How to buy & use test
lamps and meters (multimeters,
volt-ohm-ammeters, ETC!)
14 multimeters.htm
Purpose and recommendations:
The purpose of this article is
to explain how to purchase a test lamp and a meter; a small amount on how they
work. The major intent is to inform you as to how to use them on your Airhead
motorcycle; various hints and tricks & troubleshooting using these devices.
I
HIGHLY recommend that you read certain other articles on this website before,
and after reading this one...so as to gain a good working knowledge of your
airhead with regards to troubleshooting problems. Do not take this
note lightly!
As you read this article through completely, it may
raise questions; or may fill in some blanks, in what the other articles
say. There is an entire larg section on electricals on this
website. However, the MORE pertinent articles, for use with THIS one, are:
14A; 14B; 15; 15B; 16B; 28; 33. PLEASE read all of them!!
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 even at home. Sometimes 'simple' technology is very good!
You can make a test lamp with most any small low power 12 (approximate) volt 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 too low in some situations. I
recommend that you purchase (about $3)
a commercially made test lamp probe. Below is a photo of two of my own
commercially made test lamp probes. Either one would fit in your Airhead tool tray if you wanted
to take one along with you. Note that both of these test probes
contain a sharp tip of a contacting 'probe' end. It is best to keep the
tip structure 95% covered with
something like electrical insulation: 'electrical spaghetti'.. or 'shrink
tubing'...to avoid inadvertent short circuits, let alone 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, and whilst those
battery types ARE available, you should NOT purchase that type. As
noted, ...on many of
these probes, the metal tip end of these is long and bare metal. To avoid
causing an electrical short-circuit to something next to your testing area, cover
the probe metal nearly to the sharp tip, with some shrink tubing.
In the above photo, I have removed
the protective 'spaghetti' over the sharpened tip/body
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, a cylinder fin
(one of my favorite places). It might be a ground lug of the frame or anything
else; whatever is a good grounding point and CONVENIENT, yet out of the way of
hot exhaust pipe, ETC. BMW uses solid
brown 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.
Unless otherwise stated, it is ASSUMED HERE that you have the alligator clip
connected to a solid ground; but not a brown wire that might be iffy in its own
connection to ground (some care needed...if you DO use such).
There are many things
a test lamp can do. Here are a few:
1. 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...another reason for some 'spaghetti' over much of
the metal tip shank.
You can probe the contact connections at a switch to see if
power is going to...AND through it. Switches are used, basically, in one of
two ways, depending on how the switch is wired in the motorcycle:
(a) Power goes to the switch, and when the switch is turned
on, power is supposed to go through the switch. 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.
(b) Power goes to whatever is the item or device, and the GROUND
side of the device goes to the switch, and when the switch is closed, the item
or device is powered by the circuit being completed. Horns are often wired this way.
With the switch
in the grounding side of the device, 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. 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.
(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, so we begin by assuming the pod lamp
and its pod connections are OK.
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...if the GEN lamp then lights, the rotor is open. 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.
(d) In a few rarer instances you may want to apply a
small current, not the entire battery current availability, to something.
The lamp probe can act as a series resistance. You can also do this with
an AC transformer on your workbench, to identify bad diodes in the diode
board....a more vigorous test than an ohmmeter testing of the diodes in or out
of the bike.
(e)
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). 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,
there should be current coming from the battery. 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 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 is faulty. Additionally, if you suspected
a bad battery, you could probe the + (red wires) left-end terminal of the
battery. The lamp should light up under any condition of engine on or off
or starter use or lights. If the lamp did not light up (assuming
chassis grounding for the lamp alligator clip), you could put the lamp alligator
clip onto the battery negative terminal and probe the + terminal again. If
the lamp lights up, but not properly if the alligator is on the chassis (say, a
cylinder fin), then the battery negative cable is defective, just another way of
doing what was described previously. 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 the battery. 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.
On 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.
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.
These types of tests are
THE prime use for a test lamp probe....is power, full power, someplace it should be when it
should be??
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, top gear, slow jerks in forward direction...), the EXACT place where the
ignition points JUST BARELY start to open, is the ignition timing, normally done
at idle with a strobe, on a
stock Airhead (S mark on 'flywheel'). You can determine this point quite
closely by touching the probe to the points connection or
capacitor/condenser...perhaps with another
short jumper wire...or, put probe tip in a nearby case hole, and the alligator
clip to the points. Rotate engine VERY slowly, in very TINY
amounts. When the lamp glows, the points are open. The reasoning is
that power goes from battery to ignition switch to ignition coil (or two coils) and through
that coil (or two 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. Same
idea as the two-terminal horn I described previously....in this instance, the
points are the 'switch' to ground. With the points open
and the test lamp glowing, you now rotate the tire backwards
a small amount 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. DO
NOT use too much force on that 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 a POWER
diode....such as the ones in the diode board. While a preliminary
test can be done with an ohmmeter, to measure forward and reverse resistance,
the 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 volts, 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 (needle moves above a numbered
tick-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 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 ohmmeter section batteries, typically a common AA of
1.5 volts, and perhaps a common 9 volt battery. That enables the analog
meter to measure very high resistances. Some digital meters require more
than one battery too. Don't leave your analog meter on the ohms
range...if the leads should short, the battery will slowly drain. Don't
leave the digital meter turned on for excessively long periods...that drains its
battery too...whether on ohms functions or not.
These analog and digital multi-meters are generally used for the same type of
testing as test lamps, PLUS they are capable of providing much more
information. You may NOT need such 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 'exact' voltage drops, current flow, and so on.
The photo, below,
shows three types of common meters. ALL ARE CHEAP. There are hundreds of types like
these. At the upper left is a RadioShack analog VOM (volt-ohm-milliameter).
Milliameter means it measures thousandths of amperes. While it also has ampere
scales, it is not called a volt-ohm-ammeter but a volt-ohm-milliameter simply
from conventional practice. This is a type of meter where YOU select the RANGE of the meter (not
just its function). This meter measures AC and DC volts and DC
current, and has some additional higher current ranges, up to 10 amperes (full
scale reading); and a continuity test function (internal beeper tone).
This is the meter I use all the time on my workbench. I've
managed to NOT break the glass, nor ruin the meter from overloading it, which
sometimes surprises me, as it has been in use for many years.
To its right is
another inexpensive meter, but more than adequate. This one is
a DIGITAL meter (meter is off, display is blank in photo), that measures the
same sort of thing as the analog one to its left, 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 meters complete 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.
The lower meter is a
RadioShack meter. 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'. 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 a long life mini-cell battery. The one
illustrated does not measure current. It's primary drawback is the
short leads and no current measuring (not need very often anyway).
What about that 'continuity' testing
function on some meters? You COULD use an ohmmeter, it will tell you
MORE about a continuity (connection-through...), but for quicky 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 meters 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.
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 fairly rugged. Analog meters are NOT accurate
enough to measure or set charging voltages. Many are not very sensitive
either.
Digital meters that are under $35 are all that the average
person, and even a shop needs. Meters with over 4 digits 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 the ohms function to test diodes.
HOWEVER>>>>some rather
expensive digital meters do not put enough current (or voltage is too low) into a
diode you want to test, to 'turn the diode on'. 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. In the other direction, the reading will be perhaps 10 to 50
ohms. You may have to rezero the meter for each meter range. Meters vary in what they indicate in that 'forward biased'
direction. You
really do NOT, and for reasons stated, may not work properly, some fancy $$ meters like those made by Fluke.
The digital meters you can purchase under $30 are usually more than rugged
enough and accurate enough. I've seen meters that were on sale at under $20 that
were just fine.
How to
use a multi-meter:
Note: On an analog meter,
you must turn a knob to set the function, and the function range. You also
may have to select certain pin jacks. Typically the meter scale reads 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. Thus you might set this knob for
DC volts, 25 volt range (measures zero to 25 volts). These meters tend to
be more accurate at higher readings. It is generally NOT that
big a difference, until you are reading a low value where the needle is hardly moved off zero much
at all. 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 of these meters have a screw to adjust the needle to the zero
point, mechanically. Be sure to check the meter, in its
physical position you use it at, for zeroing.
BE
CAREFUL in using a meter on the CURRENT ranges. Current is NOT to be
measured from a power source to ground! Current is measured
with a series connection (the meter is connected between the power source and
the device; or, between the device and chassis ground). Failure to follow
this advice can either blow a meter's fuse (if it has one) or ruin the
meter! In the same way, you must be careful about the
OHMS ranges. NEVER have ANY power connected to the device you are testing
with the ohmmeter function!!
Many digital meters are autoranging....you select
the function, the meter automatically selects the range. There is no great
advantage or disadvantage, but it CAN make the meter more compact, with many
fewer settings for its switch. NOTE that if a meter has a multiple
selection switch, that switch can get old and tired and have its own contacts
problems. For that reason alone, many prefer the digital meter.
(1) OHMMETER FUNCTIONS:
An ohmmeter measures electrical resistance by having an internal 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 redo this
adjustment as you change RANGES. On most digital meters, there is no such
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 negative
voltage, not positive, which is generally only of importance if you are using
the ohmmeter to test a diode. 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. Apply 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. 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 had 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 coil.
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...this 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. NOTE that D- must be about 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 distance...even back home where you can
install new brushes. Articles on doing that are on the Club website and on
this website.
If the rotor shows a short circuit, or
the more common 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 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- connection is not adequate for
this test. The reading must be VERY high, if any.....in 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 as in (3) of the test lamp section, well above. 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. 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 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, 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 50
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 10-50
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 10-50 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 device, a semi-conductor
device, something like part of a transistor. A property of diodes as
used in our Airheads, is that when used in a circuit to which D.C. voltage is
applied, that the diode will conduct electricity 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. 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 inside of some of the
relays on your Airhead. If you replace a diode, pay attention
to that line printed on the diode! In the 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 leads are
connected. Bosch did it this way so 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 + direction, so that can go to the battery
for charging.
Your ohmmeter contains a small battery, with some resistors,
etc., and meter connections, that are so designed that a small voltage and 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 are the more nerdy type like me. If you were doing
more serious testing, such as of transistors (yes, an ohmmeter can test them) or
of special diodes, it might be necessary to know. Connecting another
voltmeter to your ohmmeter will tell the story.
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....the line end is officially called the CATHODE), then the diode will
CONDUCT electricity, and your ohmmeter indicates a low value. The
diode does NOT begin to conduct until, or unless, the voltage from your ohmmeter
is at least 0.5 or maybe even 0.7 volt. Thus, the ohmmeter will apply
considerably 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.
(f) An ohmmeter 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).
(g)
(2) VOLTMETER FUNCTIONS:
(a) Voltmeters will, of course, measure voltage
directly. This can be AC, DC, or a mixture of AC and DC. Analog voltmeters
require, generally, more current FROM the circuit under test, to enable a
reading. 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
normally in work on your Airhead.
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, the analog meter's
loading is also NOT of ANY importance. 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!...it won't injure anything, but it doesn't give you ANY
meaningful information. This has confused many.
(a) Voltmeters can measure voltage drops DIRECTLY: This is one
of those things that few seem to know, and understand. Many a time 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...WHERE is the
problem, and HOW to find it? You can check with your digital meter
at the dash voltmeter itself, see if the dash meter has an error. Probably
little.
What the AVERAGE person does to read drops .....is to use the meter with its negative lead to the
chassis, and the positive (usually colored RED) lead is touched to various
points. To find a 1/2 volt drop, you may have to try to subtract a reading
of 11.7 from a reading of 12.2. As you probe 'down the line' from
the battery, to switch, to circuit....or whatever the job requires....you 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 one on the left), and use the negative voltmeter lead to probe any place you
need to. The meter measures the voltage drop directly at any point
in the + lead system!
(b) A voltmeter can let you set your voltage regulator
(if yours is adjustable) by measuring at the battery after the engine is warmed
up and the battery is fully charged. 14.25 volts is a nice
value....the stock settings or calibration is often too low.
(c)
(3) AMMETER FUNCTIONS
An ammeter 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 in parallel with a power source. You would NEVER want to connect
an ammeter ACROSS a battery, as one example. You can burn out some meters by overloading
the current section. Do NOT apply the
test leads directly across a battery or power source, when the meter is set for the 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..
(a) There are not a lot of places you really would use
an 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
amp&coltmeter.htm
(b) Once in a great while someone wants
to install an ammeter to measure, temporarily, the current drain out of the
battery, perhaps with engine off or idling. This can be done in the
battery negative or positive terminal (easier at the negative wire), but the
engine needs to be started, and some ammeters have too high a resistance to
allow that. A full treatment of this is beyond the intent of this
article. See the hyperlink, above, for more on this.
(c)
(3) DIODE TEST FUNCTION:
See Ohmmeter testing of diodes, above. Additionally, most
multimeters that 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.
Release: 03/19/2007
Revisions:
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