, ignition off, then
ignition on, then when attempting to crank the engine. The battery may not crank at all,
or you hear just a simple clunk if quite bad in this respect, yet the headlight MAY seem to
operate normally.
The headlight MIGHT dim to nearly or
completely OUT, when cranking. In a few instances, once the engine IS started and
run, the battery MAY seem to act OK....until the next cold
startup. It is rare, but I have even seen a battery that was
temperature sensitive, regarding the opening and closing of an
internal connection. Another common failure mode is a shorted cell,
usually only one fully shorts, but, it can be partial.
The battery MAY never charge up to proper voltage, and if this is
seen when using an EXternal charger with a voltmeter attached, the battery can be considered worthless, and must be
replaced. IF partial, the battery might charge up to the correct voltage,
yet after standing a half hour or hour, have its 'open terminal' voltage fall
towards 12, rather than maybe 12.5 to 12.7. That battery
should be discarded too. There are dynamic battery testers
at most repair shops, that can USUALLY determine if a battery has
the proper cranking power. These are usually called
Battery Load Testers.
It is possible for bad
starter motor to 'pull' or 'draw' so much current from the battery, that the
battery appears faulty. This is commonly seen with the early Valeo starters in which the pole pieces,
which were glued in place, come loose and lock up the starter. The Bosch starters
are FAR better than early Valeo starters, and the Bosch are easily re-buildable anyplace...they are interchangeable, but
need the end support plate, and the early versions were 8 teeth, later 9 teeth,
to match a change in flywheels...and you MUST use the correct
number of teeth version. Late model Valeo
starters are OK, and draw less current than the Bosch. Euromotoelectrics has them reasonably priced....see my references
page.
The best test of a battery is the mentioned old-fashioned
mentioned battery LOAD test, done at a battery shop with the
proper equipment. Few
motorcycle shops have such testing apparatus. Some
car/truck battery shops have this equipment...and a load test at
about 80 amperes, whilst watching the battery voltage, is about
correct for the Airhead batteries (either size). The load
test is done by reading the voltage accurately at 15 seconds of loading time. There
is a chart available for the lower voltage limit, versus
temperature....and usually the load tester comes with the chart.
2. Diode board and rubber
mounts: On those models with rubber diode
board mounts, the mounts will eventually deteriorate, and then
allow the diode board to fall downward and cause shorting or
other problems.
In
MY OPINION ALL rubber mounted diode board
models should have the diode board mounts changed to
all-metal. Not only is reliability improved,
but grounding and output is
improved too! There is some goodly
evidence that the diode boards stay cooler and more reliable.
Rubber mounts are a BAD IDEA! Diode board failures
can cause everything from low output and charging to no output
and charging.
The Airheads that already should have the solid metal diode board mounts (check your
bike, no matter the model) are:
/5 models; /6 models; 1978-1987 R65 and R80 models.
Once in awhile I hear of someone installing solid mounts; or, for some other reason, they have had the diode board out of the bike, and when replaced, there is no charging. They have probably mis-wired at the rear of the diode board. No charging, and a bright GEN lamp, can come from improperly re-assembling the rotor connections white-colored brush connection block....the insulating washers must be properly assembled at the Df terminal!!!
NOTE: BMW has had a lot of various problems with diode boards, grounding, the rubber mounts, etc. PLEASE read the article on the diode boards. PLEASE read the other electrical articles on this website....#14, #15, #15-A, etc. Some of the problems were also caused by extra heating due to the fairings on some models, and the change in 1981 (most countries) to the square air filter...which included a change to the cooling air flow through/past the diode board....into the starter area.
NOTE: Oak Okleshen published an extensive article on
testing the diode board using a transformer and lamp, in the Club publication,
AIRMAIL. June 1999 was a most comprehensive article, but there are many
others. Buy the Airtech Index from Oak: askoak@aol.com
Diodes are usually tested (disconnect the bike battery first!!!) by using an ohmmeter,
first with the leads in one direction, and then reversing the ohmmeter leads, so
that forward conducting resistance is measured (ohmmeters have batteries and
pass a small current through the probes) in one direction, and hopefully no or
extremely high resistance in the other (NON-conductive) direction. A test
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.
3. Voltage Regulator:
Usually a failure here means an opening of the series pass
transistor (electronic versions of the regulator) and no
output/charging. Other failures have been seen. The
mechanical regulators usually fail by slowly deteriorating the
output voltage. A failure to charge, in which the
regulator is suspected, can be PROVEN, by bypassing the
regulator. This is done with a jumper wire with male spades in each end. Remove
the regulator plug, and insert the jumper into the opposing (NOT BROWN WIRE!!!)
plug connections. If the system now charges, replace the VR.
Rarely the internal regulator series transistor short-circuits, and the
indication is always a vastly too high charging voltage at higher rpm.
Try Rockypointcycle.com
for adjustable voltage regulators.
4. Stators: Stators almost never fail unless abused. Abuse is usually nicking or otherwise injuring the wires during stator/housing removal. Due to the low resistance winding, ohmmeter tests are not usually indicative. There must be no continuity to ground from any stator terminal (stator disconnected from anything else). There is a difference in the windings after 1990, the resistance changed slightly, but this is mostly a point of nerdy discussion. Stator failure means low output/charging. The group of three wires to the one area of the stator can be in any order. Some models have a molded plug for these three. Stators can be tested with alternating current applied. Stators can be checked for resistance between windings, and to ground, but except for the ground type, the tests are not definitive, unless grossly wrong.
5. Rotors: Rotors rather commonly fail, they rotate fast and can have large centripetal forces on the windings when up and down shifting as rpm change can be abrupt. I have theorized that some types of clutch and shifting of gears can cause very high reversing forces on the rotors, particularly susceptible will be the old non-epoxy impregnated ones. Rotors are in an area that gets hot, and the current flowing into the rotor adds more heat....and thus there are heat-cycling effects. Same can be said for the diode board, which is susceptible over the very long term to this type of heat cycling effect. In the rotor, its wires are soldered to the slip rings. Solder joints can fail. Original rotors were sealed in a type of varnish, and are not as good as later rotors with high temperature epoxy (hopefully by vacuum impregnation)....and thus the wires can move about from vibration and centripetal forces on original old rotors and be damaged. Rotors usually OPEN, and a simple ohmmeter test across the slip rings will identify that. An open rotor or brush will usually not allow the GEN lamp to light up. A rotor can work (sort-of) at low or high rpm, and not at the other rpm.
Once in awhile a rotor will short circuit, and this may or may not be
such that an ohmmeter can show a short. Another form of rotor
shorting is slightly different: one wire passes through the rotor steel core through
a small hole, a place for short circuits if the rotor is not very carefully
assembled during the rewinding process. The ohmmeter WILL show
up a short to the rotor steel frame. To properly test for this whilst the
rotor is in place on the motorcycle, you must slide a piece of paper under both
brushes. I have seen
rewound rotors improperly tested before sale, that were
shorted. Rotors are easy to remove, but you
MUST use a
hardened factory, or HARDENED home-made tool. An article on how to make
this hardened tool, is on the airheads club website. http://www.airheads.org
See the Technical Tips section on that website, the article is entitled
"Alternator Rotor Puller".
Under NO circumstances should you use a common automotive 'legs' type of tool,
nor a NON-hardened bolt!!!
DO NOT fail to use a hardened tool!!!!
The BMW sold tool
is 88-88-6-123-600
6. Brushes: Brush
failure is common (there is an article on the website on replacing the
brushes). Brushes are a hard conductive carbon material and very slowly
wear. When the brush is short enough, the snail spring that presses the
brush against the rotor slip ring will bottom out on the brush
holder, and thus
there is no longer any...or only slight...brush pressure against the
rotor. You may have strange indications on the GEN lamp...it may seem to
light up properly, it may not light up properly, it may not light up at
all. It may be rpm sensitive. If the slip rings show approximately the proper resistance, yet
the resistance measurement at the brushes themselves (Df to D-) is not
approximately 3/4 of an ohm higher, the brushes are likely not contacting
properly...and a physical inspection is needed. Brushes last
~80,000
miles....depending on the dust in the atmosphere, amount of wattage normally
used, etc. See the brush replacement article.
NOTE AGAIN!....as the brushes wear to the usable limits, the snail spring starts bottoming in its slot. At just the right amount of wear, the very very slight sideways wobble of the rotor while the engine is running (at some particular rpm usually) will allow the brushes to either contact.....or not contact....causing very irregular GEN lamp and charging. Be aware of this phenomena.
NOTE! It is rare, but happens....one replaces the
brushes, and now there is zero charging! You probably removed the brush
holder assembly, and failed to reassemble it correctly. The D- terminal is
grounded to the case, the Df is insulated from the case, even though a first
glance shows them identical. You ALSO MIGHT have
put the two push-on wires on the wrong terminals. BROWN is ALWAYS ground
(D-, in this instance).
NOTE!....if you had the diode board out, and you have no
charging, you probably failed to connect one wire at the rear of
the diode board!
NOTE!....the BROWN wire MUST be to the brush D- terminal!...if
you reverse the brush terminal wires, the alternator will not
work!
7. Miscellaneous
items:
Key switches may develop intermittents and excessive
resistance, which can confuse the voltage regulator operation.
Wires can
get pinched.
A rare short circuit in the /7 and later bikes... at the
two-wire connector near the top front of the engine...the blue wire.... may fray and
short to the frame.
Another wire problem may be the blue-black wire from
the Df brush holder to the voltage regulator Df
terminal.
Wires (well, the end connections) can overheat and therefore have essentially too
much resistance. This is seen at two places commonly: The alternator
stator terminals (the group of three)...but is not seen at the center tap of the
stator windings (/6 and later). It is seen at the other end of the group
of three wires....behind the diode board.
Sometimes one sees
overheating at the larger gauge RED
wire at the right lower side of the diode board (as you face the
board from in front of the bike)....that is the battery direct connection.
The female spade connectors can be carefully squeezed to fit much
tighter.
It is rather common to find hairline cracks in the
flexible printed circuit board material inside the instrument pod, which, if in
the GEN lamp circuit, can cause the GEN lamp to not work, and thus NO charging.
Fuses: Fuses are generally not used in the charging
system, certainly not in the output from the diode board to the battery.
There is one exception, and it is the later /5 model that has
fuses in the headlight shell, and some /6 models. On these, one of the
fuses opening will shut down the GEN lamp.
Addendum:
The following graphic came
from: http://www.thunderchild-design.com/images/charg1.gif
I have modified the sketch with some identifications of wire
colors and small diodes.
www.buchanan1.net/charge.shtml:
There is information on that page, but there ARE ERRORS. I
am listing it here just because some know about it. Please
do not use.
A very complete book, written by OAK, on the entire electrical
system of all the models from somewhat before the /5, to the end
of production, is obtainable from the Chicago Club. This
contains all the schematic diagrams of the entire bike systems,
as well as very detailed individual parts and systems drawings,
schematics, descriptions and operations, etc. This book is
really a training manual, and does contain some troubleshooting
information. I consider it a must have item.
See my URL page on this book, and there is a critique of it on
this website too!
A small book on the airhead charging system, but quite adequate, especially for
troubleshooting, with illustrations and practical advice, is available
from Motorrad Elektrik.
http://www.motoelekt.com
(256) 442-8886
Revisions:
Final release: 01-03-2004
11-14-2005: add hyperlink for additional schematic
02/23/2009: added some clarification to rotors and stators
05/09/2009: recheck, fix minor typos and clarify a few details.
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.
©Copyright,
2011, R. Fleischer
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