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Ampmeters (ammeters) versus Voltmeters.
Explanations & Problems.
Relative merits of ammeter vs. voltmeter. Use both?
Voltmeter reading problems with the stock... or aftermarket... voltmeters?
Why not just the stock GEN light?
I see little reason in having having an ammeter permanently installed on any BMW motorcycle, unless you have very special circumstances. I foresee many drawbacks, not the least of which is a potential fire hazard, which includes burning up your bike's wiring; this is besides the usual situation of less reliability with many modifications; especially with added electrical items. However, in this article I will lay out the benefits, and the problems, for both ammeters and voltmeters. The GEN lamp provides quite a bit of information, if you interpret it correctly ....but it may not be enough for some of you. Those who are measuring over-all system current drain; or, who are trying to trace down a parasitic current drain, perhaps one that is depleting the battery charge, can certainly temporarily install an ammeter (or, milliampmeter).
First ....I will address the GEN lamp and some things about old style metering:
The GEN lamp has several functions. It provides an illuminated method to, in most situations, to instantly see if the alternator is producing electricity & possibly provide information regarding the alternator being intermittent with RPM or brush wear, or diode board problems. It might indicate, by a dim indication, problems with connections, ETC. It does not indicate the level of output, or even if truly charging the battery.
Long ago, cars and motorcycles had ammeters, typically they had an engine-off centered-needle indication. These ammeters would have the needles indicating to the left for discharge, and to the right for charging.
Alternators are more efficient at producing electricity (particularly at/near idle rpm), so alternators replaced generators long ago, but the abbreviated word GEN might have been kept, as BMW did. The GEN lamp indicates an electrical generating function. Alternator-equipped cars, long ago, were mostly equipped with GEN or ALT lamps, and not ammeters nor voltmeters. Some cars still have GEN or ALT function lamps, & do not have voltmeters, although many modern cars have voltmeters or even low charging or other fault indicators. Lamp ON, engine is running ....there is a gross failure. Lamp intermittently ON .....there is a gross failure coming soon. Simplified descriptions, but fairly accurate.
You might want a meter, or meters, that indicate more than minimum information ....as opposed to a lamp. The vast majority of old BMW motorcycles had only the GEN lamp indication, with very modern displays types having such as a voltmeter function ....and ALL seem to put on huge numbers of reliable miles. There is NO question that a voltmeter gives much more information, often information that is well in advance of electrical system problems.
BMW elected to incorporate voltmeters in certain models while still keeping the GEN lamp. R90S, R100S, R100RS, R100RT, optional in R80RT (depending on country and if a clock or not), etc.
The GEN lamp; or, think of it as a ALT lamp, provides other functions than just indications, one such in your Airhead motorcycle is to provide a controlled low current to the alternator rotor, to magnetize it, for initiating alternator output.
I have installed ammeters on large powerful alternator conversions I have done for others. I instrument-up Airheads with ammeters to do charging system maintenance or engineering work. I did have a permanent amp-meter installed on one of my pre-Airhead bikes, a R60/2, but it had an enormous outboard alternator installed, as there was a Ham Radio rig and two huge Lucas Flamethrower lights with very powerful bulbs. The ammeter was not really needed; helpful only during the engineering work, but I left it installed. In analyzing my customer's bikes electrical problems, I might temporarily install an ammeter or milliameter. If you are trying to trace down a small battery drain, you can do this, in the battery negative lead, simply by temporarily connecting your own multimeter, shorted during starting.
In order to connect or install most amp-meters (also called ammeters), you must cut into a major wire or connection, and insert the amp meter IN SERIES connection. This lengthens the wiring which can add a voltage drop (small if with large wires and good connections), and certainly can create potential problems with shorts, grounds, fires!, etc, particularly if done in the + side of the system, as opposed to the - side, better known as the grounding side.
It is possible to install an ammeter in such a way that there are no problems, or any potential ones are minor, such as putting the ammeter in the grounding lead of the system. I will discuss some of these things, below.
It is possible to make a crude amp-meter by connecting a sensitive voltmeter, that is, it will measure millivolts (thousandths)... by connecting it across the battery ground cable, using the resistance of the large ground cable & fittings/connections, for the voltage drop to drive the meter. Ideally, that millivoltmeter is connected slightly away from the cable end fittings, so that readings are not affected by 'end effects' of those fittings. The meter needs to be calibrated, but that is simple if you have the equipment to do so. Every installation will require specific calibration for that particular connection. This type of ammeter setup is called a SHUNT system.
There are commercially-available "shunts", basically a piece of calibrated metal, calibration means that it has a known value of resistance (which is extremely low) between specific terminal connections on it, of which there are four. Commercial shunts are always rated for industry-wide accepted values of either 50mv or 100mv for a specified current, it is easy to match them with a meter. You connect a millivoltmeter across the proper two terminals. This method can be very accurate, & laboratory grade meter readings are standard. A shunt type system can be installed to be relatively safe, because you can put the shunt into the grounding wire or lead connection. For an Airhead, you'd put it in series with the large black lead that goes from battery to the speedometer cable hollow bolt grounding place. In order for this type of connection to work properly any bike negative battery terminal connections (besides that shunt and cable to the negative battery terminal) would be moved to a convenient frame ground. This is the method I used on most of my super-high alternator installations; and, for nearly all my alternator output connections setups. It gives quite accurate measurements.
A bit of history:
There is a type of in-dash-located meter that is not seen in modern vehicles. Modern vehicles all have some method of telling you about your charging system, such as a lamp, or other dash indication. The very old type of meter works on the principle of the magnetic field that surrounds any wire that has current flowing in it. Modern versions can be seen as clamp-around measuring instrument with openable jaws, if used in an automotive repair shop, or by someone working on home or industrial power lines. The are available cheaply for use on A.C., and at additional cost for D.C.
But, there was a type of D.C. ammeter used on old cars and old motorcycles. Many decades ago, most motorized farm equipment and most all cars, trucks, and motorcycles with lighting equipment and batteries and generators, and many with later equipped with alternators, did have ammeters. The ammeter nearly always had a zero-center, and indicated discharging to the left of that zero center, charging to the right of that zero. Many of these ammeters were not installed by direct wiring ...which could add a fair amount of expense and could be a major safety problem, not to mention introducing the generator or alternator output into the driver's compartment, bringing with it generating noise, ignition noises, etc, all of which interfered with radios, etc. These ammeters generally had a ONE TURN LOOP OF METAL on the back of the ammeter, that, in the vehicle, the main power wire passed through, without any direct connection at all. The magnetic field made the meter work. The meters were not very accurate, but adequate. Typically, for cars, they had scales of something like 30-0-30 amperes or maybe 50-0-50 amperes. Scales of down to 10-0-10, and as high as 80-0-80 or more were common. Many old English motorcycles used these types too....Triumphs, etc. Scales for the bike types might be 10-0-10 amperes. They typically had indicator needles that tend to vibrate and move about as you ride ....although some meters had built-in dampening. These meters were cheaply made, but adequate to show charge or discharge. Some of these types of meters were put in the generator or alternator OUTPUT ONLY, therefore usually did not have a zero-center, and indicated ONLY that the generator or alternator had an output.
A person well-versed in electrical charging systems will get more information from an ammeter than someone else; but I do NOT think an ammeter tells you everything; nor does it tell you enough to warrant installing one permanently, except in special circumstances. This section gets more deeply into what ammeters (ampmeters) can tell you.
An ammeter, placed in the usual and customary position in a vehicle's electrical system, monitors all the current flow to and from the battery except starter motor drain, which is much too high to include. It is possible for a circuit to be made up that monitors even the starter motor drain when it is in use; but it is complex and I see no advantage unless testing the starter motor for current drain, a rare event for anyone to need such a test. You could borrow a DC current clamp-over meter (which are not very commonly owned, as opposed to the A.C. type). Use of the starter motor as a load testing device is discussed here: https://bmwmotorcycletech.info/startingprobs.htm
There is nothing inherently wrong with the use of an ammeter, it was the method used for decades on very old cars and trucks and old British motorcycles. Ammeters of the simple car types were cheap to incorporate, although the long wires were, of necessity, quite thick gauge, and copper has been relatively expensive. Generally voltmeters cost more, but manufacturer's prefer the safety, and additional information, and no radio interference, of using a voltmeter, whether an individual meter; or, a computer-run dash display. Use of ammeters generally, but not always, requires heavy gauge cables, and these must pass by the engine, driver/passenger areas, etc, that might thereby introduce electrical noises in unwarranted places. There is also the increased risk of electrical fires. The engineering will be more difficult due to the large current flows at some times....meaning fusing could be problematical.
A number of car and motorcycle manufacturer's, some time ago (and, your Airhead RS and RT, and other models) have gone to actual voltmeters of the expanded scale type, even though they cost more than using a light to tell you that your alternator is no longer working properly (the lamp is also included though). The truth is that voltmeters give more truly usable information & eliminate the need for costly large diameter cables with various safety problems into the driver-passenger compartment. Such large cables carry large currents and electrical noises, that might get into radios, music players, and be dangerous, and a fire hazard, etc. Thus, over-all, the cost of the voltmeter may be lower, and gives better results in several ways. Voltmeters really do give more over-all information, if one knows how to use their information. They do not tell you directly if the system is draining or charging, although that can certainly be easily seen from the voltage reading. Voltmeters can be very useful ...particularly digital types with good accuracy. Those are not stock but can be installed, often in the same hole as an expanded scale round meter ...as used on many BMW Airhead motorcycles.
Ammeters are available in several popular types. One of the best uses a millivoltmeter for readout in conjunction with a FOUR-connection calibrated shunt. The millivoltmeter, whose scale calibrated in current, not voltage, is wired to the shunt. I've discussed that type already. A good one of these is quite accurate. A simple, yet adequate type for most uses, although not generally capable of tiny current measurements, is the old fashioned types used in quite old cars and trucks. Those either have studs and nuts posts and have some sort of real coil and moving vane mechanism inside; ...or are really cheap, and are simply a clip or loop that the wire passes through, with no metallic connection, the 'meter' operating on the magnetic field in the wire caused by current flow. These are accurate enough for many uses. These types of meters are inexpensive and still available. An old car junkyard might have one for free or a buck or two. They are excellent for testing purposes, the clip types in particular are reasonably rugged. One can place one of these in your bike's alternator circuit (diode board output) if you wish to do it that way, to measure the alternator output; or, you could place it in the circuit after the ignition switch; or, elsewhere's. The ideal place, as it will tell you if the system is charging or discharging the battery ....is in series with the NON-starter wire in the battery circuit, so that means in the red smaller gauge wire (not the starter motor heavy gauge wire) at the battery + post.
On an Airhead with ammeter installation, if you saw the ammeter indicating discharging when it should show charging, then you would suspect that the electrical load was too high; or, there was a problem in the alternator system. A large discharge ....and possibly a lighted GEN lamp, would probably indicate a total charging system failure.
The GEN lamp used on all Airheads actually tells you quite a bit about the charging system. When that lamp is lit at idle, and then goes off at some rpm around 1500, you can probably rightly assume that the system is working, maybe not perfectly, you don't know at this point. For the majority of Airhead owners, this is enough information.
A voltmeter will give some indication of the state of charge of the battery & system performance. An Ammeter will give a bit more information, mostly unnecessary. Some few folks may "want" all three: GEN lamp, voltmeter, ammeter. For the most part, 'these folks' will be hard-pressed to give knowledgeable information about WHY, especially if asked about potential problems.
If you have a voltmeter, especially a highly accurate one (although the stock expanded scale meter is accurate enough)....it will tell you if the battery voltage is high enough for you to assume it is charged; or not. A voltmeter indicates, approximately, the battery voltage, so if the voltmeter reading is much too low, the battery has been discharging, and if much too high, the battery is being overcharged. At least, that is the theory.
You can use an ammeter in a way that shows only alternator output, should you want to, by wiring it into the alternator rectifiers output lead. On an Airhead that is the larger spade on the diode board on the RIGHT, as you face the front of the motorcycle. The ammeter will only have an indication if the alternator is supplying current.
The ammeter can also be placed in the main electrical connection (not including starter motor). This would be in the modest gauge wire from the + battery terminal, or other point where the starter wire and system come together. That could be at the ignition switch, etc. For such a connection, a normal ammeter indication would be a discharge at engine off but ignition on ....or at idle rpm. That discharge would be from current draws from the headlight, taillight, running and indicating lights, clothing, radio, & the ignition. As much as 15 amperes for the stock ignition & lamps, especially if the brake lamp & a turn signal was on; more with a heated vest or a farkle. As rpm rises that discharge indication would decrease and become a charging indication, & with rpm high enough, perhaps 3000+ rpm, that charge value would be initially high, perhaps 10 amperes if you have the alternator capacity. Later, as the battery was replenished & rpm continued reasonably high enough, the charging value would slowly decrease to a smaller charge amount. The battery condition & charging voltage determines what the charge-maintaining rate, also called the floating rate, will be once the battery is fully charged. It can be an ampere or two, to 4 amperes or so, assuming reasonable battery condition. Note that the ammeter needs a zero center, since there will be discharge and charging currents to the battery. Care must be taken as to where the ammeter is connected, to avoid wrong readings. For some, the only important thing is charging current, and if the needle was to go backwards (non-center type meter), there is no harm.
****If the battery discharge rate was 10 amperes at a stop light ...and you sat there for one minute, ...you should expect longer than one minute at any cruising rpm from maybe 3-6000 rpm, to replenish the power taken out of the battery. This is due to inefficiency of charging as the battery is not all that efficient as a chemical converter of electricity. Your only indication of the replenishment occurring is the slow decrease of the ammeter charging reading ....and/or a voltmeter slowly rising. The headlight on the road at night will usually be brighter as soon as rpm increases from idle, giving some indication of charging (or, at least less discharging).
Voltmeters indicate the system voltage at the point where connected. This is not necessarily the true battery terminals voltage unless connected right to the battery terminals themselves; which is not a good idea as the meter would be constantly, if very slowly, draining the battery with its own tiny current flow, key off. In the stock BMW Airhead motorcycle, with a factory dash voltmeter, the meter will indicate ~0.3 volts less than battery terminal voltage, due to circuit voltage drops, which include drops in the wiring, connections, & ignition switch. If the wires connecting to the battery (or system) are corroded or not otherwise in good condition, the voltage drop will show up as even more.
Due to a battery being a chemical change storage device (and in some types of batteries this is complex), batteries are 'charged' during riding to a voltage a fair amount higher than absolutely needed to maintain a full charge. This is a required function. There are limits to this. If the voltmeter reads over about 14.5 (over 14.9 on some types of batteries), the battery is possibly being overcharged & possibly being damaged. If the voltage is only 13.5, that is marginal on the low side for re-charging speed. VR output is, or should be, designed for slight voltage increase as temperature DEcreases. Batteries are made in various chemistries and voltage needs vary some. In general, lead-acid batteries are best recharged to a floating voltage of about 14.2; and this applies to all lead-acid battery types.
If the voltmeter falls BELOW approximately 10.5 perhaps during cranking; or, below perhaps 11.5 sitting at idle with the headlight on, then something (connections or switch contacts?) in the system is bad and/or perhaps the battery is getting ready to fail or is just very heavily discharged. It is said that a lead-acid battery has almost no charge left at those voltages, assuming no load to a quite light. These things can confuse you, and I am not going to get into it here, except with a statement...and one example:
(1) A battery, resting for some hours with no charging or discharging, must show at least 12.3 volts as a MINIMUM. Anything lower means the battery is discharged some. The voltage really should be 12.5 or a bit more.
(2) If the battery is old and sulfated, or perhaps an inter-cell-connection has cracked and is barely making connection, the battery unloaded or lightly loaded voltage may be good, but a heavier load will decrease the voltage considerably. The internal battery effect is just as if one had a resistor in series with the battery. That could also be described as the same as if a quite thin wire instead of the normal very large wires, were the wires connecting to the battery. In an egregious case, the motorcycle lights may work fine, but the starter may not work, or hardly work. The voltage sags greatly with the large starter load on the battery. An ammeter does not tell you hardly anything in this instance.
A truly large variation when the blinkers (turn signal lamps) are operating might indicate a bad battery or poor connections in the system ...or a car voltmeter, not a bike voltmeter. BMW-installed motorcycle voltmeters are damped ...that is ...smoothed/averaged ...in reading from sudden modest and expected changes in voltage value from the blinkers loads. The damping has been known to fail. Damping also helps reduce needle movement from road effects. If you are replacing a BMW motorcycle Airhead voltmeter of the round expanded scale type, be sure to get the motorcycle version. If you do not care about having the original part, I HIGHLY suggest you use a DIGITAL voltmeter. A digital meter can be extra-useful, can even be used to adjust the VR, by having one lead connectable to the battery itself, when you want it that way. I have known some (few) folks who put a digital meter in the dash, & use a connector to it in such a way that they can disconnect the voltmeter from the bike & attach a test cable! If you purchase a digital voltmeter of any type for your bike, get a two wire digital meter of the type that does not need a lamp. A voltmeter that reads to TENTHS of a volt is appropriate...do not get one reading in hundredths.
By watching the voltage during various actions: idle, lights on, cranking, recharging, etc. ....one can get a pretty good idea of what is going on in the system. Used with the GEN lamp indication, even more information is available.
One particular advantage of the voltmeter method is that no large diameter cables are needed ....this means no potentially failing connections carrying high currents are needed. High currents can cause fires or other serious damage. The likelihood of radio noise is usually non-existent. With the voltmeter method, as opposed to the ammeter method, the voltmeter is 'turned off' by the ignition switch; while the ammeter method typically will remain electrically hot all the time.
The voltmeter tells you what is really going on much better than just an ammeter. An ammeter has no way of telling you the actual state of charge in the battery. In all honesty, the voltmeter can not either, at least not in all instances. With one or more failing single cells in a battery, of the shorting-type failure (not all that uncommon), an ammeter might well show a wonderfully nice charging whilst under way, yet the battery is about to fail. In this case, a voltmeter would indicate a decreased voltage, indicating a problem. It is common to see a battery with a failing cell or two, not wanting to charge up to as high a voltage as it should. Problems can come from even a single bad diode in the diode board, a failing regulator, bad brushes, etc. Another type of error can come about because batteries can fail in such a way that they seem to charge up to a quite decent voltage, yet can not start the bike.
If the battery had high resistance in one or more cells (another common failure mode, & sometimes this is a sudden total & catastrophic battery failure, is from a cracked inter-cell connection), the bike would not start well if at all, yet the headlight may work fine...but the voltmeter would go WAY down during starting. In this type of battery failure the voltmeter might indicate just fine at decent rpm, but the voltage will usually sag quickly when the alternator is not charging, even from the loads of the lights. In most of these types of battery failure, the lights are OK, but they dim, greatly or completely, when the starter is used, or you try to. On MOST of our bikes after the /5 series, the headlight is automatically turned off during cranking; the headlight is a larger load than a taillight, so this is perhaps not seen. Note also, that a battery can be fully charged & quite good in a load test, but it will NOT start your bike....because the cables to the battery, especially the + cable, is corroded away. The VOLTAGE can look fine at the battery terminals...but NOT at the starter motor DURING cranking.
The major drawbacks of an ammeter are usually involved with the method of attachment, particularly if you want a real amperes readout. Some folks use the existing battery negative cable as a shunt...as a resistor if you will... and read the millivolts drop across it. This was noted earlier in this article. Nothing is needed but relatively thin wires to the millivoltmeter. It is almost always very SAFE to do it this way too, being in the ground side of things..... after all, all the parts are essentially at ground, which means engine-case voltage, which is essentially zero. This method tells you if the battery is being discharged, or charged, and if the millivoltmeter is calibrated in actual equivalent amperes in that cable, by how much in amperes. Unfortunately, the method is not all that reliable for accuracy, and you need a meter to calibrate your setup.
If you are interested in a very cheap way of installing or connection an ammeter, that can be somewhat useful and expected to remain so over time, and needs no connections, try to find an ammeter, the zero center car type, that is has no terminals & the only connection is the wire passing through a rear circular clip or similar. There are also versions that simply snap over a wire. This type of ammeter was used in some old cars and trucks, and is still being manufactured. It works by the magnetic field surrounding a wire ...any wire ....that has DC current flowing through it. You can multiply the sensitivity of this very simple metal loop meter by looping the wire more times through/by the meter if it has the loop room or a place to do so. More loops give more sensitivity. Thus a 60-0-60 meter can be made relatively useful, and you could make it more sensitive. A rough idea of the ammeter sensitivity to try for when it is all connected, would be ...perhaps ... 20-0-20 amperes.
The installation of a commercial shunt & associated meter is rather costly, most will never want to do this, including me, except when I need laboratory quality measurements. You could make a sensitive version of such a commercial shunt, of course.
The terminal connection type of ammeter can be connected in your Airhead in several places:
1). In series connection with the smaller red wire at the battery + terminal (if there is such) ...or from the starter relay pass-through connection from the battery ...or from the ignition switch.
2) In series connection with the red wire from the alternator diode board ...that means the LARGE spade lug on the right side of the diode board, as you face the board from the front of the motorcycle.
Be very careful with wiring, mountings, & connections. If one installs wired-in ammeters, that CAN BE a fire danger. YOU MUST do things with thought, neatness, and good techniques. Additionally, if you effectively add resistance, output can be lowered, voltage regulation made poorer, etc. In other words, you need to know what you are doing, electrically and mechanically.
BY FAR the most convenient method, for temporary testing, is the clamp-on DC current probe.
These are generally much pricier than the AC current clamp-on probe meters. The DC clamp-on
type can be digital, or analog. These are usually instruments for the serious technician. They are not for permanent installation. Over-all, considering everything I've mentioned, I believe that a voltmeter (with stock lamp) is by far best indicator & that BMW got it right when it began offering them for installation in separate pods, and then installing them on some models, such as the R90S, R100RT & R100RS. VOLTMETER PROBLEMS: When a battery is getting old or getting closer to failure (even not very old batteries can start dying), if your stock BMW dash voltmeter is wildly swinging during use of the directional's flasher, and you have already checked the wiring and connections at various places, the battery may need replacing. Load Test the battery. IF OK, then again check for poor connections in the electrical system. This can even be at the terminals or inside jumper in the starter relay, or inside the ignition switch. For an aftermarket voltmeter, the round 2-1/16th inch case (through-panel measurement) size fits OK. I prefer a digital. Summit Racing sells one similar to the digital one I designed and offered some years ago ...and it fits in the Airhead dash in place of the original. It is self-powered from the 12 volt power source, the original lamp is not needed. Digits are available in at least two sizes, and two colors (green, red).
Some have fancy pushbutton functions to capture maximum or other readings. You do NOT need those functions, and the buttons can let moisture into the interior. Datron makes a truly nice and neat small rectangular one, that is potted, and never ever fails from vibration or anything else, very good quality, but it won't fit in the dash hole, unless you make a mounting plate, which is not difficult to do. It can be mounted elsewhere's, if you do not have an RS or RT, or? An excellent substitute for the BMW fairing voltmeter is the VDO 332103, available from Summit Racing, and many other places. Standard 2-1/16th inch size; black face, red pointer, white numerals, 8-16 volts, accurate, backlit, reliable, and much cheaper than from BMW. Digital replacements of my own design are no longer available from me because I could not get the original manufacturer to make more, instead, they ripped off my design, made them, and sold them in racing auto-parts stores. Note again that the stock reading is typically 0.3 or so volt less than the actual voltage at the battery. The reason is where the voltmeter is connected, downstream from the battery, and the battery has a more direct charging route from the alternator...in fact, that stock charging route bypasses the ignition switch! ...and is not fused. Voltmeters previously OK & now swinging, may be the meter (try tapping) ...or a circuit problem. One way to analyze the situation; especially if the use of the turn lights (blinkers) is causing a much larger voltmeter swing than it used to, is to use a non-digital fairly fast reacting cheapo voltmeter, and measure the voltage, during flashers blinking, at the battery posts themselves,
and then at various other places ...such as the engine side of the ignition switch, ending up at the voltmeters own input wire. The problem is often either the battery, or the connections from the battery and at perhaps the starter relay internals or its socket (if it has one). A wildly swinging voltmeter can be a failed voltmeter; but, it can also be an indication of oncoming more serious problems, most problems like that are fixed simply by cleaning connections. I have seen it from a worn ignition switch, however. Typically, once this sort of thing starts up (voltmeter OK, connections poor), it is only a matter of time before you will have problems with charging, lights, etc. Find out if the problem is the meter...or a connection! Revisions: © Copyright 2021, R. Fleischer
The stock, as installed by BMW, expanded-scale meter, are reasonably reliable and usually accurate (as a standalone meter). However, as installed, they read about 0.3 volt less than battery voltage, due to voltage drops in wiring and switches. They tend to get old-age needle bearing problems, that is, they get sticky .... try tapping on the glass with a fingernail to prove this, as they age. The stock Airhead voltmeters are similar to the ones in a BMW car (and others) except that a damped mechanism is used. That dampening averages and smooths out the needles swings ...because motorcycles vibrate and tilt ...and the dash vibrates a lot ...and the blinker (signal flashers) connections would otherwise cause a lot of swinging. Do not use a car voltmeter, even if it outwardly looks the same. If you have your unit repaired, or purchase another, be sure to specify the damped motorcycle meter. Most would not bother to have one repaired, they would buy another, or a digital meter.
05/11/2003: Add .htm title.
05/14/2003: Clarifications & hints.
01/16/2009: Clarity and accuracy.
05/12/2011: Updated for easier reading and additional clarity.
07/22/2012: Clean up a bit more.
09/19/2012: Final edit; add QR code; change google code
04/27/2014: minor editing, again 08/29/2015
01/03/2016: Increase font; update meta-codes; clean up article; narrowing.
05/08/2016: Update of meta-codes, scripts, layout, colors, fonts. Many explanation updates & expansion of information.
12/05/2017: Layout. Reduce excessive html. Reduce colors and fonts changes. Some clarifications.
Over-all, considering everything I've mentioned, I believe that a voltmeter (with stock lamp) is by far best indicator & that BMW got it right when it began offering them for installation in separate pods, and then installing them on some models, such as the R90S, R100RT & R100RS.
When a battery is getting old or getting closer to failure (even not very old batteries can start dying), if your stock BMW dash voltmeter is wildly swinging during use of the directional's flasher, and you have already checked the wiring and connections at various places, the battery may need replacing. Load Test the battery. IF OK, then again check for poor connections in the electrical system. This can even be at the terminals or inside jumper in the starter relay, or inside the ignition switch.
For an aftermarket voltmeter, the round 2-1/16th inch case (through-panel measurement) size fits OK. I prefer a digital. Summit Racing sells one similar to the digital one I designed and offered some years ago ...and it fits in the Airhead dash in place of the original. It is self-powered from the 12 volt power source, the original lamp is not needed. Digits are available in at least two sizes, and two colors (green, red). Some have fancy pushbutton functions to capture maximum or other readings. You do NOT need those functions, and the buttons can let moisture into the interior. Datron makes a truly nice and neat small rectangular one, that is potted, and never ever fails from vibration or anything else, very good quality, but it won't fit in the dash hole, unless you make a mounting plate, which is not difficult to do. It can be mounted elsewhere's, if you do not have an RS or RT, or? An excellent substitute for the BMW fairing voltmeter is the VDO 332103, available from Summit Racing, and many other places. Standard 2-1/16th inch size; black face, red pointer, white numerals, 8-16 volts, accurate, backlit, reliable, and much cheaper than from BMW.
Digital replacements of my own design are no longer available from me because I could not get the original manufacturer to make more, instead, they ripped off my design, made them, and sold them in racing auto-parts stores.
Note again that the stock reading is typically 0.3 or so volt less than the actual voltage at the battery. The reason is where the voltmeter is connected, downstream from the battery, and the battery has a more direct charging route from the alternator...in fact, that stock charging route bypasses the ignition switch! ...and is not fused.
Voltmeters previously OK & now swinging, may be the meter (try tapping) ...or a circuit problem. One way to analyze the situation; especially if the use of the turn lights (blinkers) is causing a much larger voltmeter swing than it used to, is to use a non-digital fairly fast reacting cheapo voltmeter, and measure the voltage, during flashers blinking, at the battery posts themselves, and then at various other places ...such as the engine side of the ignition switch, ending up at the voltmeters own input wire. The problem is often either the battery, or the connections from the battery and at perhaps the starter relay internals or its socket (if it has one). A wildly swinging voltmeter can be a failed voltmeter; but, it can also be an indication of oncoming more serious problems, most problems like that are fixed simply by cleaning connections. I have seen it from a worn ignition switch, however. Typically, once this sort of thing starts up (voltmeter OK, connections poor), it is only a matter of time before you will have problems with charging, lights, etc. Find out if the problem is the meter...or a connection!
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Last check/edit: Sunday, February 28, 2021