The Alternator/Charging System,
How it Works.....AND....Troubleshooting!
©Copyright, 2014, R. Fleischer
This article is NOT a substitute for Articles 14, 14A, 15A, 15B, 15E, 17, 18. This article is to be used in addition to those articles. >>> I WANT YOU to read ALL OF these articles...and more....!!!
You may also want to refer to one of the articles on the Airhead Voltage Regulators, etc.
Description of the Charging System:
The maximum alternator wattage output varied by model & year.... and particularly with what parts are in any particular motorcycle, since most 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. 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. Solid metal aftermarket mounts perform electrically much better, and may offer slight additional diode board cooling; and there is no rubber mountings to deteriorate. This is a VERY worthwhile upgrade. There is an article on this website about these mounts and the necessary wiring modifications: http://bmwmotorcycletech.info/diodebds&grdgwires.htm
BMW has used a variety of battery ampere-hour sizes, with two basic sizes of battery case over the years.Any of these batteries will start and operate the engine. The difference is the amount of reserve capacity; and cranking ability at cold temperatures. There is a large article about batteries on this website: http://bmwmotorcycletech.info/newbattery.htm
You could always, at more expense, purchase a brand-new later board, or an aftermarket high power diode board. The stock board is certainly adequate... contrary to a lot of false ideas & misleading statements. You might find an original diode board in very good condition...for very low $, as they are often removed when still OK.
The electronic regulator should be used on the 1981 & later bikes, as they have need for 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 or better yet an aftermarket adjustable one. You can also service that mechanical regulator, article is on this website. 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 asRockyPointCycle.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. Whatever you do, the 14.2 +- volt regulators are better for your battery, as opposed to the original 13.8 volt types. DISregard those that say the headlights burn out quicker. What IS true is that on a FLOODED battery, the higher voltage VR's do use a bit more water over time.
NERDY: 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. Earlier regulators MAY hold up.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 (probably NOT necessary). Interestingly, it is difficult to find the current rating of any of the VR's....but the power transistor does have numbers and thus a rating for current.
How it all works:
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 ~
the regulation set voltage. This on/off of the contacts tends to happen fairly fast. While maintaining the voltage the regulator was set for, the VR must turn on and off rapidly, once the battery voltage reaches the desired set voltage, to maintain the voltage within a narrow range. The VR
points spark gently during this actual oscillating 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. The VR can be serviced, article on this website.
Poor charging can come about from quite a few reasons. Sometimes more than one reason
at the same time, which makes finding the problem(s) more involved.
What fails? Anything and everything has failed at one time or the other.
GEN LAMP PROBLEMS:
Troubleshooting a dead (not lighted) GEN lamp: The problem may not be a bad lamp. The GEN lamp SELDOM burns out. 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. More often the rotor has opened electrically; or a brush, or both brushes, is very badly worn. Worn brushes frequently cause problems, but usually the lamp is then intermittent, especially with rotor RPM. The reason rotor RPM is involved is that there is always some slip ring runout, which varies with RPM....so typically one brush, worn more than the other, gets intermittent in its contact with the slip ring.
Here are some simple tests:
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 still 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 now lights-up, then the problem is in the rotor, brushes or brush wires. To determine which, remove the jumper wire or screwdriver. Use a copper penny or a screwdriver, etc., and short the two slip rings together...be careful that you only connect one slip ring to the other, and are not touching the case meta. If the lamp lights up the ROTOR IS OPEN AND BAD.
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.
How to determine if the voltage regulator is 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 are a good 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. If you would like to test the system maximum possible output, you can leave that jumpering in the plug, and start the engine and monitor the battery voltage as you raise RPM.
Do NOT forget to disconnect the battery before replacing the front cover.
Troubleshooting a constantly on 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, & 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:
http://bmwmotorcycletech.info/multimeters.htm.That article covers both multimeters AND test lamps usage.
1. The brushes may be well-worn. Usually when the brushes are quite worn, one of the brushes will wear faster than the other, sometimes considerably, sometimes not eyeball visible. The snail spring on the brush usually can 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 allow the spring to press on the brush & not the plastic holder, until you can get around to installing new brushes. Brushes do not fail suddenly (unless its flexible wire breaks...which is rather rare). Rather, brushes get short enough that they make INTERMITTENT contact with the rotor slip rings; usually that 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. Now and then a diode board is fried by someone trying to save a few $$, and installs a wrong type of battery (reversed polarity terminals). SOMETIMES the motorcycle is one with the diode board RUBBER MOUNTS, with mounts failures 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 each brush. 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 probably took the brush holder apart, misplaced the correct order & installation of the insulating washers in the Df brushholder section. Testing with an ohmmeter, D- and Df wires OFF, & 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.
Rotor failures are typically due to aging of various types. Rotors are either discarded for new ones (which may be from BMW, or aftermarket). Rotors whose windings were
unsealed, or sealed with lacquers or shellac, tend to fail due to stresses
on the windings
during larger RPM changes, typically when downshifting into first gear. This is particularly so when the rotor is quite hot.
Sometimes a rotor winding will short to another winding, or short to the metal magnetic pole pieces material. Now and then a rotor solder joint fails.
I have seen TWO 'out of the box' rotors that were bad. One was a rebuilt, improperly tested,
because there was continuity between slip rings and the magnetic metal, amounting to a
direct short of Df to ground.
Without measurements, you may not know what rotor you have. For the most part, rotors are being rewound with the later lower resistances.
These work better than earlier 7 ohm 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; but, it will work for awhile with the mechanical VR. I suggest you avoid
using a 2.8 ohm rotor with early stators; the charging output might be inadequate. This can
be a puzzling thing, in trying to figure out what the problem is from low output.
Stator failures are NOT common; rarely from age failed insulation. Some failed because owners removing & replacing rotors nicked the stator windings since to replace a rotor you must remove the stator housing. NOTE that the stator windings are done on multiple thin special varnish insulated sheets of steel generally called laminations. If you scratch across these sheets accidentally by using metal tools, you can have a stator that seems to work fine, but has "somewhat" REDUCED performance from what is called Eddy Currents.
Rotor failures, diode board and/or rubber mount failures, wiring failures, voltage regulator failures....and ignition switch and kill switch failures....all are seen now and then. Often found are loose connections at the battery, or starter motor solenoid, or battery ground lead at the speedometer cable hollow bolt (DO NOT over-tighten that one!), or starter relay connections (or, internal in that relay). More on that relay in the next section after the horizontal line.
Failure to disconnect the battery before removing the outer cover of the timing chest, has caused electric sparks & failures of the diode board. Also be careful when removing the starter cover.
Failure of the voltage regulator is usually indicated by poor or no charging or wrong voltage. VR failure is hardly the only failure to exhibit these problems. TESTING: You can easily bypass the VR, using a jumper across the opposing plug connections (NO connection is made to the BROWN wire!). That will produce maximum output from the alternator, do not let the voltage get too high.
Occasionally a VR will fail in the Full Maximum Output condition; system voltage will be very excessive. 99% of the time the VR is at fault. Substitute a good VR.
Failure of the diode board is usually indicated by lower charging (lower watts) capability, or MUCH lower voltage output under load of such as the headlight (check battery voltage at ~4000 rpm with headlight on, and off). Just ONE "open" large diode will so indicate by vastly lowered charging with the headlight turned on, yet charging may be OK, headlight OFF.
Problems with the small diodes rarely occur, and can usually be seen by poor charging or 'funny' GEN lamp indications.
Almost anything that goes wrong with the charging system components can result in lower voltage, or low or no charging. Overheated, loose, or slightly dirty or corroded connections ARE COMMON. GEN lamps that can be seen glowing dimly at night while cruising are usually an indication of poor connections someplace, or several places. I usually see such a glowing lamp at night and my first assumption is that electrical wiring has never been cleaned or checked for tightness of connections, etc.
After the /5 the flexible printed circuit board that the GEN lamp fit into can get microscopic cracks, and the lamp connection to the board can fail. You can carefully repair the board....or replace it (IF you can find a replacement!.....new ones NLA from BMW). Do NOT yank lamps out roughly.....press on the printed material with a fingertip, rock the lamp out carefully.
From 1976, most Airheads have a major connection of the battery to the electrical system
located at the starter relay under the fuel tank. The red wires are, essentially, ALL connected together via the relay INTERNALS. On the metal can starter relays you can often carefully UNcrimp the edges and fix the internals....but that is a rarer fault than simply corrosion (of the male spades of the relay and the femaled socket connectors). If the problem is internal to the relay, fix it, and
I recommend you SEAL the top after re-crimping it.
There have been instances of a slightly loose rivet in this relay 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. Total loss of electric power is because the INTERNALS of the relay, even if the relay is
NOT being actuated for the starter motor, 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.
The starter relay has some 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 UNplug the relay & plug it back in. That wipes the spades.
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 corrosion is very visible and nasty you will have to install
new female connections. There is a modification that can help. Used with relatively
brightly cleaned male and female spade connections, this will likely be a 100%
permanent FIX! First, on all three RED wires, determine a place near the plug socket where you can CAREFULLY remove ~3/4" of the red insulation from the wires adjacent to each other. I suggest you use a very sharp
thin blade on a very flat angle. I use a hobbyists Xacto knife. DO NOT nick/cut any
copper strands. Tightly wrap some shiny copper wire around the insulation-less area. 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. Solder the wrapped connection using 60-40 common electronics solder (the type with a rosin core). Be SURE to use a 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.
Charging system failures; further discussion:
Another common failure mode is a shorted cell, usually only one fully shorts, but, it can be partial for one or more cells. The battery MAY never charge up to proper voltage, and if this is seen when using an EXternal charger with a voltmeter attached, the battery can be considered worthless, and must be replaced. IF partial, the battery might charge up to the correct voltage, yet after standing a half hour or hour, have its 'open terminal' voltage fall towards 12, rather than maybe 12.5 to 12.7. Falling from 12.6 or so, to closer to 12, is also seen when the headlight is turned-on, perhaps quickly after what seems like a full charging. That battery should be discarded too. There are dynamic battery testers at most repair shops, that can USUALLY determine if a battery has the proper cranking power...and LIFE left. These are usually called Battery Load Testers. You can get your own at a quite reasonable price from Harbor Freight Company. I test all my vehicles' batteries once or twice a year, and, yes, with a Harbor Freight Company Load tester. I suggest you purchase the slightly more expensive version (they sell TWO types); which has TWO METERS. It is much more versatile and works great. It is on sale now and then, and you may be lucky & combine it with a 20% off coupon! Really worth the $50-$80 you will pay. A load test at about 90 amperes on the 28 or 30 AH batteries (and 70 on the smaller batteries), while watching the battery voltage, is about correct for the Airhead batteries. The load test is done by reading the voltage accurately at ~15 seconds of loading time. That is done AUTOMATICALLY by the HF 2-meter model. That HF unit is ADJUSTABLE to match battery capacity size! There is a chart available for the lower voltage limit, versus temperature....and the load tester comes with the chart.
It is possible for bad starter motor to 'pull' or 'draw' so much current from the battery, that the battery appears faulty. While any well-used/worn starter motor can exhibit this problem from use, it was especially egregious on early Valeo starters (which were installed by BMW instead of the Bosch starters, in the later eighties) in which the permanent magent 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. Euromotoelectrics.com has them reasonably priced....see my references page: http://bmwmotorcycletech.info/references.htm. There are also other sources for starters. http://beemershop.com is my recommendation. No matter what starter you install, be sure it has the proper 8 or 9 tooth drive gear. Be sure to read my article on installation (even if the SAME type starter you are replacing), as there has been some slight metal interference problems, EASILY fixed. The following article covers the various starters, including aftermarket ones like the Valeo & Denso and the fitting problems: http://bmwmotorcycletech.info/boschvaleostarter.htm.
2. Diode board, rubber mounts, brush-holders, etc:On those models with rubber diode board mounts, the mounts will eventually deteriorate, allowing the diode board to fall downward & cause shorting or other problems. In MY OPINION ALL rubber mounted diode board models should have the diode board mounts changed to all-metal. Not only is reliability improved, but grounding, output, & voltage regulation is improved too! There is some evidence that the diode boards stay a bit cooler and are more reliable. Rubber mounts are a BAD IDEA! Diode board failures can cause everything from low to NO charging.
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 better test is 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. This is an especially VERY good test, and HIGHLY recommended. 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. I've conveniently made testers using an old desk high intensity lamp and its transformer. 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 the ohmmeter and/or diode function test on multimeters. 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: http://bmwmotorcycletech.info/diodebds&grdgwires.htm
Read the other electrical articles on this website....#14, #15, #15-A, 17, 18, etc.
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 at 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!!): http://www.euromotoelectrics.com http://www.rockypointcycle.com/
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/scratching across 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. 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 used
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. 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 can be done with the system on the bike, with the phases (& centertap, but 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 or differences may mean anything.
b. With Df harness wire disconnected & D- grounded & Df terminal connected to battery +; measure phases with an A.C. Voltmeter, at ~2000 rpm. The voltage should be about the same between any connection type of the three terminals stator output group at the SAME rpm.
c. Repeat b. but with A.C. Ammeter....properly connected! Few of you have an A.C. ammeter, so this test is seldom done by anyone.
d. Use the series lamp and transformer tester you made for testing diodes. Apply to one phase, see how the lamp lights, or doesn't, and also test to the case (no illumination), and when onto one phase, check all phase connections for equal A.C. voltage.
e. The tests can also be run with a common old-fashioned sealed-beam headlight, 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.
f. For /6 and later types: disconnect the 3 phases & center-tap connections (pull off the connectors). GROUND the centertap. Use a test light on each of the three stator terminals, the other end of the test lamp to chassis. Use somewhat more than idle rpm. 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. A shorted turn makes a rather gross difference in output from a phase.
5. Rotors: Rotors rather commonly fail, they rotate fast & have large inertial and 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 quite 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 fail by OPEN circuit, and a simple
ohmmeter test across the slip rings will identify that. 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 on many (not all) rotors 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 to the steel of the rotor. Some rotors are being made with the wire NOT going through a hole.
The BMW-sold tool is 88-88-6-123-600. You can get an equivalent tool from aftermarket suppliers.They are NOT expensive. You can make a tool from 8.8 bolts and a roller bearing....ask on the Airheads LIST. I am NOT in favor of this two-piece tool method....due to the possibility of wrong use, INCLUDING canting of the inner part, causing you HUGE problems.
RARE situation, a reverse magnetized rotor:
Brushes are a medium-hard conductive carbon material and wear slowly. When a brush is worn short enough, the snail spring (that presses the brush against the rotor's copper 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 and/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 replace the brushes, and then: 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 connectors, 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 break, that connects to the holder terminals, 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, & even 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 by the female wire connector. Female spade connectors can be carefully squeezed just a wee bit with a pliers, and thus they will fit much tighter.
i. Other wire connectors, beside the one above 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). 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 them.
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. There is an article on this website about modifying the lamp circuit so even if a lamp fails, charging will occur. http://bmwmotorcycletech.info/genlampresistor.htm
k. Occasionaly there is a problem with the pod CABLE PLUG, or the pod side of it.
l. 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.
m. /5 Front METAL had poor ventilation. Using a later cover will improve reliability.
n. Front Fiberglas covers (RT, RS): The early versions had NO louvres. That reduced the cooling to the diode board. You can replace it with a later type, or, cut into your solid one and make slits in it, or your initials, ETC.
A smaller 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
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. In entire article add material & 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: Entire article: Simplify some testing for ease in understanding & speed. Edit article as needed for clarity. Update sources, links, etc.
01/18/2015: Add section on rotor magnetization; which came from a response I did on the Airheads LIST.
07/07/2015: Add new section on gen lamp always on, clean up some of the rest of the article (more should be done, when I get to it....)
12/27/2015: Go through entire article. Clarify many details & 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.
04/25/2016: Go through entire article. Clarify many details and add more hyperlinks. Update metacodes; fonts, layout, etc.
©Copyright, 2014, R. Fleischer
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Last edit of THIS page: Tuesday, April 26, 2016