Alternator Brushes & Rotor
©
Copyright, 2011, R. Fleischer
altbrushrotor.htm-17
Removing and replacing BMW Airhead motorcycle alternator brushes and rotor
Skill level required: Non ham-fisted Beginner or
better
Introduction:
BMW Airhead motorcycle alternator brushes are usually worn to the 'too short and intermittently contacting' point by AROUND 80,000 miles. That mileage can be highly variable, depending on brush snail spring tension, how much dirt is in the air where you ride, how much of the alternator output you use. When the brushes fail to contact the rotor slip rings properly, you will usually have problems with the GEN lamp lighting, intermittent charging, and eventually no charging at all. Quite often the problem shows up as a dim lamp or lowered charging voltage as rpm goes up, due to slight irregularities in the run-out of the slip rings, causing brush bounce. When at least one brush fails to contact at all, the GEN lamp will not light at all.
Tools required:
BMW standard on-bike tool kit.
(8 mm Spintite or modified nut driver or modified socket..,...only if you decide
to remove the brush holder, which is NOT necessary!)
Good sized soldering iron, such as 250 watt soldering
gun, or 100 watt large tip iron; rosin core solder. It
is possible to use a soldering iron as
small as 25 watts if the tip area
is massive enough.
Long nose pliers or other device to help to avoid solder wicking; may not be
needed.
torque wrench and allen adapter is
OPTIONAL
Parts required:
* Brush set, 12-31-1-244-480 (one set = 2 each). Some older
numbers are 12-31-1-350-787; 12-31-1-243-004; 12-31-1-244-389. Your new
brush set might not come with the insulating sleeves, so you should save your
old ones.
Brush dimensions: .670" overall length; specification is 16.5 mm from concave to top (.650"); brush is .385" x .190"
* Brush holder if you are not careful, 12-31-1-243-003
* Snail springs (two), IF yours are lacking enough force, note that not much
is needed... 12-31-1-350-786. Many folks automatically replace
these. I have not found that necessary.
***CAUTION!!
BE SURE TO DISCONNECT ALL THE BATTERY NEGATIVE TERMINAL WIRES
FIRST! If you have only
one wire at the negative
battery post, it likely
goes to the stock position, which is the speedometer cable
hollow bolt; if so, then you can disconnect it at that bolt. The battery MUST be
disconnected one way or the other. Only then is it safe to remove the outer cover of
the timing chest. Remove the outer timing chest cover,
usually just two long allen screws (three on early models). There may be a locating peg at the
bottom. Wiggle off the cover. If you have a pre-1979 model, be
cautious about the rubber around the points cavity and perhaps the tachometer
drive. If you have a BMW oil cooler on your
bike, and the cooler is mounted at the front top of the
cover, you must remove the cooler first. I suggest you
remove ONLY the left and the right 10 mm hex bolts holding
the cooler to its bracket. If you have a faired model,
you must remove the outr painted cover before you can remove
anything else.
When the brush length is such that
troubles begin, they are actually more than
long enough to work OK, if it was not for
the circular snail spring that applies
pressure against the brush.
The snail spring can
be mechanically prevented from pushing enough, or at all, on
the brush, when the
brush wear is enough to allow the snail spring to
hang-up, bottomed out actually, in its slot in the brush holder.
When worn ABOUT half way or somewhat more, the brushes are usually not working
correctly, or, soon won't be. You can tell by looking CAREFULLY
at the
snail spring, and see if it is BOTTOMING on the HOLDER. If it
is NOT, there should be brush life left IF the spring is OK, and
applying pressure. You do NOT need a lot of pressure, and
excessive pressure is NOT a good idea. ...but neither is too little
pressure!
HINT! In an emergency, you can put a teeny piece of paper, such as from a common matchbook, under the spring tip. This will allow a LOT of miles of charging.
***There are two basic styles of brush holder, & they
are interchangeable. They look nearly the same unless you
look carefully. The difference is the SIDE slot for the
snail spring. Also there
are two styles of brushes, also interchangeable. The stranded
wire may come from the END of the brush, or the SIDE of
the brush. Some brush holders have a groove for the brush style in
which the wire comes out the side. Be sure to look carefully,
assemble properly.
On some brush holders the soldering point is easy to
get to on both brushes without removing the stator, but
the rear brush and snail spring is somewhat of a bear
to get to without removing the stator. You will see what
needs to be done by a look-see. On other brush holders,
the soldering points are available conveniently ONLY by removing the
stator.
Both brushes are in a single, somewhat breakable (especially when old
and brittle), white-colored plastic holder. The outer (forward) brush is
relatively easy to get to, not so the inner brush.
Due to irregularities in brushes,
circular spring pressure, dirt, etc., the inner one MIGHT
wear faster, so take the time to find out.
The snail springs are
not
to be tightened so much that they are nearly wound up solid.
A little known problem can occur with the
snail springs, mostly I see this now and then on the inside
hidden spring (rearward one), but can happen on the forward
spring. The snail spring can be installed with the
steel mounting plate
TAB on either side of the center part of the snail spring. IF the
tab is in the wrong place then the outer part of the snail spring MIGHT rub on
the white plastic housing, this is because the spring is a bit
off center..... this can cause the BRUSH to NOT slide smoothly in
the holder. This will cause improper brush operation.
Check that the brushes move in and out
smoothly, when you have the stator off the bike and you can
easily push the brush with
your finger. Do NOT think you can test for this by pulling on the brush
wires with the stator mounted, you can not.
I recommend cleaning the
brush holder after the old brush is removed, with a Q-tip and a good solvent.
The brush flexible stranded wire is soldered to the brush
holder metal plate. You will need a soldering iron with a
substantial size soldering tip to hold enough heat during
the soldering. Large heavy duty soldering guns also work OK.
When replacing the brushes you have two choices:
Method #1: Cut the old wires NEAR the point where they
solder to the plate, leaving a bit for
soldering
to the new brush's wire. You have to be careful when soldering
these wires as solder wicking can occur, making a considerable
length of the wire stiff, a bad idea, that does not allow proper brush movement, and
makes for a potential for breaking from vibration. Long nose
pliers will help prevent solder wicking. This procedure, attaching to a part of the old existing wire is really not the best,
but it does work just fine,
and the inner brush can be fun to get to...and you can break
the
plastic brush holder, necessitating doing Method #2 anyway. I really do not like this Method #1, but I could argue that
there are times where it is preferable, proper tools not being available
being one reason.
Method #2: Remove the case/stator of the alternator. Many folks
struggle with, and even break, the plastic holder, when trying
to replace the brushes...especially the inner brush. This can
be avoided and a neater job done, by removing the outer
case/stator assembly. This is the preferred and frankly usually
much easier method. You can now easily replace the brushes
properly.
This is the method described below.
***NOTE that you do NOT have to remove the rotor!! You do
NOT
have to unsolder any stator wires!!
1. Unplug all wires at the alternator. They are push on/pull
off types. Take note of any that are not fitting tightly
or are overheated....and fix these problems before reassembly,
by cleaning with a fine tipped tool of some sort and squeezing
the female connector parts slightly. Failure to ensure clean
and tight connections will result in them overheating, and likely lower
electricity output.
2. NOTE where the brush holder and their respective wires connect
to...you will be putting the alternator stator/cover/wires
as one assembly,
back in the same position upon reassembly. HOWEVER, of the
THREE
stator connection wires right next to each other, their order
is UNimportant. Do NOT mix up the two rotor wires. D- is ALWAYS BROWN.
3. Remove
the THREE long screws holding the alternator case to the timing
chest. NOTE that when reassembling, those screws should be equally tightened, in
back and forth patterns.
4. Using a small piece of soft wood in one hand....NOT a metal
screwdriver!!
and SUPPORTING the outer alternator/stator case with the other
hand....to keep it from dropping on the floor when it suddenly
releases....CAREFULLY LIGHTLY pry, evenly all around, at the
open areas along the REAR side of the stator, where it meets
the
engine. AVOID TOUCHING/PRYING ON THE STATOR WIRES!!!! DO NOT
scratch or nick the steel laminations of the stator!!!!
Pry points are the timing chest metal to the alternator stator
multi-layered metal stack. Typically prying a SMALL amount,
relatively evenly around, at the three opposing places along the
circumference is all that is necessary. Try to remove the
assembly straight out horizontally.
***NOTE!...Avoid any damage to the brush holder.
Be careful, noting that the brushes might get hung up on the slip rings during removal, so go at this stator removal, outwards, bit by bit. If the brushes hang up, things can break: pull up on a brush wire as needed, while slowly removing the stator assembly.
****NOTE!...To avoid breaking the brushes and brush holder during reinstallation, the brushes MUST be lifted away from the rotor slip rings AS you install the case/stator. You do this one brush at a time, during refitting. DO NOT forget this!
5. With the case/stator/brush assembly in your hand and away
from the engine, all will be self explanatory upon inspection at
the inside of the stator assembly.
6. For SOME brush holders, you need to remove the inside nuts to remove
the holder for soldering, on other holders this is not
necessary. The nuts holding the brush holder to the case are thin, and
you may have to improvise a nut-driver. I use a SpinTite,
with the end ground flat to avoid any internal taper. A thin
socket, suitably end ground, will also work fine.
7. When replacing the case/stator, be SURE the mating surfaces of the stator
housing and the matching engine area are cleaned of all filth. Corrosion on the steel part of
the stator can be cleaned with a common file.
DO NOT nick or bend any metal of the stator, its housing, or the
stator/engine mounting area. If you decide to clean
the slip rings, DO NOT use emery paper. You can use a MILD
plastics type scouring pad from the kitchen if
needed. A bit of carbon won't hurt, but no grease allowed. You can
just use a rag with a few drops of some fast drying solvent.
I often use a very fine grit of sandpaper. I PREFER a clean
and shiny set of slip rings.
8.
****NOTE! It is altogether too easy, many have been fooled..... to forget that one of the
brushes is an electrically hot brush, and the other one
is grounded.
IF you have to remove the brush holder, do
not fail to note where the washers are located, and how
assembled!! Failure to reinstall the insulating washers at the proper
places,
in the proper order, with any washer ridges in proper
position,
can result in NO charging! There are markings on the stator
housing.
The D- marking is the grounded brush, and the BROWN wire
connects there.
Df is the hot, INSULATED brush. You will probably find the
Df brush wire to be blue/black.
9.
Make a final inspection after assembly, and tighten the long
screws EVENLY, and do not over-tighten. Attach all the wires
by pushing them onto their respective male spades, and as cautioned
previously, be sure that the
grouping of three at the stator are very tight fitting, and it
makes no difference what order these three are attached.
Go for a 30K ride...then inspect the brushes again!
:-)
CRITICAL!!!!
If you are removing and replacing a ROTOR, be
advised that use of ONLY a
HARDENED tool should be used,
DO NOT use a conventional fingered puller
at all!!!!....use either the BMW
special remover bolt (it is hardened), or you can make one, as many
times described by Joe ('Cuda), and published under the Technical Tips section at:
http://www.airheads.org. The TAPERS,
male and
female,
of the crankshaft, and the rotor, must be CLEAN AND
DRY,,,,no
finger prints either!!...and do NOT use gasoline as the
degreaser. Do NOT over-tighten the rotor bolt when replacing it.
BMW's tightening specification is a bit too high, in my
estimation. The official specification for the tightening of the stock bolt in the center of
the rotor is 23-27 Nm, which is 17-20 foot pounds. I never torque them
with a wrench, but I think
that 12 foot pounds is plenty. You really need very little torque on that
bolt.
Revisions:
02/03/2003: add brush dimensions
03/30/2003: add ****NOTE #1: and
****NOTE #2:; add altbrushrotor.htm
at top; minor other editing for clarity
07/13/2005: torque on center bolt of rotor added
10/26/2006: clarifications of no real importance
05/10/2011: Update article for clarity
© Copyright, 2011, R. Fleischer
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