HOW tO uSe a multimeteR G

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WORKSHOP
Multimeter Masterclass
How to use
a multimeter
This essential bit of garage
equipment has more uses
than you might think…
Words by Dave Rogers Photography by Gez Hughes
G
et a good multimeter and it’ll be
one of the most versatile pieces
of kit in your classic car toolbox
armoury. If you don’t have one
then you should get one! They’re not
expensive and good basic digital meters start
from around £20.
The question is, how do you use it
productively and what can it tell you? This
beginners guide will help you find your way
round the most common features of a
typical digital multimeter, how to make
common measurements and how to
interpret the readings. So read on and
become a multimeter master.
What to pay
■ Less than £10: Check it’s got DC capability
and ranges that will be relevant to the sort of
thing you’ll be checking on classics.
■ £20-£50: Decent range of auto features.
■ £50 plus: Full range of features.
Bear in mind that the more functions a
multimeter has, the easier the method to
switch between them has to be.
Voltage (DC)
1
Select sockets
Voltage is a measure of the system
‘pressure’ – you need this to push the
current around. To measure it, connect the leads
to the pictured jack sockets. Select the range
– between 10-20 volts. If the meter is capable
of auto ranging, just select DC volts.
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2
Connect leads
Now connect the leads. For negativeearth cars, black lead to a good earth,
red lead to the test point. For positive-earth
cars, it’s the other way round. Here, we’re
measuring across the battery.
3
Voltage reading
You’re now connected, so you can
observe the reading. The display will
show the voltage potential at the test point. The
test point should show a voltage reading similar
to battery volts.
DECEMBER 2010 CLASSICS MONTHLY
69
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Resistance (Ohms)
4
Correct sockets
Any current-carrying conductor will
have some resistance to current flow.
This is measured in ohms. To measure the
resistance of the primary circuit of a coil, remove
and/or disconnect the coil. Connect the meter
leads to the jacks for resistance measurement.
Diode check
5
Dial it up
Select ohms on the dial. If necessary
select the correct range – for an
ignition coil, the reading will be quite low.
6
Measure resistance
The meter applies a voltage to the
component and measures the current
flowing through it. From this, the resistance can
be established. The reading for our ignition coil
is 3.3 ohms, which is correct.
Current (DC amps)
7
Let it flow
Current is electrons flowing down a
cable. The unit used is amps. Nearly
all digital meters can measure up to about 10
amps. This is enough to check smaller devices
– we’ll measure the current to the blower motor.
The meter must be connected ‘in-circuit’.
Sound the buzzer
This enables you to establish an open
or closed circuit between the meter
probes – useful for switches and suchlike. If
there’s a closed circuit, the buzzer will sound;
open circuit equals no sound. Select ‘continuity
test’ on the dial.
70 CLASSICS MONTHLY DECEMBER 2010
Set the dial
This can be useful for jobs such as
checking rectifier packs employed in
alternators. To do this, the rectifier must be
removed from the alternator itself. Connect the
meter leads to the correct jacks and select
diode test on the dial.
14
One-way street
17
Connect leads
A diode is an electrical one-way valve,
so the meter works similar to
continuity-check mode – ie, you check the diode
to see that current flows in one direction only.
15
Swap it around
18
Fire it up
For each diode, connect the meter
leads one way then the other to
ensure that you get open-circuit and closedcircuit readings respectively.
DWELL
8
Set the range
Set the meter range to amps – highest
range first if not auto ranging. Connect
the leads to the correct jacks on the meter. Note
that often there’s a separate jack for measuring
current at 10 amps.
9
Feel the energy
Energise the circuit and note the
reading on the display. For lower
current readings, most meters are capable of
reading milliamps (one thousandth of an amp)
with the appropriate range selected.
Continuity
10
13
16
Select range
The dwell reading relates to the
amount of time the contact breaker
points are closed. This indicates if the points
gap is correct without having to remove or
disturb the distributor. Variation in the dwell
reading with speed indicates a worn distributor.
The multimeter leads are connected
to the coil negative/points side and
earth respectively.
Tachometer
11
Test it first
Before going any further, test the
meter – join the leads together and
make sure the buzzer sounds, or the meter
reads ‘zero’.
12
Do it for real
Connect the test piece. If the circuit’s
completed, the buzzer will sound and
your bulb, fuse, or switch is okay.
19
Measure revs
For an engine-speed reading, the
meter measures the time difference
between ignition pulses. Connect the clamp
around an HT lead, then connect the clamp
cable to the appropriate jack on the meter and
set the range/function switch.
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Start the engine and monitor the
dwell reading – vary the engine speed
and look for variations.
cm says…
20
Instant reading
Start the engine. The meter should
display engine speed accordingly. If
you get no reading, check that the clamp’s fully
secured around the HT lead.
A digital multimeter is a vital piece of kit,
particularly one with the handy extras for
automotive use. If you haven’t got one in
your toolkit, you’ll find yourself
hamstrung when it comes to electrical
diagnosis. However, you don’t need to
spend a fortune. All meters will read
volts, ohms and amps, and those are the
basic functions you need for electricalsystem fault finding.
The most important things to look for
in a multimeter are a good-quality,
durable unit with a protective case or
holder that will stand up to the garage
environment. Long leads are essential for
use around the vehicle (look for leads of
approximately one metre), in addition to
a large, clear display.
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