Updated and revised by the author, 12/08/2007
We will start off with very basic electricity, which is fundamental to understand, when you are working on your BMW Airhead. This article not only will cover what basics you need to know about your Airhead motorcycle electrical system, but will also cover some common faults, failures, problems, and what the causes and fixes are. This article is NOT any sort of Engineer's guide to electricity; nor, is it a complete primer on all Airhead electrical faults. The Author's website contains vastly more information. <http://bmwmotorcycletech.info>
The above mentioned website contains articles #14 through #38, specifically for your motorcycle.
When writing this this lenthy article, below, I have taken some liberties in explaining how some things operate; to make things simpler for the average person who is unlikely to be an electrical or electronics engineer; or to have knowledge much beyond a laypersons. SO, PLEASE, no E-mails about quanta movement; elecron packet flow, holes versus electrons, and electron movement via wave or field theory!...OK?
Electricity is a flow of electrons, whirling parts of atomic structure. These little bits have a 'charge'. When these little charges move through a conductor, typically a copper wire or other metal, you have a flow of CURRENT. If enough are flowing through such as the wire inside a lamp, the lamp's wire, usually called a 'filament', will heat up, and when there is enough current and thus the filament is hot enough, it radiates heat and light. If there is too much flow the lamp burns out, just like a fuse, which blows for excessive current flow.
Some examples: If a diode in your Airhead's alternator diode board should have an internal fault, it might overheat from excessive current flow. If you crank the engine for a minute or so continuously, that may be too much current flow over too long a period of time for the starter motor, it may overheat, and will most certainly also cause battery heating. If your voltage regulator is shorted, or otherwise fails in the 'too high alternator output mode', it will cause excessive charging, and overheating of several components. If you carelessly drop a metal tool across the battery terminals, or possibly many other places on your Airhead, sparks may lead to large heating effects.
Conversely, there is a certain amount of current flow that is NORMAL, in a given design of something. The 1981+ ignition module is designed for a certain current flow over the rpm range; and, it develops heating from that flow. If the heat-sink paste on its underside is very old and not conducting heat very well, the module may overheat. When a module overheats, it usually shuts off the ignition, only to regain it when it cools. Enough such heating-cooling cycles, and the module will totally fail.
Another example, this time of current flowing where it should not flow. If you were to remove a spark plug cap from a spark plug while the engine was operating, the high voltage in the ignition coil would have no easy place to jump to the chassis. The voltage would rise to a value much higher than normal, and then probably jump across wires or wires to case, INside the coil. This can lead to a partially shorted coil, and that damage might NOT show up for a long time.
Current flow is measured in amperes, and in many cases, very tiny parts of amperes, such as milliamperes (thousandths of; or microamperes, millionths of). You don't really need to know what an ampere really is.
It is still popular to use common household water pipes to explain electricity. I find that often is very confusing to folks. But, for those that wish to use the analogy, I will just say that water pressure is the force, similar to voltage, that ALLOWS more flow (current) from the faucet, at a given faucet opening. That faucet handle might well be adjusted for less flow than when fully opened...so the restricted faucet opening IS quite similar to electrical resistance (ohms). The rest of the usual analogy does not fit well, so I won't get into it any further.
In order to have electrical current flowing, it must begin someplace, travel 'through a circuit' and BE RETURNED TO THE SOURCE. Electricity, in this respect, is not really like water flowing from a open pipe. The circuit need not be actual wires or metal, although that is overwhelmingly more common. An example of this not needing a metal connection would be in the Ignition 'circuit' ...which includes the spark plug gap.
A battery may have an excess of electrons at one terminal, ready, willing, and able to do something; compared to the other terminal, but NO current (other than internal leakage) is flowing. If you connected the terminals to each other directly, like accidentally dropping a pliers across them, a huge current will flow. If you connected the terminals to each through a lamp, the lamp will glow and much less current is flowing than if the terminals were directly connected to each other by that dropped pliers. To restate this differently: You need to have the device to be powered, a lamp for instance, connected somehow to BOTH battery terminals, for electrons to flow THROUGH the lamp, the wires, the connections, the switches...and..back to and through the battery too. The the current MUST flow completely around and through the 'circuit'. Nothing much happens unless there is a 'complete circuit'.
That is a very important concept to grasp, it should always be in the background of your mind when one is doing electrical repairs or testing. The battery has its negative post connected to the engine/transmission, usually at the speedometer cable hollow bolt at the right rear of the transmission. Any place in the system that has battery + voltage on it, can flow current from that point to any point on the frame, engine, etc....because that is the path BACK TO the battery negative post. It you find this hard to understand, re-read it again.
When electrons flow through something that allows such a flow (usually metallic, most often a wire or engine metal), the properties of that CONDUCTOR are such that the conductor itself offers SOME 'resistance' to the flow. Examples: A thin wire would offer much more resistance to flow to your starter motor, than a much thicker wire. A thin wire would be bad here, as you WANT high current to be able to flow into the powerful starter motor. If the starter motor large nut holding the large size wire from the battery is looser than fairly tight, the starter may not function well. That slightly loose connection can be said to have excessive resistance.
SOME airheads came from the factory with painted surfaces on the timing chest inner cover where it mated to the engine....that paint resistance was enough, even with the various through-bolts, etc., to cause low or unstable charging voltage and BMW installed a 'spider-cable' to better ground the diode board to the engine case. BMW also used that same spider cable to better ground the models with the rubber diode board mounts (which are awful, and should be replaced with aftermarket solid metal ones).
Electrical resistance is generally undesirable in our boxers. However, you can't get away from it.
Corrosion is commonly seen at lamp socket bases...making for high resistance at times and intermittent lamp operation. The same thing can happen at almost any connection, plugs, etc. Poor connections at the diode board, alternator, system plugs and connections, will make for electrical problems due to the added resistance of/at the connections.
This sort of thing is THE common cause for the dim glow of the GEN lamp, usually seen at night. That dim glow is warning you, in effect, to service your connections; although it can come about from other problems.
MANY electrical problems in our Airheads are due to failure to clean and service connections at reasonable time and mileage.
Electronics devices such as 'RESISTORS' are used to purposely restrict electron flow. There are some of these in your Airhead, no matter what year and model.
In our Airheads, the resistance of the GEN lamp, which serves as an indicator of sorts for charging; has an additional, not usually mentioned purpose, of restricting the maximum possible current flow to the ROTOR during initialization of charging. It does this in combination with resistance internal to the Voltage Regulator.
The BMW airhead comes with a Bosch alternator. It must have a certain number of turns of wire on the STATOR (the non-rotating part), in order to obtain proper output VOLTAGE. To increase the output power of the alternator beyond what the stock alternator can provide, there are several methods. One; we might want to reduce the wire resistance (the unit of measurement is the OHM) of the stator windings. We'd want to keep the same physical size of stator so it would fit. To reduce the resistance we need either a lower resistance wire via using larger diameter wire (meaning less turns, can't do THAT or voltage is too low); or, have a metal that flows electrons with less resistance. That is impractical as the only conductor that is better than copper is almost pure silver wire! If the an alternator physical size increase could be accommodated, then we could have more turns, and larger size copper wire at the same time.
So far I have mentioned amperes, volts, and ohms. When current (amperes) is flowing in a resistance (ohms), due to being forced through the conductor by pressure (volts), HEAT is produced from electron movement. It can be a very small amount of heat, or a very noticeable amount. In some cases the heat is desirable or necessary, or accepted, like in an incandescent lamp. In other cases the heat is not desirable, even a bad thing. Semiconductors such as diodes and transistors do NOT like heat. More specifically they do not like excessive heat, nor do they like to be cycled, cold/hot/cold. This cycling tends to bring about failures from molecular sized faults in the manufacturing process and normal changes in dimensions due to temperature changes. In electronics equipment excessive heat may cause circuitry to fail, sometimes in intermittent ways. This happens to the ignition module under the tank if it overheats due to lack of heat conducting paste...at least in THAT instance, fresh paste usually fixes things ..without replacing the module...unless it is allowed to overheat enough times to cause a permanent failure. Of the many semiconductor items in the Airhead motorcycles, the most highly stressed are some of the diodes on the diode board. Semiconductors can be over-stressed by other means, such as voltage 'spikes', stray radiation, etc.....so it is VERBOTEN (forbidden) to remove the spark plug caps when the engine is running, or ignition turned on. It is perfectly OK to ground the spark plug caps, securely. An old-time practice of lifting spark plug caps for help in synchronizing carburetors will over-stress the ignition coils. That practice, which was perfectly acceptable on the old magneto BMW motorcycles (they had a protective spark gap), is absolutely BAD and FORBIDDEN, on models from the /5 to the end of production; and, is especially bad on the 1981+ models where damage to very pricey components other than the coils is LIKELY. If your 'manual' says otherwise, the manual is WRONG!, and doing so will lead to an expensive lesson for you; and, worse, that lesson may not show up for some time!
When current flows, heat, or work, or whatever, is done...whichever way you want to think about it. Energy is being moved about, and such 'work' is called WATTS. Even the slight (or more) heating of wires from this current flow can be thought of as being in watts. It just so happens that there are some very specific relationships between amperes, volts, ohms, and watts:
Volts multiplied by amperes equals watts.
Ohms equals volts divided by amperes.
From these, any value can be obtained from any two known values!
Also note some other relationships:
(1) 746 American watts is one American horsepower. You might see your Airhead's power output expressed in Kw (kilowatt, or one thousand watts), besides horsepower.
(2) In a few instances, TIME becomes a factor. There are places that TIME is of concern on your airhead motorcycle, at least as far as simple basic electricity is concerned. One such is, of course, the 'timing' of your ignition spark to occur at exactly the correct mount of piston travel at particular RPM. Another place for TIME is with the battery. The battery is rated to 'deliver' a NOMINAL voltage (12) at its terminals (the voltage very slowly lowers as the battery becomes discharged, so it is a nominal, not exactly fixed value). The battery, when called upon to deliver CURRENT, must deliver it at some needed value, for a period of time, before the battery is discharged, and no longer capable of such current flow. One of the specifications for a battery is the ampere-hour rating. So many amperes, constantly, for so many hours. The larger Airhead battery may be rated at 28 AH. That is supposed to mean that the battery can deliver 28 amperes for one hour; or 2.8 amperes for 10 hours, and so on. Those of you with a quick grasp on this and what I spoke about earlier may suddenly be thinking that if the battery is nominally rated at, let us say, 12 volts, and rated for 28 ampere-hours; THAT could mean 12 times 28 or 336 WATTS (for ONE hour)….or maybe 33.6 watts for 28 hours. Heck, you might also be thinking that 336 watts is nearly half a horsepower…..for the starter motor…. read on, for lots more on this.
The battery is used to power your starter motor to rotate your engine for starting purposes. The starter motor may be rated at, let us say, 0.6 horsepower; that would be its maximum rated capability. The starter, however, is not 100% efficient, and may require more than '0.6 horsepower' of electricity. So, perhaps the starter requires 1200 watts to rotate your engine. That would not be unusual on a cold day. In FACT, your starter motor has a maximum current rating too, likely that is 320 amperes. Usually that is taken to mean the maximum the starter could draw, just before the engine begins rotation. A very cold high compression engine could require very high output from the starter. 320 amperes at 12 volts is 3840 watts of energy; and if you divide that by 746 (remember, 746 watts is one horsepower of energy), that is a tad over 5.0 horsepower of energy to the starter.
But...let us assume you have a nice warm engine, nice thinned warmed oil, an easy starting engine. If you have been paying attention to the volts-amperes-watts relationships I mentioned earlier; then, you can see that you can divide 1200 watts by 12 volts, and get 100 amperes. The battery can, supposedly, run the starter for 28 ampere-hours divided by 100 amperes, giving the result in hours. That is about 17 minutes. Things would overheat, of course, if the starter was used that long. This is somewhat simplified, in that in a 'storage battery' the AH rating of the battery is decreased, as the current demand increases. Thus, a 28AH battery might be only a 15AH, for starter ONLY use. The battery capacity also is reduced with DEcreasing temperature.
If we refer back to that cold engine and 320 amperes maximum allowed cranking amperes, we could calculate that the battery might only last 5 or 6 minutes...and likely much less! AND, this is for a NEW battery, in perfect condition, at 70°F! Batteries have other ratings besides storage capacity; one of the ratings is Cold Cranking Amperes (CCA). Battery manufacturer's sometimes make outrageous claims, withOUT noting all the important specifications.
If the flow of electricity is restricted by such as a too thin wire ...maybe some broken or corroded strands; especially common slightly up the + cable of the battery; or, badly corroded connections, sulfated battery...etc....then we say that 'there is excessive resistance'. With excessive resistance, ENOUGH current flow may be UNobtainable.
Besides those faults, now and then one sees a battery in which at least one of the inter-cell connectors, inside, are cracked and hardly thick in actual conductor size due to that crack. The battery could then be said to have high resistance, and you MIGHT see the effect as everything working fine...or, maybe the lights dim some...but the starter won't work right.
*******************
Voltage is typically measured by allowing a small amount of current to be diverted from a circuit under test, and applying that to circuitry, in such a way as to have a calibrated reading. So, the metering device usually, in itself, uses a small of current. That fact can, in SOME more rare circumstances, affect the meter reading, depending on the type of meter. Digital meters generally have almost no such effect, but many digital meters can be 'confused' by rapidly varying voltages.
Resistance (Ohms) is typically measured by applying a small voltage to the part under test by the internal meter circuitry, in such a way that current flow is indicated on the meter, but the meter readout is for the effective resistance in the circuit. That is why ohmmeters contain one or two small batteries. For measuring very low resistances, if the meter is capable of such, you must short together the meter's leads, and subtract that reading from the circuit reading. Most meters have a control knob to 'zero' the reading, but some, particularly digital types, do not. NEVER EVER connect a meter on its ohmmeter function to a circuit having power on it!! Always disconnect the vehicle battery.
Some devices, such as diodes, are often tested by means of an ohmmeter. A diode must pass current in one direction, and not in the other (or, very little). If the ohmmeter does not apply enough voltage from its built-in battery and circuitry to the diode, the diode may not 'turn on' in the so-called 'forward direction'. This DOES happen on SOME usually expensive digital meters. These, on purpose, use very low currents, to avoid damaging sensitive devices that might be connected to the meter. Do NOT purchase a meter unless it tests diodes adequately, either on ohmmeter functions, or a specific diode testing function. The readings on an ohmmeter with too low an applied voltage might be so weird as to be unusable. In many instances, a CHEAP digital volt-ohm-milliampere meter (or volt-ohm-ampere meter or Multimeter) is the BETTER one to purchase.
For the most common types of diodes, the applied voltage at the diode, must be slightly more than half a volt for the diode to 'turn on' in the 'forward' direction. Some types of diodes are specifically made for some 'strange' functions. A 'Zener' diode is used in your electronics type voltage regulator, and some tachometers, to regulate the voltage...or provide a reference for voltage. There are diodes used in your CD player, called laser diodes. These emit a type of infrared narrow beam of light. Other diodes give visible light. Light Emitting Diodes are VERY common these days.
Diodes are used in our airheads at the alternator diode board. Diodes are also used, depending on year and model, in such as the voltage regulator, headlight and starter relays, clutch and neutral switches circuitry; and in the tachometer.
Diodes, in the forward, turned-on direction, can be thought of as having an inherent internal resistance. Another way to think about this is that just about every diode has a forward voltage drop that is fairly constant , thus, let us say, one always has approximately 0.5 volt across the diode when it is in conducting operation. This is a gross simplification for many purposes, just accept it here, for now. Hence, diodes can, with enough current flowing, develop a lot of heat. After all, if an alternator diode is passing 10 amperes, and the diode has a ½ volt drop, that is 10 times ½….or 5 watts of HEAT. There are SIX of these major power diodes in the diode board for the alternator. This can be a LOT of heat, on top of engine heat; and, frankly, more than that heat is possible under high output conditions. Therefore you can see that the rubber insulating diode board mounts used on some airhead models, are not a good idea; were a BMW goof, and should be replaced with aftermarket metal ones...which provide a bit better heat conduction, besides the vastly better electrical grounding too. BMW thought it had a vibration problem with some diode boards, which is why BMW changed from the cast-in metal mounts to the rubber mounts…but BMW was WRONG….the problem was improperly made Wherle brand boards. The rubber mounts needed a spider-grounding-harness too….compounding the error (although it helped the painted case grounding problem).
Although your motorcycle may have a lamp marked GEN, it is really an ALTernator lamp indicator. GENerator is sort of a common term for any PRODUCING source of energy, typically this means any mechanical source (but not a storage battery). In days gone by, cars had real 'generators', which are quite different from alternators, and so there can be some confusion in this area.
When the world was first being electrified by Edison for street lamps and home lamps, current flowed in one direction, and was called DC, which stands for Direct Current. This was very limiting, as when you had enough homes and factories using electricity, the wires must get larger and larger to keep the resistance down, and soon are very unwieldy. It was almost impossible to move lots of electricity long distances if it was DC. That is where Edison personally failed, from stubbornness. He gave all sorts of false arguments to bolster his DC position; as, opposed to A.C., which, of course, won out, eventually. Enter Mr. Westinghouse! You may find the history of DC and AC to be interesting....do a Google search.
Over a hundred years ago, some farmers used windmills mechanically coupled to DC generators, which then charged batteries. Some of these windmills also pumped water. Thus, many a farmers house WAS run on DC. Today, few such homes exist. There are motorhomes, of course, with DC (and often AC too) lamps and other items. Some few modern cars come equipped with converters and produce AC for our nominal 120 volt things. Aftermarket 'inverters' that convert 12 volts DC to 120 volts AC are cheap and commonplace these days.
The electricity in YOUR home is NOT DC; rather, it is AC (Alternating Current). What this means is, that for any portion of TIME, the power at the wall socket is such that its VOLTAGE is constantly varying, going up and going down. It follows a mathematical curve called a Sine-Wave. To simplify this, I will simply state that the voltage rises following a type of curved bell-shape towards a positive value, follow the same bell shape back towards and then through zero to a negative value via a similar bell-shape, and back towards zero again. The 'cycle' then repeats, SIXTY times per SECOND. When the voltage goes from zero to maximum positive, down to zero and down further to maximum negative, and back to zero, that is called 'ONE CYCLE'. You might want to think of this as an S shape, lying on its side, with a zero voltage line horizontally down the middle. Cycles per second gave way many years ago to the term HERTZ, to honor Mr. Hertz. This value, 60 Hz in the United States for homes and most factories, is kept VERY accurately by your power company...so accurately that your old-fashioned mechanical-electric clocks run correctly, and TV pictures stay steady. Some countries use 50 cycles per second (50Hz). Some Aircraft used to use 400 Hz....which has advantages for minimalizing the size of components.
Alternating current (AC) has one HUGE advantage over Direct Current, it can be 'transformed'. In the common electric power industry, and in the street someplace outside or otherwise near your home, a device called a transformer is normally used. A transformer is nothing more than a specially designed magnetic steel laminations structure, usually with two separated coils of wire, one having many more turns than the other coil. In the transformer near your home, the high voltage input from the very high voltage power lines (usually the TOP most on power poles) is to the many turn coil; the output to your home is from the other, lesser turn coil. If the input is to the larger number of turns, and the output (to your house) from the smaller number of turns, whatever the input voltage was, is reduced in the smaller number of turns coil by a specific known ratio. The relationship is a direct one, that is, it is directly proportional to the turns ratio of the coils. Depending on how any transformer is designed and used, one can change an input voltage to a lower or higher output voltage of any amount. Thus, you might have a 12,000 volt input, and a 120 volt output.
You have learned that it is the CURRENT, in amperes, that causes heating in wires (in the wire's resistance). To reduce heating losses, which ends up as a voltage loss and/or power loss, it is desirable to have either very thick copper conductors; or, much higher voltages. That is why power plants are designed, and HUGE transformers used, to deliver super-high voltages (up to 3/4 of a million volts!!) to distribution centers and eventually to near our homes and factories, where more transformers reduce these voltages. In other words, for the vast distances of sending electricity from power plants to your home, the voltage needs to be, for economical reasons, to be VERY high voltage indeed. Since electricity at higher and higher voltages will have more propensity to 'jump' and make sparks, etc., those very high voltage transformers and power lines have very large and long insulation parts. That super high voltage is transformed initially downward in one or two steps, in 'sub-stations'. Then the modestly high voltage is sent to your neighborhood, to be transformed downward again. Somewhere near your home there may be that mentioned transformer on a power pole; or, buried. Much smaller transformers are also VERY common in your home, in all sorts of devices.
Since you have learned in this article that POWER (watts) is voltage times amperes, this means that if we can transform the POWER of an electrical generating power plant to a super high voltage, we can get the same POWER at a much lower CURRENT (amperes). Thus we would need only a much thinner wire than otherwise, to carry energy for many miles, where at the other end, we could transform the voltage downward...say to 220 or 110. Otherwise, the wiring to our neighborhoods might, in some cases, be as thick as many feet! As you can imagine, it is this area where Mr. Edison goofed, horribly.
Transformation is actually done in your Airhead alternator magnetically, from the induced field from the rotor, and some other effects in the stator...but this is far too complicated to explain in this article. There is another place in your Airhead motorcycle where transformation is done. This type of transformation is done in the ignition coil, which, by trickery, has a DC voltage applied that is made to act like a form of AC, and is applied to a moderately low number of PRIMARY winding turns....and the SECONDARY winding has many thousands of turns, and steps-up the primary voltage.
It is a property of transformers that turns are directly proportional. Thus, a one turn primary and a 1000 turn secondary is a multiplication of 1:1000 in voltage step up (and a corresponding step-down, or drop, in CURRENT... which is ONE reason of many why ignition parts need to be in good condition)...ETC. No need here to go deeper into this...other than to state that transformers are sometimes used to step current, not voltage, up and down.
In your Airhead, the primary source of electricity is the battery, which, if in good condition, has an INTERNAL RESISTANCE which is very low, a small fraction of an ohm...which is why dangerous currents (like melting things type of levels) can flow with short circuits. The battery must be constantly recharged. The reason it must be recharged is that it 'holds' a finite amount of watts (it actually does not hold it, as such, except as a potential to deliver, from an electro-chemical reaction); and, some of that power is lost through INTERNAL leakage, over time…especially in hot weather, although some batteries are MUCH better at it. Recharging on the motorcycle is done by the alternator in conjunction with the diode board and the voltage regulator. The battery's inherent self discharge during the time the motorcycle is not driven can be substantial ...often as much as 1/3 of its charge is lost in a very hot Summer month with common flooded batteries, especially when helped along by having an electric clock. The battery supplies electricity via a chemical reaction to the headlight and other lights and the ignition, when the alternator is not putting out enough...such as at low rpm.
A true generator produces DC, and it would be less efficient and likely much larger, and its brushes would not last very long, and they tend to spark a lot and create other problems, so BMW elected to use a type of generator called an 'alternator' in our Airheads. "Alternator" means that its output is alternating current...that sine wave thing again. The frequency (number of CYCLES per second) varies with engine speed. When the frequency is high enough, and your radio power connections are not electrically filtered well, some of the alternator electrical noise, helped along by a funny characteristic of diodes, and even maybe some air radiated energy, may show up on your radio as a whine. Poor or bad diodes in the alternator's diode board might cause your electronic tachometer to act up, from electrical noise traveling in the bike's wiring. A bad diode can also make the radio whining noise very much louder. That radio whining noise tends to be more intense at the low end of the AM broadcast band frequency dial; and, on some tape players, due to their sensitivity of their magnetic heads.
In order to charge the battery, the AC from the alternator must be RECTIFIED. This means it must be converted to DC (Direct Current). This is done by the large diodes in the diode board. The uppermost diode section of three diodes allows only the negative half of the AC to go to the battery negative (-) post (via the engine metal case). The lower diode section of three diodes allows only the positive half of the AC to go to the battery positive (+) post, by means of wiring. The stock Airhead alternator does not produce just one sine wave output...but, for efficiency...is designed to have three...'THREE PHASES'. The alternator, in effect, has three separate coils of wire, that is, 6 wires. Each coil has two wire ends, and there is ONE phase per coil. The coils are mechanically and electrically arranged so that the coils produce the electricity at NOT exactly the full amount at exactly the same TIME. One can describe a single cycle of waveform as 360 degrees. If one does this, then the alternator, WITH REFERENCE TO any ONE of its output coils, is producing output every 120 degrees from the other coils;...during that 360 degree period. This is why there are six diodes, two for each 'phase' winding. If you picture in your mind that S curve lying on its side that I mentioned much earlier, and draw a line horizontally down the center; and put two more S curves on top of that one, but separated horizontally somewhat…you will see what I am talking about.
These three waveforms are rectified by the three positive and the three negative diodes. /6 and later have an additional set of diodes, smaller ones, connected to the center junction connection of the three stator coils, we won't discuss them here, too complicated. Their purpose is to capture a bit more electricity.
The diode board also has some other small diodes which do exactly the same thing as the six large ones, but are only used to provide a SMALL amount of current, for 'sensing' the voltage of the alternator, and comparing that sensed output to the battery voltage. The voltage regulator works with the sensed voltage. Yes, that is true, the regulator does not exactly and directly sense the battery voltage. Nor does it do this at the big diode outputs. This needed complication can, on RARE occasions, be the cause for some head scratching to determine what is wrong.
If a large diode in the diode board 'FULLY short-circuits', it allows the entire PORTION of the AC waveform applied to it to pass to the battery, causing an improper current flow, and a substantial decrease in total available alternator output. If just one of the six large diodes OPENS OR SHORTS, then you will more than just 1/6th of the alternator output, due to complex interaction of the waveforms, diodes, and magnetic fields. This type of failure is hard to diagnose with the diode board still in the motorcycle, until disconnected or removed. Symptoms might be a battery that MIGHT seem to fully charge up with correct voltage.......yet, when enough load is put on the bike's system...such as the headlight being turned on, or heated clothing...etc....the voltage will not come up far enough, even though the rpm is plenty high enough and the rated alternator output hardly being exceeded. Since other faults can mimic this one, it takes some sleuthing.
An open large diode is a rare event, but does happen. The worst case is a diode that OPENS ONLY at elevated temperature, hard to find that one without serious tests, the diode board usually needs to be removed from the bike. Diodes boards can have SOME testing done in the bike, but are best checked OUT of the motorcycle...and with both an ohmmeter for forward and reverse resistance, and by an alternating voltage source with a lamp. See Oak's June 1999 article on Diode Boards, in AIRMAIL. One rather common open diode defect is on early Wehrle diode boards, where the lack of a bent-over lead properly soldered causes the diode to, in effect, disconnect. Usually this causes discoloration at the solder joint on the diode board, rather easily seen most of the time, through the paint coating.
Specific diode problems in Airheads, OTHER THAN in the diode board:
If the diode in the headlamp relay shorts, the motorcycle will not turn off with the key switch, only by disconnecting a battery wire or stalling the engine. The process repeats after the next start.
If your neutral light has weird things going on, such as being OUT in neutral, ON in gear with the clutch lever is pulled in; and, maybe ON at every clutch usage..... you have a bad diode that is located on the UNDERside of the board inside the headlight shell. This applies to all twin shock models, EXCEPT the R45 and R65 where the diode is plugged into the wiring below the starter relay on pre-1981 R45/65, and IN the starter relay in post 1981 R45/65 ....AND in all MONOLEVER models.
These are hardly all the various diode problems found in our Airheads. More are listed in the Author's website.
There are many sources of electrical problems in the Airhead, from such as connection problems in the instrument pods (the printed flex material is famous for almost invisible cracks in the copper), corrosion in various connectors including lamp sockets, loose electrical connections such as at the starter motor, etc. A series of very detailed articles on every aspect of the Airhead electrical systems will be found on the author's website:
http://bmwmotorcycletech.info
see articles #14 through #38
Also see other articles on this Airheads.org website.
rev. of 12-08-2007
Snowbum
ABC 1843
