Multi-meters:
a newbie guide
By lala
Disclaimer:
I am not responsible for the actions of individuals who use this information
for any use whatsoever. This is only a guide, and as such, it is your own
responsibility with whatever you do with it. So, if you mess up, it’s not my
problem. EDUCATIONAL USE ONLY.
Overview:
When working with a vehicles electrical system, a multi-meter is a great
tool
to use. It can help you find problems one can’t normally see with the naked
eye. But without understanding the basics of how one operates, one can find
great difficulty in operating such a device. In this day and age, we have
various flavors of multi-meters, but they generally boil down to two kinds:
Analog and Digital.
For simplicity, DMM stands for Digital Multi Meter, and AMM stands for
Analog
Multi Meter.
Analog:
An AMM is one you can find with a mechanical dial and a printed face. It
uses
discrete, solid state components to generate a result that you can read on
the
dials face. One disadvantage of this is parallax error. You’ll never really
read an exact number because of the angle of your eyes relative to the
needle.
But, these little guys have no computer chips in them. You can probably
totally
rebuild/repair one from parts at Radio Shack or you favorite electronics
hobby/industrial store. For vehicle repair, there’s nothing wrong with using
one, as long as has the limits/settings you need for your situation.
Usually,
they always do.
Digital:
A DMM is one you can find with a digital readout. No needle. Some can give
you
graphs, others just numbers. Some really expensive ones can connect to your
pc.
They can be as precise up to (but not always including) the last digit, but
are
much more complicated in construction. Usually, they are hard to break by
accident, but even stupid abuse can destroy one of these, and you’re not
going
to the Radio Shack for the parts. Not that easy, unless all you need is
leads.
Great tool, once again, only limited by what settings it offers. For
vehicles:
awesome.
Meter Settings: Overview
First and foremost: know your meter. Read the manual. Ask the store clerk if
unsure about anything. The best way to dive into this is by reading the
literature provided by the manufacturer, and/or getting a professional to
give
you advice. This guide can help you too, but don’t assume anything until you
know for a fact.
Usually, a MM will have a big knob or dial on it. The dial will let you pick
various settings such as AC volts, AC current, DC volts, DC current,
Resistance
and possibly hfe. (It may not show all of these,
so
when shopping, make sure you get the one that has what you need.)
The biggest number in that section is pretty much what the meter will go up
to
for that measurement at that setting. So, if you’ve got your meter set to 20
VAC, don’t try to measure the 120 volts coming out of your wall! Always set
the
meter to a setting higher than you’d expect for voltage and current
measuring,
and then turn it down if you need more accuracy (if you should). Also, use
the MM’s correct setting for the application. Don’t use AC
for
DC sources and vice-versa. Checking for resistance in a live a circuit, or other stupidity is a no-no too.
Meter Settings: Leads and other stuff
Ok, so you want to measure something, but how? First thing to know is what
the
lead colors mean. This is determined by your setting. Example, for DC and
resistance applications, red means +, black means -. For AC, red is hot,
black
is neutral.
Because this is a guide geared for automotive applications, one does not
need
to use the AC settings. Most MM’s are usually
tuned
for 60hz AC applications, and wont do you any good.
What most people need is DC volts (anything over 15 volts), and resistance
(anything up to a Mega-ohm). DC current can be used, but only for those who
know what they are doing. I’ll explain more about this later in the
document.
For those wondering hfe is used for measuring
properties of transistors. Usually, a MM with hfe
has
a small “pod” with several holes in it marked C, E, or B. This is not
required
either, and is beyond the scope of this document.
A MM will also have a battery. Make sure it’s always good. An AMM only needs
a
battery for resistance, but a DMM wont work at all without one.
MM’s have fuses. You’ll find one if you open it
up.
Make sure its always good if your meter’s acting
strange, or not reading at all. If the meter lead
holes say “un-fused” that means its not using the fuse provided. Use at your
own risk. Don’t defeat the fuse in the MM either. It is there to save your
MM
and your life.
Finally, as a note of safety, keep an eye on where you trail with your
leads.
Beware of spinning fans, hot surfaces, places to pinch the wires, etc. Also,
don’t use your probes as screwdrivers, prying devices or toothpicks. Bad.
Meter Settings: Resistance
For checking wires: use the smallest resistance setting possible.
This setting is found usually with a small arrow pointing at a vertical
line.
Sometimes it has some round lines that look like comic book motion lines.
That
means it should beep when it finds continuity (the resistance it is
measuring
is that similar to a good wire).
Resistance is measured in Ohms. The, more resistance something has, the
higher
the ohms. A wire can be considered (for our purposes) anywhere between 0 to
2
ohm or the needle should go all the way up. Even
the
beep will work. Anymore than that, you found a problem. Anything else you
measure depends on the object.
Something that doesn’t usually conduct (like an insulator) is said to have
“infinite” resistance. Its just higher than what
the
meter can read at the max setting. An AMM will show no needle movement,
where a
DMM will continuously show a “1”, basically like you did nothing.
Everything has resistance, including you. Depending on humidity, and other
things you could be sitting at 600 kilo-ohms to a few Mega-ohms. This is
pretty
high (and good). If you want to check, turn your meter at a high resistance
setting, and hold a lead in each hand. You’ll get something. You can even
try
it across your tongue. It will be less than your hands/chest.
This is because the meter is actually applying a minute charge across
whatever its touching, and measuring it. This charge, keep in
mind,
is polarized with respect to the leads. It’s so small, it will not damage
any
components (or you), unless you were to keep the circuit powered while
measuring.
NEVER measure for resistance while your circuit is alive. You can damage all
sorts of things and make a real mess.
Also, make sure you never touch the leads with your skin. Putting your body
into a circuit while measuring for resistance can give false results. This
happens when your body becomes part of the loop. If you have to touch a
lead,
try to touch only one, and keep the rest of you as isolated as possible.
Alligator clips work wonders for this issue. Don’t be afraid to splurge an
extra $5 on ‘gators when shopping for a meter!
Meter Settings: Voltage
When measuring voltage in a vehicle, you want to
try
to set it over what you’d expect. A car battery will give you 12 volts
alone, but
while the engine runs, you’ll find a few more volts over that. I usually use
my
20vdc setting, which is always enough for me. One way to measure is by
attaching the black wire to the negative terminal of the battery, and poking
around with the red one. There are others, but the idea is to have positive
on
red, negative on black, or you’ll get a negative reading. On AMM’s, the needle goes backwards – try not to do this
too
often. Of course, normally, when measuring voltage, you need the circuit
powered.
Meter Settings: Current
When measuring current, there are usually two ways
about it: clamp or series. When using a clap, just set the meter, and put
the
clamp over the wire. It doesn’t have to touch it, and you get a reading (on
the
right setting). This is good if you got a clamp. Otherwise, one would have
to
break the circuit, and attach the MM leads (in the right polarity!) at the
breaks of a circuit, re-completing the circuit with the meter as a part of
it.
Now power the circuit, and you get a reading. This is usually not needed for
vehicle applications, and is not recommended because most MM’s
usually don’t support anything over 10 amps DC, un-fused. Unless you know
your
meter can handle what you should get, don’t bother. A good guide is to use a
setting higher than the fuse in the circuit is rated for.
Checking for Ground: A Hunters Guide
There are two ways to check for that pesky bad ground
:
by voltage and/or by resistance.
Resistance checking involves one lead to the (–) terminal and one to the
ground
in question. You should get a reading such as that of a good
wire. If not, you found a problem.
Voltage checking involves the red lead to the (+)
terminal, and poking around with the black lead. Do this while
powered
and be careful. You should get the battery voltage or running voltage
depending
on what you’re doing. If not, another problem is found.
Some ford systems will give you the usual “click-click-vrrr”
when you hit a bad ground, and replace it by using your meter as a
load/connection to ground for that intermittency. This is good. You found a
problem this way. Also, when checking the block, make sure the surface is
clean. You want good contact at all times when measuring with the
probes.
Rectifiers/Alternators
Rectifiers are a small device usually found in alternators and are used to
“rectify” alternating current into direct current (AC to DC). When damaged,
a
rectifier can discharge your battery overnight, and possibly damage your
battery while you’re running the vehicle (by feeding it ac, but it only
likes
dc).
To check this, remove the component from the housing, and set the meter to
about 2000ohms. Now, test between every lead, one at a time. When
testing
a pair, one way you get a few hundred ohm (low resistance), where if you
reverse
the leads, you’ll get much more than that (high resistance).You might have
to
increase the setting on the meter, but you’ll get something. If you get 0-3
Ohm, or a 1 (“infinite” at max setting), its bad,
toss
it, and replace it with new. Rectifiers cannot be repaired, unless you feel
like wasting a lot of time and money.
An alternator has two parts to test: field coils, and the rotor. The rotor
is
the shaft that spins. The field coils are the windings surrounding the
rotor. A
resistance measurement across the field coils will tell you if they are bad,
and you should get only a few ohms from this (not necessarily 0, but under 7
or
so ohms). The same goes across the brushes, brush leads, and the rings they
ride on. These rings are attached to the windings on the rotor, and you
should
not have an issue getting a low resistance reading.
If you’re unsure about any of this, a simple load test for the alternator
will
clarify this. These tests are usually free at your local Autozone,
Canadian Tire, or various auto-part megastores.
The
employee will apply a voltage to the field coils, and spin up the
alternator. A
load is applied to the alternator, with some meters and basically power
output
is measured. A resistance check will tell you almost the same thing: whether
it
should work or not, but only at an electrical theoretical standpoint. If the
alternator has mechanical issues (bearing/bushings etc), spinning it up will
show that. But so will spinning it by hand ;). I personally have never had
the
need to take in an alternator for a test. I find that simple inspection of
the
device will tell me if it’s adequate, assuming its stock, and designed for
that
much power delivery. If there’s an issue, my meter always found it. I would
only,(in my opinion) bring it in to know HOW MUCH
it
capable of actually delivering,!
a thing a meter cannot tell me alone. But, when it
doubt…
Relays
A relay is an electromagnetic switch. It is a way
for
one circuit to control another circuit with electrical isolation.
Remember when you were a drooling little kid? There was an old trick where
you
wrapped a lot wire around a nail, and hooked the leads to a battery? Made a
cool little electromagnet?
A relay has the same component, with a reed switch in the magnetic field.
When
energizing the coil, the switch will either close (turn on) or open (turn
off).
To know which one, the leads are marked Common, (
C ),
Normally Open (N.O.) and Normally Closed (N.C.). When the relay is
off,
the resistance between ( C ) and (N.C.) is that of
a
wire, where between ( C ) & (N.O.) should be infinite. When energized,
the
states change, because the switch has flipped.
Two other leads on the relay are used for the coil. You should always have a
few ohm across that, sometimes even 1 ohm.
Otherwise,
punt it.
Make sure that when the relay is energized properly, you hear a click.
Putting
your finger on the casing while energizing is good too, and you should feel
it.
No click? Punt it. If you have to give it a tap to click, punt it.
People have asked “If it looks like a wire won’t it be a short?” Well, if
the
wire is wrapped in a circle, it creates something called an inductor, where
the
power it absorbs gets converted into electromagnetic power, or “flux”.
Hence,
the device is no longer a short but something that can act like a load.
Starters
A starter has two parts to test: a solenoid and a
rotor. Sometimes it may come with field coils, but those are rare. The rotor
will have several sections the brushes ride on the rotor, all leading to
back
to its counterpart, the section 180 degrees away in question. Check those
windings by resistance, just like the alternator. If one’s burnt out, you’ll
know it.
The solenoid is just an electromagnetic coil/switch, like a big relay. Check
for continuity across the solenoid. Only a few ohms expected here too, or
punt
it.
When the solenoid arm is pushed to the “active” position, a connection is
made
between the battery terminal post on the starter and the rotor itself. Make
sure that’s good too. Anything else is mechanical. That can be tested by
running the battery across it out of the engine. All you need to do is spin
it
up momentarily. The pinion gear should jut out and spin like crazy, then
retract after you kill the power. Don’t let it spin too fast! A starter
motor
is wired up in series and it WILL spin itself to death if it has no load to
go
up against!
Once again, if unsure, go get it check at a megastore.
They usually do starters for free too.
Shorts
A short is when a wire is connected directly to
ground
(or power) from the opposite source. No wire should have little or no
resistance between + and – unless there is an inductor involved. Best way to
check this is by checking the resistance of a source wire, detached from the
load, un-powered, and measuring against the opposite source. So, if you want
to
check if a + cable is shorted, read the resistance between the wire and
ground.
It should be infinite. Otherwise, there’s the short.
Batteries
Batteries are hard to test with a meter. You can charge one, get voltage and
bright lights, but not turn over a starter because of a lack of amps. The
results with a meter can be deceiving. The best thing to do it is just get
it
load tested at the store. Otherwise, all a meter can tell you is if a battery is providing voltage. Don’t check the
resistance across the posts, and DON’T check DC current across the battery
with
the meter alone! Really, really bad….
The End?
This guide was to clarify some questions newbies
tend
to ask about meters. It is not finished, and I don’t ever plan to. I instead
hope that other users, at the discretion of the moderator will add to this
guide with hints, tips and tricks. The moderator(s) may edit/add to this
guide,
but all I ask is that all those involved have their name on it.