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How to buy & use:
Test lamps & multi-meters.

Copyright, 2014, R. Fleischer


The purpose of this article is to explain how to purchase, and the basic use, of test lamps & multi-meters (VOM); provide some information on how they work; & inform you about use for your motorcycle, car, home, etc. 

Various hints are provided plus some tricks & troubleshooting.  

If you are a near beginner in tracing electrical problems, doing measurements, etc., you may want to read the following articles, among others in my electrical articles list:  14-A; 15; 15B; 16B.   I HIGHLY recommend that you read those articles on this website before and after reading this one as to gain a good working knowledge of your airhead with regards to troubleshooting problems.   There is an large section on electrical's on this website. 

Test Lamps (test lamp probes):

This simple device CAN tell you a LOT, but you need to know how to use it properly.  For many bikers, one of these is in the on-bike tool tray, and not a multimeter.  Some carry both.    Do NOT be put off by the simplicity of a Test Lamp ...they can do MOST of what you might need on the road, and can be very helpful in your home shop!  These test lamps save troubleshooting time (we Pro's use them all the time!), and using them is not at all difficult to understand.  

Sometimes 'simple' technology is very good ...and FASTER than other methods!

You can make a test lamp from most any small low power 12 volt (approximate) lamp, and some test leads soldered to it.  I recommend you DO NOT make one from an LED, but that you use an incandescent lamp.  An LED might illuminate confusingly, as its current drain is very low (or think of that as the LED will light up with tiny amounts of leakage current, that may be even normal in your testing area) in some situations, and most are polarity sensitive.   I recommend that you do not make your first test lamp, but purchase it (about $1-$4).   Get a commercially made test lamp probe; any auto-parts store, or Harbor Freight company.  Below is a photo of two of my own commercially made test lamp probes.  Either one would fit in your Airhead tool tray.   Note that both of these test probes contain a sharp tip contacting probe end.  It is best to keep the tip structure covered with something like 'electrical spaghetti'.. or 'shrink tubing' avoid puncturing something in the tool tray.  These probes contain a 12 volt (nominal) lamp, and a length of fairly flexible multi-strand wire with an alligator clip.  There is NO BATTERY in these probes.  Battery types ARE available, but you should NOT purchase that type, as their prime purpose is ONLY for continuity testing.   

Here I have removed the protective 'spaghetti' over the sharp tip.

In using a test lamp, 99% of the time you will have the ground clip wire from the test lamp connected to a convenient unpainted metal part of the motorcycle.  That might be the battery negative terminal, or a cylinder fin (one of my favorite places); or any other convenient grounding point.....wherever is a good grounding point and it is convenient, yet out of the way of hot exhaust pipe, ETC.  A 6 foot or longer lead on the test lamp may make it easier to use, the ones in the photo are not that long, as these are the ones I use in my electronics repairs shop, not for my bikes.

Inside the headlight shell of most Airheads is typically a headlight relay and a turn signal flasher relay. The bolt holding these in place is NOT a good grounding place, as the shell is painted, and the electrical ground may be there, or non-existent due to the paint coating.  USUALLY a place in the headlight shell with ALL-brown wires going to a screw or rivet place on the shell is OK. If that is a poor connection, all sorts of problems can arise.  If any questions, install a good wire from there to the grounding point on the frame backbone ...check near the ignition coils for a convenient place ...and the older bikes standard ground WAS at a coil bolt.

BMW uses solid brown (NO stripe) colored insulation to signify a grounding wire; if you see such a wire going to a terminal that has a bolt or screw into a frame tab, etc., that IS likely a grounding point.  

There are many things a test lamp can do.  Unless otherwise stated, it is ASSUMED HERE that you have the alligator clip connected to a good ground ...which means a cylinder fin, or a bolt that is grounded somehow to the chassis, ETC.

1.  A test lamp can tell you if a circuit has power on it or not.  This is its main function.   Touch the probe tip to the place in question.   If the lamp lights, you have power there.   This has many dozens of applications, not just checking a bulb socket or some other simple test.  NOTE that if the metal shank of your test lamp is UNinsulated over considerable length, you could accidentally short circuit a lamp socket.  Because of this, if your probe tip is bare metal over its full length, you may want to put some shrink tubing, etc., over a fair amount of its length, leaving just the sharp tip exposed.   Some test lamps come with such insulation.

You can probe the contact connections at a switch to see if power is going to...AND through it.  
(a) Power goes to the switch or connection point, etc., and when the switch is turned on, power is supposed to go through the switch or connection.  Probe both connections (power in, and supposedly power out), see if the power really does go through with the switch turned on.  If the switch is shorted, that will show up by the power going through with the switch turned off. Most of the time the problem one is trying to trace down is a LACK of power going to or through something.  In some instances, something is intermittent.  Wiggling connections, wires, etc., can show that problem, the lamp will flicker.

(b) Power goes to whatever is the item or device, and the grounding side of the device (lamp, etc) goes to the switch, with the other side of the switch going to ground......and when the switch is closed, the item or device is powered by the circuit being completed.   Horns are often wired this way, as are neutral switches and oil pressure switches.  With the switch in the grounding side of the circuit, with the switch being OFF, the non-ground wire at the switch should light your test lamp (assuming power is ON, of course), and the test lamp go OFF when the switch is ON.    The reason the lamp lights when the switch is off and your test lamp is connected to the grounding side of the device, is that with the switch off, that side is NOT grounded.  Thusly, power goes to the device, then goes through the device, and then to the grounding terminal which is not grounded if the switch is off.  On some horns there is only a single connection, the horn body is grounded.  That type is tested at the horn terminal when the horn button is pressed, as power goes to the horn via the switch; this means that the test is done like the prior paragraph (a).   On BMW's, an earthed (fully grounded) wire connection is always brown, no stripes or other colors included, just plain brown.   ONE exception is the large size battery negative wire, which is usually black.   BMW may use BLACK, and occasionally other colors, for wires that will be connected to ground by some function or some switch, or some relay, etc....but other than the battery ground, all wires connecting DIRECTLY to a ground terminal are always SOLID BROWN.

(c)  You can test the voltage regulator circuit to see if there is a bad GEN lamp; or, a bad voltage regulator.  It is much easier to get to the alternator than the pod lamp.  Normally, if the GEN lamp does NOT light up, one uses some bit of metal or screwdriver, etc., to short the two rotor slip rings together (or one grounds the black wire at the alternator D+ terminal)...if the GEN lamp then lights, the rotor is open, assuming brushes are making contact.  If the GEN lamp does not illuminate with the slip rings shorted to each other, then short the D- and Df terminals of the alternator together (wires still connected).  If the GEN lamp lights, then the brushes are not making contact...provided you already found that shorting the slip rings did NOT light the GEN lamp.  If the Gen lamp won't light with you shorting the D- and Df push-on connection wires themselves, then the problem is the GEN lamp, its socket, its wiring to the regulator or the Df terminal at the alternator, or the regulator itself.   You can short across the OPPOSING two voltage regulator harness plug terminals (unplug first).  If the GEN lamp lights, the regulator is faulty.  If not, check the GEN lamp and its socket...a very common problem is the socket connections in the instrument pods of the /6 and later.  You can use the test lamp to probe areas to find out where the power is, and is not.

(d)  In rare instances you may want to apply a small current, not the entire vehicle battery current availability, to something.  The lamp probe can act as a series safety resistance.  You can also do this with an AC transformer and that lamp, on your workbench, to identify bad diodes in the diode board ...a more vigorous test than an ohmmeter testing of the diodes. 
HINT:   Need to apply more current, but not too much?  Using an old headlight lamp, any type, with leads, can allow more current to flow than the standard small bulb of a test lamp, but not enough to burn up wiring, and this can be a much better test for the 6 large diode board diodes.  This is a particularly clever method if fuses are blowing, you simply substitute the test lamp that pulls a bit of current if there is a short circuit, but won't allow circuit damage. 

There are tests you can perform on wires themselves with a test lamp.  If one end of the wire is probed, and then the other, you can draw some conclusions.  If the wire is expected to have +12 volts on it, and it IS at one end of the wire, and NOT at the other end...the conclusion must be that the wire has an open circuit (a break in the wire someplace).  This often shows up the rare condition of a bad crimped connection.   An advanced version of this test, in a reverse type of way of thinking... is at the large battery negative cable.  Once in a great while they fail at the battery terminal, or speedometer bolt terminal.   With the ignition switch ON, and headlight switch on and headlight illuminated, you can probe the + wire, and you can probe the wire at either end of the negative cable too.  For the negative cable, normally you would NOT see the lamp light up in the slightest, as this wire IS a heavy sized grounded wire.  If you had a suspicion that the wires inside that black negative cable might be iffy, you could probe each end.  If you thought the cable might fail only with a very large starter motor load, you could use the starter motor, and whilst the starter is engaged, probe both ends of the wire.   If either end of the cable causes your test light to illuminate, no matter if it is bright or dimly...then the wire or connection fitting area is faulty.    Additionally, if you suspected a bad battery, you could probe the + (red wires) (left side terminal) of the battery.  The lamp should light up under any condition of engine on or off
or starter use or lights.  Note that if the + battery terminal ITSELF when probed lights the lamp, but the connection lugs at, say, the battery + terminal, do not, or get very dim (with lights on, and especially with starter button pressed)...then you have a bad connection at those battery connections.   Another example is if the battery and the leads at the battery test fine....but maybe your headlight is dim.  Probe the battery +, then the key switch.....the idea is to 'follow the power...'....then continue down the circuitry towards the lamp.

HINT:  One particular test may be of interest to you.   Suppose your starter is not functioning, or not properly.  Is it the battery ...or the starter ...or the starter relay ...or the starter solenoid??   A test lamp CAN tell you, and quickly!  Just follow the power from the battery + terminal to starter relay to solenoid to starter.

It is possible to probe the printed circuit board material in the instrument pod of the /6 and later bikes, to find out if it is cracked and open-circuited (commonly seen at a lamp socket).  Be careful of the printed material.

2.  A test lamp probe can tell you when the ignition points open (engine not running but ignition switch on).  This enables what is called 'static' timing....close enough to start the engine and have it run OK before you rev the engine and use a stroboscopic timing light at the 'flywheel'.   When the engine is rotated very slowly by hand (rear wheel, higher numbered transmission gear, slow jerks in forward direction...), the EXACT place where the ignition points JUST BARELY start to open (maybe a gap of .001") is the static (or idle rpm) ignition timing point.  Normally this point is found by using a strobe light and is done at idle.  This corresponds to the S mark on the  flywheel (called clutch carrier on 1981+ models).   I dislike turning the alternator bolt for the purpose of rotating the engine.   You can determine this JUST BARELY points opening position quite closely by touching the probe to the points connection or capacitor/condenser...perhaps with another short jumper wire...or, put probe tip in or connected to a nearby case hole/place for the grounding, and the alligator clip to the points/condenser.   Rotate engine VERY  slowly, in very TINY amounts.  When the lamp glows, the points are open. You back up, try again, until you find the smallest engine rotation amount that JUST opens the points (lamp will light up).  Obviously the ignition needs to be ON for this.  Do not leave it on for long periods of time.  A few seconds at a time are fine.    Description: battery power goes from battery to ignition switch to and through ignition coils to the points.  Thus, when the points are open, the low resistance of the primary's of the coil's are vastly lower than the resistance of a small lamp, so the lamp lights at basically normal brilliance as if connected directly to the battery and not the points.     With the points open and the test lamp glowing, you now rotate the rear tire/wheel backwards a small amount, best using driveline slop to jerk it back.... until the lamp is off, then retry forward direction VERY slowly.  When the lamp JUST glows, check the flywheel marking.  Some folks remove the spark plugs and use an allen wrench into the alternator bolt to turn the engine; I just don't like that method, as occasionally it over-tightens the bolt.

3.  A test lamp can be used in series with a small low voltage transformer, to test diodes.  This is THE best method of dynamically testing diodes!....such as the ones in the diode board.   While a preliminary test can be done with an ohmmeter (disconnect the battery in the bike before probing a diode board!), to measure forward and reverse resistance, this test lamp dynamic method is vastly better.   When the series circuit of lamp and transformer has its probe wires shorted, the lamp lights up fully.  When connected across a diode, the lamp should be much less bright, but NOT off.   That is because the diode only conducts on half the incoming AC current.

Multi-meters, voltmeters, ammeters, ohmmeters, ETC:

This covers both digital and analog types.

You can purchase separate meters for just voltage, just amperes, and just ohms, but most people will purchase a type of meter called a Multi-meter (Multiple Function Meter).  These are available in analog types (needle moves above a numbered marked scale covered with glass or plastic) and in digital types.   The analog meter, even though old-fashioned, does have some uses that the digital is hard-pressed to show you.   The analog meter, having some inertia and thereby being naturally 'damped' in action, can TEND to show how a voltage or current is changing during measurement. A digital can do that only within certain limits of its sampling rate, etc.  There are quite a few reasons for using an analog meter, and I will describe such uses now and then in this article.  The analog meter does not need a battery for any of its internal electronics (except, as on all meters, analog or digital, for the OHMS functions).  Some analog meters have two or even three ohmmeter section batteries, typically one or two common AA of 1.5 volts, and a common 9 volt battery.  That 9 volt battery enables the analog meter to measure very high resistances.  Some digital meters also require more than one battery.  

Don't leave your analog meter on the ohms range, the battery can possibly slowly drain ...on some meters, leads do not have to be shorted to slowly drain the battery on the ohms function.   Don't leave a digital meter turned on for excessively long periods ...that drains its battery too ...whether on ohms functions or not.   

I do not want to give the impression that YOU should get an analog meter and not a digital meter.  For almost all of you, a relatively inexpensive digital meter is THE best way to go.

These analog and digital multi-meters are generally used for some of the same types of testing as test lamps, PLUS they are capable of providing much more information; but lack the ability to do SOME of the test lamp testing.  You may NOT need such meters information.  In some instances "digital" meters are absolutely needed, such as in testing or adjusting the 'exact' output of your Airhead voltage regulator; or, where you need to measure voltage more precisely, etc.   

The photos below, show various types of common meters.  ALL ARE CHEAP.  You can get perfectly good and reliable meters for a few dollars (occasionally FREE with a coupon from Harbor Freight company). There are probably many dozens of types like these.

SOME industrial multimeters, often the types that have an additional function, a CLAMP-OVER device to measure current flow in a wire, do not measure in milliamperes, only amperes.  Many of these do not measure DC amperes, only AC amperes.  They are NOT the type I recommend for you.

For the type of multi-meters under discussion for YOU, these INexpensive meters are typically the type where YOU select the FUNCTION and the RANGE of the meter by means of turning a switch.    A typical analog meter measures Resistance (ohms), AC volts, DC volts, DC current, and usually has one additional higher DC current range, typically 10 amperes full scale reading, although a 20 ampere range would be nicer.  The 10 or 20 ampere range typically uses a different plug hole on the meter.  Many meters include a continuity test function with an internal beeper tone.

The other meter in the small photograph is a DIGITAL meter (meter is powered-off, display is blank in photo), that measures the same sort of thing as the analog one, but FAR more accurately, and this particular digital meter is a type used for automotive use; and, unfortunately, has no current ranges.  It has a number of ranges for RPM and DWELL, and has a diode test function.   I have seen these sorts of meters with 10 or 20 ampere DC measuring ranges, plus RPM and DWELL for multiple cylinder engines (you CAN use the 4 cylinder function on a 2 cylinder BMW, with 50% correction factor), and even AC volts and diode testing.....all for under $35.   These can be useful around the home too.  To be really useful for something besides vehicles, the meter needs AC volts measurement capability too.

The last meter in the small photograph is a RadioShack product.  This is one I have recommended for years for bikers, and is shown with its folding lid opened.  It is very small, a digital type, quite amazingly accurate, and is auto-ranging. Auto-ranging means that for volts or ohms you select only the function with the knob, the meter itself selects the appropriate 'range'.  That function is called autoranging.    It measures ohms,  AC and DC volts, checks diodes for forward voltage, and has a continuity test function (beeper).   It is thin when folded up, the leads shown wrap around pegs INside the top lid...and this meter will easily fit an Airhead tool tray.  It is powered by 2 long-life mini-cell batteries of the type used in watches, a slight drawback.   The one illustrated does not measure current.   It's primary drawback is the short leads that do not plug into the meter; and, no current measuring (not needed very often anyway).  Radio Shack no longer sells this model.  NOTE that the Triplett 2030C meter is cheap, about the same size, and measures quite a bit more than this Radio Shack meter.  Triplett is a good brand.

What about that 'continuity' testing function on some multimeters?   You COULD use an ohmmeter, it will tell you MORE about a continuity (connection-through...), but for quickie tests, on non-powered circuits, a continuity test can be handy.  I use this function mostly on my workbench when making up cords with many many connections, to be sure that the wire I THINK I am working with, is the proper one (especially when no positive color coding is available).  When the continuity function is selected, shorting the meter leads causes a beep.  NOTE, however, that you can often have a considerable resistance in a circuit and the beep still sound.  That is why the continuity function is not as useful as you might think, for motorcycle troubleshooting.   You also usually have to have the bike battery disconnected.  At the upper left in the below photo is a RadioShack analog VOM (volt-ohm-milliameter).  Milliameter means it measures thousandths of an ampere.  While it also has ampere scales, it is not called a volt-ohm-ammeter but a volt-ohm-milliameter simply from conventional practice.  Analog meters have some uses, but most will be better-off with a digital meter.

Below is a nice reasonably small meter, accurate, and reliable.  This one is the Radio Shack 22-810.  It is not quite as small as the clamshell meter in the lower part of the above photo, but those are totally discontinued.  This meter, below photo, does have a diode test function.  This easily fits in your Airhead tool tray.   About $20.00.  The only drawback is that the battery is not a cheap single 9 volt type or 1.5 volt AA, etc.


I am saving the best deal (and a good meter) for last!    I had previously mentioned the sometimes free (with a coupon), or often cheap Harbor Freight digital multi-meters.  I have tested many of these, and all were accurate-enough.   I specifically went to Harbor Freight with a 'coupon' to get one of these for FREE, to take a photo of, for this article.  This particular one is still in its plastic packaging, so the photo is not great, with some reflections and is slightly out-of-focus.  This meter is not auto-ranging (I prefer them NOT to be), and measures AC volts, DC volts, DC current (has a 10 ampere range), resistance, checks diodes, light load checks common dry batteries of the 1.5 and 9 volt types, and measures gain of transistors.   It has what is called a 3-1/2 digit function, which is what you want.  It runs on ONE battery, a very common 9 volt need to go try to find the exact 'watch' battery.   The battery lasts a LONG time too.   This meter is small enough to fit most tool trays, has leads which are long enough for most uses, it is only 3-1/2" wide, 5" tall, and less than 1" thick.   This free (often) meter from Harbor Freight is usually sold under their CEN-TECH brand, and this one happens to be their "item 98025".  Just because it is often a freebie, do NOT think it is not OK, because it IS.   If you use it properly, and do not test live circuits with the ohmmeter, nor over-power the 10 ampere current range, the chances are good it will last for MANY years.  Those same cautions are for nearly any meter.   This one does not contain the safety fuses covering most functions, of some pricey meters.  These do have one internal fuse.
HIGHLY recommended!


Some advice about purchasing meters:
I don't recommend analog meters unless you have a real need.  If so, get a taut-band type, which are reasonably rugged.  Analog meters are NOT accurate enough to measure or set charging voltages.  Many are not very sensitive either, which is not overly important.  Analog meters are not as rugged as digital meters, in general far less rugged.  Digital meters that are under $35 (often WAY under, or free) are all that the average person, and even most shops need.  Meters with a full 4 digits or more are NOT necessary and usually a waste of money.  Some meters are called 3-1/2 digit, those ARE FINE ...YOU DO NOT NEED MORE.  Some digital meters have a diode test function, and on those that do not, you use usually use the ohms function to test diodes.

Some rather expensive digital meters do not put enough current (or internal battery supplied voltage is too low) into a diode you want to test, to 'turn the diode on', when using the ohmmeter function.   Those meters are designed to test very sensitive electronics circuits that might be injured by the higher test current on the ohms functions of other meters.   AVOID meters that won't turn diodes on, from either not having a diode function; or, the ohmmeter current is too low.   You can tell if a meter, without a specific diode test function on the panel, will test a diode, by simply having the meter on an ohms function (if multiple range type, use the lowest ohms range) and testing a diode, in both lead-connection directions.  In one direction there will be no reading, or one that is in the millions of ohms (or beyond the meter's range).  In the other direction, the reading will be perhaps 10 to 150 ohms.   Meters vary in what they indicate in that 'forward biased' lower ohms direction.    The digital meters you can purchase under $35 are usually more than rugged enough and accurate enough. I've seen meters that were on sale at under $20 that were just fine. 
Harbor Freight sells meters that do just fine, for $10 or less, and as noted above, often gives them away as freebies if you have one of their coupons; or make a very minimum purchase of something else.  Get on their mailing list.  I have tested numerous ones, they are accurate enough. As a convenience, I usually keep some of them in new packaging, in my shop for those needing one.

Buy a meter that does at least the minimum (or get the freebie which is MY advice).  I recommend that the meter measure AC and DC volts, & it must have an ohmmeter function.  It is a PLUS if the meter measures DC current, especially if the range goes as high as 10 or 20 amperes.  Meters that only measure in the milliampere range are not as useful.  There IS a way to measure DC or AC current with a resistor, by measuring the voltage across it, the resistor placed in series with the circuit, kind of a PIA for most of you though.   Just ONE reason for DC current ranges is the often heard complaint: "ignition off, lights off, excessive current drain from the battery".  That is, when the battery is going dead too quickly when the bike is stored or parked.

Current measuring ranges do other things, you can learn about them later on. 

Meters that measure frequency, A.C. current, capacitance, etc., are VERY SELDOM ever needed by anyone working on vehicles.  This INCLUDES meters that measure ignition DWELL.  You just do NOT likely need those things.   

The Continuity function, which gives a BEEP, is useful at times on UNpowered circuits. 
 Having a separate function to test diodes is useful, although your ohmmeter function can substitute well enough ...but test the particular meter you are thinking about purchasing by using an actual known-good diode, so you know what a good diode reads on that meter.

An auto-ranging meter is nice to have for some folks.  So is one with automatic off, although both features are minor things.  I somewhat prefer NOT to have auto-ranging, although it is nearly mandatory on the smallest 'pocket' meters, because they don't have ROOM for a large multiple-position switch.

TEST any meter you propose purchasing, with a diode, before buying.  See if it measures the diode OK.


How to actually use a multi-meter:

On an analog meter (and some digital meters), you must turn a knob to set the function, and the function range.  You also may have to select certain pin jacks, especially for a highest current function, on digital meters as well as analog meters.  Some require using certain pin jacks for resistance (ohms) functions.  

Typically the analog meter scales read zero on the left (except for ohms), where the needle rests with no input.  The needle is deflected during use, and the maximum reading is USUALLY the function RANGE setting, which is the maximum reading for that setting.  On lowest ohm functions, full right indication is often the minimum, shorted, ohms reading.

Thus you might set this knob for DC volts, 25 volt range for measuring zero to 25 volts.  Analog meters tend to be more accurate at higher readings.    This is generally not of much importance unless you are reading a low value, perhaps 10% or less of maximum reading on a range.    If you had a voltage drop in a circuit of maybe 0.5 volt, you would NOT use the 0-25 range to measure it, you would use a more sensitive range.  Many analog meters have a screw to adjust the needle to the zero point, mechanically.   Be sure to check the meter, in the physical position you use it at, for zeroing.

BE CAREFUL when using any meter on the CURRENT ranges.  Current is NOT to be measured from a power source to ground! (same for ohms ...but no electricity AT ALL in the circuit is allowed).   Current is measured with a series connection (the meter is connected between the power source and the device; or, between the device (or battery) grounding side connection and chassis ground).  Failure to follow this advice can either blow a meter's fuse (if it has one) or ruin the meter, at least on the current ranges!    The ohmmeter circuit in the meter must not have power applied from your test lead connections, or you can burn the ohms section.   If you are checking a diode board in an Airhead motorcycle, DISCONNECT the battery first, before removing the outer cover and certainly before using the ohmmeter or diode test functions.
READ:  (3)  AMMETER FUNCTIONS:      below.

Many digital meters are auto-ranging; select the function, the meter automatically selects the range for that function.  There is no great advantage or disadvantage; except for certain of us nerdy folks, but it CAN make the meter more compact and may have fewer dirty switch problems.  If a meter has a multiple selection switch, that switch can get old and tired and have its own contacts problems.    Because digital meters are so cheap, I suggest you get a NON-autoranging meter.

Some literature says, WRONGLY, that you measure voltage or current in a series circuit by disconnecting the battery negative wire, and measuring voltage between battery terminal and ground).  THIS IS FLAT OUT WRONG.  Measure current, not voltage, in this configuration.

The symbol for ohms is:  Ω  which may or may not show up properly on your computer as a Greek Omega symbol.    An ohmmeter measures electrical resistance by having an internal meter battery so connected that a low voltage at low current is applied to the test leads (usually red and black).  When the leads are shorted together, the ohmmeter should indicate a very low nearly zero value.  Analog ohmmeters will almost always have a 'zeroing' knob, that you use to set the needle to zero reading with the leads shorted.  On most analog meters, you must adjust this knob as you change RANGES or as the battery ages. When the meter can no longer be zeroed, the battery needs replacement.  On most digital meters, there is no such zeroing adjustment, and for very low resistance devices being tested, you mentally subtract the shorted-leads reading, to obtain a more accurate final resistance value.   On some ohmmeters, the red lead may be internal negative voltage, not positive, which is generally only of importance if you are using the ohmmeter to test a diode. In such a case, just mark the panel with a diode symbol, assigning the cathode and anode parts of the diode symbol to the appropriate red and black jacks.  NEVER EVER connect an ohmmeter to a live circuit.

(a) testing spark plug wires:  This is done by two methods, either measuring between the left and right spark plug cap (I consider it a quick test of limited value); or, by removing the wires from the coil (which is much more accurate) and testing them separately.   Remove the spark plug wire at the coil, and pull the resistance cap off the spark plug.  For an analog meter, select the appropriate range on the meter, short the meter leads together, zero the meter.    Meters vary, so this might be a x100 or x1000 range.     Connect the leads such that one lead is touching the inner contact of the spark plug cap, and one lead is contacting the end of the wire that went inside the coil.   Early bikes (1970-1976) had 1000 ohm nominal caps/wires, and later bikes had 5000 ohms.  It is not uncommon to find an OPEN ...or very high....resistance; which is BAD.  For the 1000 ohm caps, a rather wide tolerance is acceptable, perhaps as low as 700 and as high as 3000 or so.   For the 5000 ohm caps, they should be between about 4000 and 7000.  If over 8000, replace the wires or caps.    Peculiar engine problems can occur if the caps are open or very considerably over tolerance.

(b) testing ignition coils:  This can only be really properly done with the above wires disconnected.   Measure between the tower and one of the side terminals on single tower coils, and between the towers on the dual tower coils.   For the single tower coils, the resistance should be 6500-7000 ohms.   From 1981, BMW used both single and dual coils in an electronic ignition circuit, and the coils are different.  For the single tower coils, the resistance should be 3700-5300 ohms; and the twin tower coil should be 7500-9150 ohms.   Whilst the primary windings (the two push-on terminal lugs) almost never fail, the resistances should be about 0.7 ohm for the single tower coil and 1.2 ohm for the dual tower electronic ignition coil.  LATE style dual tower coils have even less primary resistance....they can ONLY be used with electronic ignition, which can handle the current, points can not.   NOTE that just because a coil tests OK on an ohmmeter, does NOT necessarily mean it works OK when the engine is operating.

(c) Testing rotors and brushes while engine is not rotating is done by touching the meter probe tips (check zero reading with them shorted, first) to the rotor slip rings and taking a reading.  /5 rotors might indicate about seven ohms.  /6 might indicate nearly four ohms, and later and most rewound rotors just under three ohms.    With the brushes in position, measuring at the terminals marked D- (to Df) should show the same reading PLUS nearly one more ohm due to the carbon brushes.  NOTE that D- must be zero ohms to the CASE of the alternator.  I have seen folks wrongly assemble the brush holder insulating washers and if Df is shorted to the case, the alternator will NOT charge, and the GEN lamp stays ON with the key on.  Snail springs must NOT be bottoming in the holder side or end.  You can, in emergency with very worn brushes, add something to the end of the brush, between spring and brush, and that will get you a very considerable number of miles, perhaps many thousands, until you can install new brushes. 

If the rotor shows a short circuit, or the more common failure, an OPEN circuit, then the rotor is faulty.  One ohmmeter test not done enough, and I recommend this even for brand-new or rewound rotors, is to check the rotor with one probe on one slip ring and the other probe on the rotor steel.   That can even be done in the bike by lifting the brushes away from the rotor slip rings, using a piece of paper to ensure the brushes are insulated from the slip rings. Removing the D- and Df connections is not adequate for this test.   The reading must be VERY high, if any the millions of ohms.  I have seen rotors wrongly assembled by rewinders, in which a slip ring was shorted to the metal of the taper/body/etc.  

(d) Identifying relay contacts and switch contacts and/or testing testing for low resistance ON, and high resistance OFF:    First...short the ohmmeter probes together, take a reading or zero the reading.   Place the probes in very solid contact with the switch or relay or other connections in question.   If the contacts are OPEN, the ohmmeter will have an infinite, or nearly so, reading.   When the contacts are closed, the resistance value should be VERY low, a small fraction of an ohm.  Anything higher than maybe a tenth of an ohm is questionable.

(e) Testing diodes:    Power diodes are best tested using a test lamp and transformer, as noted earlier in this article.  However, most folks do not have an AC transformer setup to test diodes, and so they check the diodes with the ohmmeter function of their multi-meter (or, the diode test function of the meter, if it has such).   In general, the diode to be tested should not be connected to anything else, not any circuitry, during such a test, to avoid false readings.  Certainly the motorcycle battery MUST ALSO be disconnected if testing the Airhead diode board.   All those things being said, for the Airhead diode board, it IS possible to test the diodes (motorcycle battery disconnected) with your ohmmeter function, and the testing will show if the diode is shorted, and whether it has reasonable front to back resistance.   It is not easy to see where some of the SMALL diodes are located, and it is somewhat difficult to get your multi-meter probes on their connections.  If the diode board is removed (not particularly easy), then this is much easier.  You simply use the ohmmeter (lowest range if such is available) and measure the relative ohms in the lead connection (reverse them, try both directions of the leads) that shows a relatively low value (10 to 150 probably).  When the leads are reversed, the ohms reading will be many hundreds, if not thousands, of times HIGHER.   If the reading is near zero in one direction AND the other, then the diode is SHORTED.  If there is NO reading in either leads connections, that is relatively in the 10-150 range, then the diode is OPEN.    If the diode board is still installed, or, you wish to do this anyway, it is perfectly acceptable to REMOVE the coating (a paint) on the forward side of the printed circuit material.   I suggest this be done with a few applications of common gel-type paint remover.  Wipe off and clean off with alcohol, thoroughly, when done.  The advantage is not only being able to get test probes on the connections easily, but you can see if any of the soldering is overheating ...and fix that.   The LARGE power diodes, there are 6 of these, are of TWO types, even though they look the same.  One type is on the top row of three, the other type is in the bottom row of three.  The difference between these diodes is that the INternal connections in them is reversed.  It matters little, except that you know this.  You can test the bottom row by placing a probe on the common aluminum heat sink structure, and the other probe onto the diode connections, one by one.  Then reverse the leads, do it again.   The same thing for the upper diode row.  The ONLY difference you SHOULD see, is that the lead connections for the forward-bias condition (the condition of 10-150 ohm readings) will be reversed. The important thing is that each diode have a low forward-bias resistance and a very high reverse-bias resistance.

WHAT is this forward and reversed bias thing I discussed, just above?
A diode is an electronic semiconductor device.   A diode will allow electricity to pass through it only if the voltage is of the correct polarity, with respect to a particular end of that diode.   Diodes are marked by a line, sometimes an arrow and a line, to indicate a certain end.  Because of the polarity sensitive nature of a diode, a diode MUST be connected in the proper direction in the circuit.    This is so on the diode board, true on the diode on the backside of later model airheads headlight bucket connection boards, ETC.  Some diodes are located inside of some of the relays on your Airhead.   There are diodes used in many places on an Airhead, and more on the models from, perhaps the early 1980's.  

If you replace a diode, pay attention to that line printed on the diode!  The end of the diode that has the LINE printed on the diode, is called the CATHODE; and the non-line end, or the end with the ARROW, is called the ANODE.   In the Airhead diode board, the top 3 power diodes conduct electricity going in one direction; this is reversed in the bottom 3 power diodes.  That is why these are different part number diodes, they are different internally in the direction the INternal leads are connected.   Bosch did it this way so all the diodes could be pressed fitted to a heat sink, which is the mounting bracket aluminum pieces.    In your Airhead, these particular 6 diodes are used to convert the A.C. coming from the 3-phase alternator, to D.C., and the diodes are arranged so the output is D.C. in the + going direction, so that can go to the battery + terminal, for charging.  

The ohmmeter contains a small battery, with some resistors, etc., and internal meter connections, that are so designed that a small voltage with a small current is available from the ohmmeter test leads.   This is applied to the diode when you are testing the diode.   Some ohmmeters have the + voltage on the red + lead, some have the - voltage on the red + lead.  The owners manual may not tell you which is which.  It is of little importance unless you were doing testing of such as of transistors (yes, an ohmmeter can test most of them) or of diodes.  When you connect your ohmmeter to a diode, if the connections are for the 'forward-bias' condition of the diode (negative voltage to the LINE printed on the diode, positive voltage to the other end), then the diode will CONDUCT electricity, and your ohmmeter indicates a low value.   The diode does NOT begin to conduct unless the voltage from your ohmmeter is at least 0.5 volt.  Thus, the ohmmeter will apply more than that voltage, to be sure the diode conducts (except on some very $$ ohmmeters, discussed near the beginning of this article, which are NOT useful for testing diodes on ohms functions).   If the leads are reversed, the diode does NOT conduct (unless faulty) and the ohmmeter will indicate a very high value, if any.   If your meter has a diode test function, in one direction of leads the meter will probably have no reading, and in the other direction, perhaps 0.4 to 0.8 volt.  In the diode test function of a meter, the circuitry inside the meter is very similar to the ohmmeter circuitry, except that the meter is so calibrated to read the voltage drop in the forward (conducting) direction of the diode.  Testing a transistor with an ohmmeter is more involved, and not dealt with here.  Some multimeters have a transistor socket and thus a transistor test function, which might not be all that clear on how to do the testing ...and usually the transistor must be removed from the circuit board.

(f) An ohmmeter or continuity tester function can be used to check light bulbs (one lead to metal body, one to the base contact; if two base contacts, one at a time).

(a) Voltmeters measure voltage which can be AC, DC, or a mixture of AC and DC.  Analog voltmeters require more current FROM the circuit under test, to enable a reading.   Digital types generally have such a high input resistance in the meter itself, that they don't 'load' the circuit under test enough to make a difference.   For MOST airhead functions, even the analog meter's loading is also NOT of ANY importance.   Taking voltage readings can be confusing if there is DC and AC mixed.   I won't get into that here, as it has no serious applications in work on your Airhead.   PLEASE NOTE, as this is often very much misunderstood, that a voltmeter connects ACROSS a circuit, NOT in series with it. You would NEVER want to connect a voltmeter in series with a wire or circuit.  DO NOT open the connection at the battery negative wire (connects to speedometer cable boot/breather bolt area) and connect a voltmeter between the battery negative terminal and the chassis.  Do NOT do that! won't injure anything, but it DOES NOT give you ANY meaningful information.   This has confused many, especially since some literature tells you to do it.

Voltmeters can
measure voltage drops DIRECTLY.   This is one of those things that few seem to know and understand.   Many times when diagnosing a motorcycle electrical system, the voltage at one place is not what it is supposed to be.  Not just totally missing ...but maybe a bit low.  Perhaps, in the simplest thing we see OFTEN, the in-dash voltmeter measures LESS than at the battery with your accurate digital meter (0.3 difference is approximately normal).   So, if yours is over 0.5 volt difference; ...WHERE is the problem, and HOW to find it? You want to fix this because an excessive voltage drop means eventual troubles.  You can FIRST check with your digital meter at the dash voltmeter leads themselves, see if the dash meter has an error.  Probably little, although the dash meters can and do fail.  

What the typical person does to read voltage drops to connect the meter black lead to the chassis, and the positive red lead is touched to various points.   As an example, the battery measures 12.52, and you find 12.07 in the headlight bucket, lights on. You subtract the readings and get 0.45 .   As you probe 'down the line' from the battery, to switch, to circuit ...or whatever the job are always subtracting different values.  Why not do it MY way, and read the drop DIRECTLY!  Put the positive (+) lead of the voltmeter on the battery + terminal (that's the terminal on the left), and use the negative voltmeter lead to probe any place you need to.   The meter will measure the voltage drop directly from battery + to any point in the system!  Another test with the meter is from the negative battery terminal to the chassis; measuring the grounding battery lead voltage drop (I often do this whilst the starter motor is cranking to check integrity of the thick negative battery cable).  Easy, EH?

(c)  A digital voltmeter can let you check your voltage regulator (and adjust, if yours is adjustable) by measuring at the battery.   14.25 volts is a nice value ...the stock settings or calibration is often too low at maybe 13.8.

An ammeter, or ampmeter, is a CURRENT measuring device, not a voltage measuring device. 
Please note, as this is often very much misunderstood, that a ammeter, when properly used, is connected IN A SERIES circuit, NOT EVER across or in parallel with a power source.  You would NEVER want to connect an ammeter ACROSS a battery, as one example.  You can burn out meters by overloading the current section. Many meters are NOT FUSED on the AMPS functions, particularly the highest range (typically 10 or 20 amperes), which may, on many meters, be on a separate meter probe jack.   Do NOT EVER apply the test leads directly across a battery or power source, when the meter is set for current (amperes, milliamperes, microamperes) functions.  

Ammeters are connected IN SERIES with a device, to measure the current drain of the device; or the current flowing TO AND THROUGH the device or section of the bike's electrical system.   An example here is when you have a battery that seems to get low rather quickly, in short term storage of the bike.  Disconnect all wires to the battery negative terminal, but if only one wire wire is there to the chassis ground, then disconnect the wire at the chassis or battery, your choice.   Connect the negative probe of the meter to the battery negative terminal and the positive terminal of the meter to the chassis grounding point.   With ignition and lights, etc., OFF, turn on the meter, on the highest DC current range to start with.  On many of these meters there is a separate panel jack for the highest current range. Start with that range.  If no reading, then try the other regular current jacks.  Progressively increase meter sensitivity to see what the offending current is.   If there are multiple wires at the negative terminal, add those wires, one by one, to the meter negative wire; a good practice is to again, start on the higher range.   See if one of those wires shows where the offender is.   BMW does not, when stock, use more than the one large grounding wire at the battery negative terminal. 

HINT!   There is ONE exception, when the factory type of power socket is installed, and its grounding is to the battery negative terminal.  IF you have such, I HIGHLY recommend that you permanently connect it to a FRAME grounding point and NOT the battery.  I recommend that for aftermarket accessories too.


HINT and a bit nerdy!     I have explained how to connect the ammeter (using a "current" setting on a multimeter, for instance), when trying to trace down an annoying battery drain.   Here is a bit of a hint, known to some few professional auto mechanics.  Techs are rather often called on to find out why there is a key-off battery drain that is excessive.   In some instances, IN PARTICULAR where the key-off drain is rather high, amperes for instance, if you disconnect the battery in order to insert a series-connected ammeter, that MIGHT "FIX" the problem, temporarily, and defeat your testing, at least at that time.    The best check, to start with, is a CLAMP-ON type of meter.  These are sometimes called inductive amp clamps.  This meter tool must be reasonably sensitive to see modestly small drains.  If you have such a meter, use it first; if you do not, you will have to use the disconnect and connect in series method.   Inductive units that plug into a meter, 30 amp type range, are such as the model CA30MA from Also see ESI's clamp part number 687.
NOTE:  such $$$ tools are for the professional, due to their cost.  MOST cheaper clamp-on meters only measure A.C. current.

There are not a lot of other places you really would use an millameter/ammeter in an Airhead.  Another article on this website discusses whether or not installing an ammeter permanently is wise, or not, and presents both sides of an ongoing now and then argument.     The article is

See Ohmmeter testing of diodes, above.   Additionally, most multimeters that do have a diode function will actually read the forward voltage drop (some do diode testing in other ways).   The probes are placed across the diode in first one direction, then the probes are reversed.   See YOUR meter's instructions for testing diodes (and transistors).

Release:  03/19/2007
10/10/2008:  edited for clarity
11/15/2009:  addition clarifications and cleanup
12/10/2010:  add omega symbol
06/22/2011:  Clean up, and do some updating/editing for clarity
06/23/2011:  Add photos of the 22-810 meter
10/05/2011:  Rework some areas of code, so displays OK in Firefox; previously it had heavily broken sentences where original tabs were used.
05/14/2012:  Clarify some details
10/10/2012:  Add QR code; add language button; update Google Ad-Sense code; clean up article a bit, fixing minor possible confusing points.
03/08/2013:  Update Google coding a bit (move 234 size mainly) plus minor stuff.
03/14/2013:  Add information on using clamp ammeters
In 2013, remove the language button, as the javascript was causing some browsers to hang.
08/11/2014:  Update first ~third, and add photo of the Harbor Freight meter.
03/08/2016:  Update meta-codes; format; narrowing, remove many hard line breaks; clarify some details.
09/06/2016:  Clean up metacodes, scripts, H.L., html, layout.  Improve clarity of some areas.

Copyright, 2014, R. Fleischer

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Last check/edit: Monday, February 27, 2017