Troubleshooting the
Alternator/Charging Problems
©
trbleshootALT.htm
15-B
FIRSTLY, and IMPORTANTLY, this article is NOT a substitute for Articles #14, #15, #15-A.
What this Article IS, is a special addendum to those above articles. ON PURPOSE, I have not placed lots of hyperlinks in THIS article (although a few GOOD ones ARE here), referring to sections in those articles. I WANT YOU to read those three articles!
You may also want to refer to the articles on the voltage regulators.
*********************************************
Brief Description of the Charging
System Parts:
The BMW charging system, as used on ALL airhead boxer models from the end
of 1969 at the introduction of the /5 series, to the end of civilian production
of these machines, in 1995.....(and with some slight modifications to the
stator, rotor, and voltage regulator for some Authorities (Police) models)....are the SAME....with only some modest
variations.
The System consists of a three-phase alternator, a diode
board, a voltage regulator, the GEN lamp, and the battery. Wattage output varied
by model and year. The original /5 had a 180 watt alternator which
can be upgraded to the 280 watt alternator by use of a very specific slightly
later alternator STATOR which will physically fit (1974 and some early 1975 production year).
You will need the /6 and later style of diode board as well. Other variations over the years include adding a center-tap to the
stator winding and adding a connection and three small diodes to the diode board
(/6 and later), mounting the diode board on rubber (bad idea, and
never done on all models, see later in this article), changing battery sizes, changing the windings on the
rotor (resulting in different resistances), and several versions of the voltage
regulator. There was a rotor diameter change during production,
early being 73.4 mm, later 73 mm. 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 whipping. The larger diameter 73.4 rotor could be used. That 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(s) as the R90S.
The diode boards are all the same in electrical function,
except that the /5 model did not have the extra three diodes for the center-tap
of the stator winding, as did the /6 and later, and thus this is just one other
reason the /5 had a lower output, besides the stator windings themselves (and
the /5 had a higher resistance rotor too).
All the diode boards interchange, but use of a /6 and later board on a /6 and
later, is mandatory to obtain full rated alternator output. In the early
1980's, BMW had a lot of problems with Wehrle brand diode boards. These
would severely overheat at the wires from the large power diodes, due to failure
of them to be bent-over before soldering. This is fixable. Aftermarket high power diode boards are also available
commercially. The stock board is adequate.
The earliest voltage regulators were mechanical, made by Bosch, in metal cans. Bosch also made an electronic regulator that is in a similar, if slightly shorter can. Those 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 must be used on the 1981 and later bikes, as they have need for a smoother, less spikey (electrically) output. The mechanical regulator slowly deteriorates, and an electronic type can be substituted, whether the Bosch, or Wehrle, or even a car type. Most any VR from a car that has the same three prongs and same case mounting, will work just fine in our bikes.
Description of how it works:
When the key is turned on, some small amount of
battery electricity flows from the battery, through the key switch, then through
the GEN lamp, and then to the D+ terminal of the voltage regulator (blue
wire). The regulator allows this small lamp limited current to travel
further,
via its Df terminal (Blue-black)... to the Df terminal at the brush holder for the
rotor. The electricity passes into the rotor, causing a slight
magnetization, and then out the rotor via the D- terminal to the engine ground,
which is the same as the battery negative (-) terminal,
electrically-speaking.
As the engine rotates, the rotor magnetic field is
transformed into alternating current electricity in the stator windings.
Six small diodes on the diode board are used
to change this small amount of electricity into Direct Current, and the
electricity is applied to the same input side of the voltage regulator that the
GEN lamp feeds. When enough rpm is reached to have about +12 volts
supplied at that point, then the alternator stator and diodes are supplying
current to the rotor,
and the lamp extinguishes, as it has +12 on one side, and +12 on the other side,
and thus no voltage DROP across it. The small diodes...as rpm
increases...produce the MUCH larger current (than the lamp could supply), through the regulator, that is
needed to fully power the rotor. Thus, the system is a merry-go-round,
supplying itself....once initiated by the lamp current. As the voltage
in the alternator stator rises, it eventually gets high enough to have another
six diodes, much larger ones (needed to pass lots of current), change the AC to
DC in the same manner, and THAT output comes from the B+ terminal of the diode
board, and goes directly to the battery, UNfused. If the rpm is high
enough, or system need low enough, the alternator could produce too high a
voltage into the battery and the rest of the bike. The voltage regulator
senses the output of the mentioned six small diodes, and begins to reduce the
current flow into the rotor.
The mechanical regulator does the regulating by slowly
separating the contacts in the regulator...vibrating them actually...thus, they
spark gently during actual voltage regulation....producing a bit of electrical
noise into the wiring...as well as deteriorating the points contacts
slowly. The later low ohms rotors are really a bit much for the
mechanical regulator to be used for long periods of time.
Performance:
The system in any of the airheads is adequate, particularly if you keep
the rpm above 3000. Those with much higher system usage...such as larger
wattage headlights, many extra other lights, heated clothing, ETC., may need to
make accommodation for same. Ask at the Airheads LIST.
What fails? Basic
answer: anything and everything has failed at one time or the other!
Today, many rotors have failed, and many rotors are very commonly
rewound....or newly made....by other companies than the original equipment
Bosch. Without measurements, you may not know what rotor you
have. For the most part, the rotors are being rewound with the later
lower resistance, approximately 2.8 ohms. These work better than earlier
rotors. It is not a great idea to install a 3.4 or 2.8 ohm rotor into a
mechanical voltage regulator bike....change to the later electronic regulator in
the plastic case. Stator failures are UNcommon. Rotor failures,
diode board and/or rubber mount failures, wiring failures, voltage regulator
failures....and ignition switch failures....all are seen now and
then. The MOST common failure is the battery.
Failure to disconnect the battery before removing the outer
cover of the timing chest, has caused electrical sparks and failures of the
diode board.
Failure of the voltage regulator is usually indicated by poor
or no charging....or just wrong voltage.
Failure of the diode board is usually indicated by lower
charging (lower watts) capability, or lower voltage output under load of such as
the headlight. ONE open large diode will so indicate by
vastly lowered charging with the headlight turned on. Problems with the small diodes, while rarer, do occur, and can
be seen by poor charging. In fact, almost anything that goes wrong
with the charging system components can result in lower voltage, or lower
output/charging. Even overheated connections.
GEN lamps that can be seen glowing dimly at night whilst
cruising are usually indicating poor connections someplace, or many
places.
GEN lamps that do not light up, and thus you have no charging
(or, maybe only at VERY high rpm), are usually a bad lamp, a bad lamp socket, a
bad regulator, or, commonly, an open rotor or excessively worn brushes.
From 1976, many models have a connection of the battery into
the 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 and socket...as 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.
If something fails, what are the
USUAL indications?, and how to determine:
1. Batteries:
A battery failure can cause an enormous amount of problems and
indications. It is QUITE possible for a battery to simply lose
capacity, that in itself is usually not a problem, as we normally do not operate
many hours of accessories with the engine off. One common failure
mode is a separation....or part separation...of an interconnection in the
battery. That means the battery acts as if there is a resistance inserted
in series. The battery may SEEM to charge up to the correct voltage, yet
be unable to properly crank the engine. The battery may not crank at all,
or a simple clunk, if quite bad in this respect, yet the headlight MAY seem to
operate normally. In a few instances, once the engine IS started and
run, the battery MAY seem to act OK....until the next cold
startup. Another common failure mode is a shorted cell,
usually only one shorts. This can be partial, or complete. The
battery MAY never charge up to proper voltage, and if this is seen by using an
EXternal charger, the battery can be considered worthless, and must be
replaced. IF partial, the battery might charge up to the correct voltage,
yet after standing a half hour or hour, have its 'open terminal' voltage fall
towards 12, rather than maybe 12.5.
It is possible for a poor or bad
starter motor to 'pull' or 'draw' so much current from the battery, that the
battery appears faulty. This is commonly seen with the early Valeo starters, in which the pole pieces
(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.
The best test of a battery is the old-fashioned LOAD
test, done at a battery shop with the proper equipment. Few motorcycle
shops have such testing apparatus. Some car/truck battery shops have this
equipment...and a load test at about 80 amperes, whilst watching the battery
voltage, is the best test.
2. Diode board and rubber
mounts: On those models with rubber diode board mounts, the
mounts will eventually deteriorate, and let the diode board fall downward and
cause shorting or other problems. ALL rubber mounted diode board
models should have the diode board mounts changed to
all-metal. Not only is reliability improved, but grounding and output is
improved too! There is some goodly evidence that the diode boards
stay cooler and more reliable. Rubber mounts are a BAD IDEA! Diode board failures can cause everything from low
output and charging to no output and charging. Diodes fail from
opening.....or shorting.
The Airheads that ALREADY SHOULD HAVE solid metal diode board mounts (check your
bike, no matter the model) are:
/5 models; /6 models; 1978-1987 R65 and R80 models.
NOTE: Once in awhile we hear of someone installing solid mounts, or, for some other reason, having the diode board out of the bike, and when replaced, there is no charging. You probably mis-wired at the diode board, especially at the rear of it.
NOTE: BMW has had a lot of various problems with diode boards, grounding, the rubber mounts, etc. PLEASE read the article on the diode boards. PLEASE read the other electrical articles on this website....#14, #15, #15-A, etc. Some of the problems were also caused by extra heating due to the fairings on some models, and the change in 1981 (most countries) to the square air filter...which included a change to the cooling air flow through/past the diode board....into the starter area.
NOTE: Oak Okleshen published an extensive article on
testing the diode board using a transformer and lamp, in the Club publication,
AIRMAIL. June 1999 was a most comprehensive article, but there are many
others. Buy the Airtech Index from Oak: askoak@aol.com
Diodes are usually tested (disconnect the bike battery first!!!) by using an ohmmeter,
first with the leads in one direction, and then reversing the ohmmeter leads, so
that forward conducting resistance is measured (ohmmeters have batteries and
pass a small current through the probes) in one direction, and hopefully no or
extremely high resistance in the other (NON-conductive) direction. A test
using a 6 or 12 volt brake or turn signal light bulb and a 6 or 12 volt AC transformer is a VERY good test, and HIGHLY
recommended, as 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.
3. Voltage Regulator: Usually
a failure here means an opening of the series pass transistor (electronic
versions of the regulator) and no output/charging. Other failures have
been seen. The mechanical regulators usually fail by slowly deteriorating
the output voltage. A failure to charge, in which the regulator is
suspected, can be PROVEN, by bypassing the regulator. This is done with a
short, perhaps 4 inch, jumper wire with male spades in each end. Remove
the regulator plug, and insert the jumper into the opposing (NOT BROWN WIRE)
plug connections. If the system now charges, replace the VR.
Rarely the internal regulator series transistor short-circuits, and the
indication is always a vastly too high charging voltage at high rpm.
4. Stators: Stators almost never fail unless abused. Abuse is usually nicking or otherwise injuring the wires during stator/housing removal. Due to the low resistance winding, ohmmeter tests are not usually indicative. There must be no continuity to ground, of course. There is a difference in the windings after 1990, the resistance changed slightly, but this is mostly a point of nerdy discussion. Stator failure means low output/charging. Wires to the stator (the group of three) can be in any order. Some models have a molded plug for these three.
5. Rotors:
Rotors rather commonly fail, they rotate fast and 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. On top of which, rotors are in an area that
gets hot, and thus there is heat-cycling effects. Same can be said for the diode
board, which is quite susceptible over the very long term to this type of
damage-effect. Wires are soldered to the slip rings.
Solder joints can fail. Original
rotors were sealed in a type of varnish, and are not as good as later rotors
with high temperature epoxy vacuum impregnation....and thus the wires can move
about from vibration on original old rotors and be damaged. Rotors usually
OPEN, and a simple ohmmeter test across the slip rings will show a problem, if
any. An open rotor or brush will usually not allow the GEN lamp to light
up. Once in awhile a rotor will short circuit, and this may or may not be
such that an ohmmeter shows a shorted few turns. Another form of rotor
shorting is different: one wire passes through the rotor steel core through
a small hole, a place for short circuits if the rotor is not very carefully
assembled during the rewinding process. The ohmmeter WILL show
up a short to the rotor steel frame. To properly test for this whilst the
rotor is in place on the motorcycle, you must slide a piece of paper under both
brushes. I have seen
rewound rotors improperly tested before sale, that were
shorted. Rotors are easy to remove, but you
MUST use a
hardened factory, or HARDENED home-made tool. An article on how to make
this hardened tool, is on the airheads club website. http://www.airheads.org
See the Technical Tips section on that website, the article is entitled
"Alternator Rotor Puller".
Under NO circumstances should you use a common automotive 'legs' type of tool,
nor a NON-hardened bolt!!!
DO NOT fail to use a hardened
tool!!!!
6. Brushes: Brush
failure is common (there is an article on the website on replacing the
brushes). Brushes are a hard conductive carbon material and very slowly
wear. When the brush is short enough, the snail spring that presses the
brush against the rotor slip ring will bottom out on the brush
holder, and thus
there is no longer any...or only slight...brush pressure against the
rotor. You may have strange indications on the GEN lamp...it may seem to
light up properly, it may not light up properly, it may not light up at
all. 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 an ohm higher, the brushes are likely not contacting
properly...and a physical inspection is needed. Brushes last about 80,000
miles....depending on the dust in the atmosphere, amount of wattage normally
used, etc. See the brush replacement article. NOTE 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).
7. Miscellaneous
items: Key switches may develop intermittents and excessive
resistance, which can confuse the voltage regulator operation. Wires can
get pinched. A rare short circuit is the /7 and later bikes at the
two-wire connector near the top front of the engine...the blue wire may fray and
short to the frame. Another wire problem may be the blue-black wire from
the Df brush holder to the voltage regulator Df
terminal.
Wires can overheat and therefore have essentially too
much resistance. This is seen at two places commonly: The alternator
stator terminals (the group of three)...but is not seen at the center tap of the
stator windings (/6 and later). It is seen at the other end of the group
of three....wires....behind the diode board. Sometimes one sees
overheating at the larger gauge RED wire at the
right lower side of the diode board (as you face the board from in front of the
bike)....that is that battery connection.
It is 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:
Here is a URL for a website page that I suggest you
IMMEDIATELY print:
An excellent schematic and discussion of the basic charging system. It
is so good, and so legible, that I SUGGEST you PRINT a copy and keep it in your
reference material...as, who knows how long it will remain on-line.
www.buchanan1.net/charge.shtml
NOTE!!...I found an error on that
schematic, so please annotate your copy. I have notified
the author, so I can't be sure if he will fix the schematic or
not. On the lower right area of the schematic are two
arrows and the words Big Diodes. Remove those arrows and
words at that area. All the other notations are correct,
although you may want to make a note that the dotted lines from
the Stator center-tap go to two small diodes.
For another URL with just the schematic: http://www.thunderchild-design.com/images/charg1.gif
A very complete book, written by OAK, on the entire electrical system of all the models from
somewhat before the /5, to the end of production, is obtainable from the Chicago
Club. This contains all the schematic diagrams of the entire bike systems,
as well as very detailed individual parts and systems drawings, schematics,
descriptions and operations, etc. This book is really a training manual,
and does contain some troubleshooting information. I consider it a must
have item. My url.com section may have the latest contact
information on how to obtain this book...or....it may be in the other items on
the website, such as section 14, 15A.
A small book on the airhead charging system, but quite adequate, especially for
troubleshooting, with illustrations and practical advice, is available
from Motorrad Elektrik.
http://www.motoelekt.com
(256) 442-8886
Revisions:
Final release: 01-03-2004
11-14-2005: add hyperlink for additional schematic
02/23/2009: added some clarification to rotors and stators
05/09/2009: recheck, fix minor typos and clarify a few details.
