Ampmeters versus Voltmeters – Airheads Beemer Club

Ampmeters (ammeters) versus Voltmeters.
For BMW Airhead motorcycles.
Explanations & Problems.
Relative merits of ammeter vs. voltmeter. Use both?
Voltmeter reading problems with the stock… or aftermarket… voltmeters?
Why not just a GEN light?

SUMMARY:
I see little reason in having having an ammeter permanently installed on any BMW motorcycle, unless you have special circumstances. I foresee several drawbacks, not the least of which is a potential fire hazard, which includes burning up your bike’s wiring; besides the usual situation of LESS reliability with many modifications; especially with added electrical items. 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 YOU.

The GEN lamp, voltmeters, and ammeters:
The GEN lamp has several functions. It provides an illumination to instantly see if the alternator is likely to be producing electricity. It may also provide information if the alternator is intermittent with RPM or brush wear, or diode board problems. It might indicate, by a DIM indication, of problems with connections. It does not indicate the level of charging or discharging; and, it is a very broad type of indication, indicating an operating charging system…or not. It is reasonably decent at doing this, and cheap for a manufacturer to include. Long ago, alternator-equipped cars were mostly equipped with such lamps, and not ammeters nor voltmeters. Many cars still have GEN or ALT function lamps, & do not have voltmeters, although many modern cars have voltmeters built-into their in-dash electronics displays. In the same way that cars that do not have manual shift gearboxes but are equipped with mechanically and electronically controlled automatic transmissions do not need tachometers, so it was with alternators and voltmeters and the charging lamps. Lamp ON, ….there is a gross failure. Lamp intermittently ON…..there is a gross failure coming soon. Simplified descriptions, but fairly accurate.

In the SAME way that many automatic transmission cars DO have tachometers; YOU may WANT a meter, or meter’s, that indicate more than minimum information….as opposed to a lamp. Remember that the vast majority of BMW motorcycles have only the GEN lamp indication, only a modest number had voltmeters. There is NO question that a voltmeter gives much more information, often information that is well in advance of actual serious electrical system failures.

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 clock or not), etc.

I have installed ammeters on very high output alternator conversions I have done for others. I often temporarily attach instruments to Airheads such as ammeters to do charging system maintenance or engineering work. I did have a permanent amp-meter install 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.

Ammeters….history….installation…basics:

In order to connect or install most ammeters (amp-meters), you must cut into a major wire or connection, and insert the 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 problems are minor, such as putting the ammeter in the grounding lead of the system. I will discuss some of these things.

It is possible to make a crude ampmeter by connecting a sensitive voltmeter, that is, it will measure MILLI-volts (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 fairly simple if you have the equipment to do so. EACH installation will REQUIRE specific calibration for THAT installation. This type of ammeter setup is called a SHUNT system.

There are actually commercially-available “shunts”, basically a piece of calibrated metal, calibration means that it has a known value of resistance between specific terminal connections on it. Some relatively small commercial shunts are available, and since they are always rated at either 50mv or 100mv, it is easy to match them with a meter. You connect a millivoltmeter across the terminals. Since the voltage drop across the shunt is known, this method can be very accurate, even laboratory grade. A shunt type system can be made to be relatively SAFE, because you ccould put the shunt INTO THE GROUND LEAD. For an Airhead, you could simply put it in series with the large black lead that goes from battery to the speedometer cable hollow bolt grounding place. However, in order for this type of connection to work best, any other motorcycle grounding connection, 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.

A bit of history:
There is another type of meter that is not seen in modern vehicles, which all either have voltmeters with or without an indicating light…or, no voltmeter but use a red light, etc.,… to notify you of ‘problems’. This other type of meter works on the principle of the magnetic field that surrounds any wire that has current flowing in it. It can be seen as a clamp-around measuring instrument with movable jaws, if used in an automotive repair shop, or by someone working on home or industrial power lines. Similarly, your strobe light for engine ignition timing purposes probably has a clamp-around probe that picks up the electrostatic field around the ignition wire. Operating in much the same way, there was a type of 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 later with alternators, DID have ammeters. The ammeter nearly always had a zero-center-reading, and indicated discharging to the left of that zero center, charging to the right of that zero. But, MANY of these ammeters were NOT installed by direct wiring…which added a fair amount of expense and could be a safety problem. These non-direct-wiring ammeters generally had a ONE TURN LOOP of metal on the back of the ammeter, that, in the vehicle, the main power wire went through, without any connection at all. The magnetic field made the meter work. Typically, for cars, they had scales of something like 30-0-30 amperes or maybe 40-0-40 or even higher. Some very old English motorcycles used these types too….Triumphs and many others. Scales for the bike types might be 10-0-10, etc. They typically have 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, and hardly much else. 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, except in special circumstances. This section gets much 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. It IS possible for one to be made up that monitors even the starter motor drain when it is in use; I see NO advantage unless testing the starter motor for current drain. Using a millivoltmeter across the battery grounding cable type would be such a way, a very safe way too, but you have to find a way to calibrate it, which actually is NOT difficult, you can simply compare against one of those FREE (often) Harbor Freight meters (but will not measure such as starter motor drain, which is much higher than the meter range, usually 20 amperes). You could also 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 old cars and trucks. Ammeters of the simple car types were very cheap, and adequate for the purpose.

A number of car manufacturer’s have installed voltmeters of the expanded scale type, even though they cost more than using a red light to tell you that your alternator is no longer working properly. The truth is that voltmeters give more truly usable information & eliminate the need for costly large diameter cables into the driver-passenger compartment for such as ammeter purposes. Such large cables would carrying large currents and electrical noises, that might get into radios, music players, etc. Thus, over-all, the cost of the voltmeter is almost always considerably 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 on vehicles with separate metering, but can be installed, often in the same hole as an expanded scale round meter…as used on many BMW motorcycles.

Ammeters are available in several popular types. The best is an old-fashioned type is called a FOUR-connection calibrated shunt, used with a millivoltmeter whose scale calibrated in current, not voltage, wired to the shunt. A good one of these is quite accurate, and somewhat better ones are laboratory type devices. A simple, yet adequate type for most uses, although not generally capable of tiny current measurements, is an old fashioned types used in 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 were described previously as 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 crudely accurate enough for many uses. BOTH of those 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. You can place these at your bike’s alternator circuit if you wish to do it that way, to measure the alternator output; OR, you could place it 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 -motor wire in the battery circuit.

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.

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’ might be hard-pressed to give coherent knowledgeable information about WHY.

If you have a voltmeter, especially a highly accurate one (although the stock expanded meter IS accurate enough)….it will tell you if the battery voltage is high enough for you to assume it is charged; or not. An ammeter will tell you if current is flowing into, or out of the battery, but will not indicate more.

You can use an ammeter in a way that shows ONLY alternator output, should you want to, wiring it into the alternator output lead (on an Airhead that is the larger spade on the diode board on the RIGHT, as you face from the front of the motorcycle).

A normal ammeter indication in the battery circuit 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. Around 10 to 12 amperes is fairly normal on a stock bike, no extras. As rpm rises that discharge indication would decrease and become a charge, & with rpm high enough that charge value would be initially high, perhaps 10 amperes or more if you have the alternator capacity and modest electrical drains. As the battery charge 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.

If the discharge was 10 amperes at a stop light… and you sat there for ONE minute, … then, you SHOULD expect LONGER than one minute at any cruising rpm from maybe 3000 rpm or higher, to replenish the power taken out of the battery. This is due to inefficiency of charging, as the battery is not efficient as a chemical converter of electricity. Your only indication of the replenishment occurring is the slow decrease of the ammeter reading….and/or a voltmeter slowly rising. Granted that the headlight on the road at night will be a 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 THERE; which isnot a good idea as it would be constantly draining the battery with a tiny current flow, even with the key being off. In the stock BMW Airhead motorcycle, with a factory dash voltmeter, the meter will indicate roughly 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 quite good condition, the voltage drop will show up as even more.

Due to a battery being a complex CHEMICAL CHANGE storage device, batteries are ‘charged’ during riding to a voltage 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 being overcharged & possibly is being damaged. If the voltage is only 13.5, that is marginal on the low side. VR output is, or should be, designed for slight voltage increase as temperature DEcreases, to compensate for slowing of chemical reactions with decreasing temperature.

If the voltmeter falls BELOW maybe 10.5 or 11, perhaps during cranking or sitting at idle with the headlight on, then something (connections or switch contacts?) in the system is bad and/or MAYBE the battery is getting ready to fail or is just very heavily discharged. A truly large variation when the blinkers (turn signal lamps flashing) are operating might indicate a bad battery or poor connections in the system …or a car voltmeter, not a bike voltmeter. BMW bike voltmeters are damped …that is…smoothed/averaged…in reading from sudden electrical changes in voltage value from the blinkers loads. The damping has been known to fail. Dampening 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 temporarily. I have known 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, even more information is available.

One particular advantage of the voltmeter method is that no large diameter cables & no potentially failing connections carrying high currents are needed. There are some other reasons not to use an ammeter, such as avoiding alternator whine noise in your radio gear & avoiding possible shorts, sparks, fires, as well as not reducing reliability.

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 QUITE common to see a battery with a failing cell or two, not wanting to charge up to as high a voltage as it should. This indication also can come from a single bad diode in the diode board, a failing regulator, bad brushes, etc.

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 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 use it. On MOST of our bikes after the /5 series, the headlight is automatically turned off during cranking. A battery can be fully charged & quite good in a formal Load Test, but 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, connections 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 different meter to calibrate your setup.

If you are interested in a cheap way of adding an ammeter, that 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 current flowing through it. You can multiply the sensitivity of the meter by looping the wire more times through/by the meter if it has the place or room to do so. More loops, more sensitivity. You can enlarge the loop in some circumstances. Thus a 60-0-60 meter can be made relatively useful, and you could make it far more sensitive. A rough idea of the ammeter sensitivity to try for when it is all connected, would be ..perhaps 25-0-25 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.

The terminal connection type of ammeter can be connected in your Airhead in several places:
1). In series connection with the smaller RED WIRE of the battery + terminal…or from the starter relay pass-through connection from the battery…or from the ignition switch.
2) In series connection with the largish 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.

Be very careful with those types of connections. ****I caution…greatly,…. if one installs wired-in ammeters, that it CAN BE a big fire danger. YOU MUST do things with thought, neatness, and good techniques. 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.

I believe that a voltmeter is by far best indicator & that BMW got it right when it began installing them on some models, such as the R90S, R100RT & RS.

VOLTMETER PROBLEMS:
Except for the ‘only fair to good’ quality of the mechanicals, in the stock and as installed by BMW, expanded-scale meter, they are reasonably reliable and fairly accurate. 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. I have seen some that had very little dampening that WERE motorcycle types…the dampening mechanism failed. 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 out the needles swings…because motorcycles vibrate and tilt…and the dash vibrates a LOT…and the blinker (signal flashers) connections can also cause a lot of dancing. Do NOT use a car unit, and if you have your unit repaired, be SURE to specify the damped motorcycle movement. Most would not bother to have one repaired, they would buy another, or a digital meter.

NOTE: 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 start checking for poor connections in the electrical system.

For an aftermarket voltmeter, the round 2-1/16th inch case (through-panel diameter measurement) size fits nicely. I prefer a digital. Summit Racing sells one similar to the one I designed and offered some years ago…and it fits in the Airhead dash in place of the original. It is self-powered, 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 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. This CAN be done, neatly, and looks OK. 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 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. The best way to prove that 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.

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:
05/20/2017: Copy to airheads.org website and edit/update.

Member Login