If you watch any expert working on the electronics of a car, the chances are
that they’ll have a multimeter close to hand. They’ll grab the meter, turn the
central knob, and then apply the probes to a spaghetti of wiring. The digital
numbers will flash up some reading, whereupon they'll nod knowingly. It can all
look like some kind of foreign ritual, but being able to follow the same routine
can make life a helluva lot easier.
What? You don’t have a multimeter? That’s bad if you do any modifications to
your car that involve a sound system, the engine management system or lights. Or
if you want to fit a turbo timer or courtesy light delay. Or an alarm. Or if you
need to find and diagnose faults in the engine management, auto trans control,
cruise control – in fact any electrical or electronics system in the car.
But first up, what exactly is a
multimeter?
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Our ‘Using a Multimeter’ article, originally published in December 1998, is one of our
best-read stories ever. Obviously, lots of people want to know how to use these
multipurpose tools! This three-part series is a revised, updated and expanded
version of that original article.
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Advanced Features
A multimeter is a test tool that can measure a variety of different
electrical factors. Some cars have a voltmeter on the dash - a multimeter can
measure volts. Some older cars had an ammeter - a multimeter can measure amps,
too. Add to this resistance, and you have the three basic parameters able to be
measured by all multimeters. However, while you might be able to pick up a basic
volts-ohms-amps meter for under AUD$20, it pays in the long run to dig deeper to
get a meter with more functions.
In fact we’ll check out the ‘advanced functions’ before going back over the
old volts-ohms-amps. These advanced functions include:
- Frequency
- Duty
cycle
- Pulse
width
- Temperature
- Continuity
Frequency measurement is used to
indicate how quickly something is being turned on and off. For example, a
factory boost control solenoid will be pulsed at a certain frequency (eg 50
times a second) and many idle air control valves are also rapidly pulsed. Being
able to measure the frequency is useful in that it shows the device is working
and because the value can then be compared with the expected reading shown in a
workshop manual.
Duty cycle indicates the
proportion of time which a pulsed actuator is on for. Using an injector as an
example, it shows what percentage of the time the injector is being held open.
This is good information to know because if the injectors in a modified car have
a duty cycle of 100 per cent during full load measurement, then the max
capability of the injectors has been reached - no more fuel can be supplied. That is, 100
per cent duty cycle means that the injectors are constantly open. If you want
more fuel to go into the engine, you’re going to need bigger injectors or higher
fuel pressure. Other valves pulsed with a varying duty cycle include those for
boost control, variable power steering weight control, auto trans line pressure,
and in some cases, idle speed control.
Pulse width is a measurement which
shows in milliseconds how long the injector (or any other pulsed actuator) is
open for. It is measured in milliseconds. It can be useful to compare pulse
width against expected readings shown in a workshop manual, but in general it is
far less important than being able to measure frequency and duty cycle.
Temperature measurement (usually
by an optional K-Type thermocouple) is very useful when working on cars -
especially turbo ones. In addition to measuring the coolant temp (good for
checking the gauge and measuring the temp at which the rad fan switches on), it
can also be used to check on engine, tranny and diff oil temps, and the temp of
the induction air after the turbo. Intercooler efficiencies can then be
measured, while the temp of the air in the induction air pick-up can be checked.
In short, this function is extremely useful!
A multimeter Continuity function
simply means that the meter has an internal buzzer that will sound when the
meter's probes are connected together. This also means that the buzzer will
sound if the probes are connected to both ends of an unbroken wire - which is
slightly more useful! In fact, continuity checks are amongst the most common of
hands-on car multimeter uses.
In addition to these advanced measurement functions, the following are
incredibly useful:
The functions are as their names suggest – the Maximum button, for example, will show
on the display the maximum value measured since it was pressed. This is a
Godsend if you’re doing some kind of balls-to-the wall blast down a back road
trying to measure max injector duty cycle! Or max intake air temp. Or max
airflow meter output voltage. Or....
Minimum is less useful (though
there are still some good applications) but Average is another brilliant one. Much
less widely available than Maximum and Minimum, if Average is available and it’s on a
fast-sampling multimeter (more on this later) the information can be really
useful. Average can be used to dial-out ‘noise’ in the readings (ie where
there’s lot of other information superimposed on what you are trying to measure)
and so is excellent in disregarding wild fluctuations which can occur in some
measurements.
In any serious hi-po auto multimeter application, we’d suggest that a Maximum
(sometimes call ‘peak-hold’) button is a must-have.
Normal Features
All multimeters measure volts, ohms and amps and there mostly aren’t any
specifics to look for in the meter specs on these.
Except for two.
Firstly, the
meter must have what is called a ‘high input impedance’. This means that when
you apply the meter to the system that you are measuring, the meter won’t draw
more than a tiny amount of current. Meters that don’t have a high input
impedance (old analog meters and some cheap digital meters) will load down the
system. For example, measuring the output of an oxygen sensor will be impossible
with a low impedance multimeter – and attempting to do so may well damage the
sensor. When looking at meter specs, the meter should have an input impedance of
at least 10 meg-ohms.
Secondly, make sure that the current measuring ability of the meter stretches
to at least 10 amps DC – and 20 amps (for at least for intermittent measurement)
is even better. 20 amps is 280 watts at a running-car voltage of 14 volts, and
so will cover the current measurement of pretty well anything but the starter
motor, alternator charge rate or the current draw of a big amplifier.
Lotsa Amps?
As discussed in the main text, having a multimeter that can measure up to 20
amps (discontinuous) is very useful. But what if you want to go higher – to
measure starter motor current draw, or alternator charging current? In that case
you need to step up to a current clamp, a device that plugs into your standard
multimeter and lets you measure really high currents. For more on these, see Current Clamps.
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Multimeter Types
Different multimeters have a different number of digits on their LCDs. This
can be readily seen by looking at the catalog picture or at the meter in the
flesh. But what you see may be not what you’re actually getting – there’s a
trick involved in understanding what the meter can actually show you.
A typical low-cost multimeter has what is called a “1999 count”. That is, it
has four digits with the last three digits able to display all numbers from 0-9,
but the first digit able to be only 0 (sometimes blanked) or ‘1’. The highest
number that can therefore be displayed is 1999 – or 1.999, 19.99, 199.9.
Confusingly, this type of display is often also called a “3½ digit” display –
the “½“ indicating that the first digit is capable of showing only ‘1’ or ‘0’.
Next up the sophistication list are “3999 count” or “3¾ digit” designs. These
have a maximum display number of 3999, 3.999, 39.99, 399.9. Really top meters go
as high as “50,000 count” or “4 4/5 digits” and can display numbers like 50000,
5.0000, 50.000, 500.00, 5000.0.
It’s easy to get lost in all of this, but remember, the higher the “count” or
“digit” number, the more detail you can read.
Multimeters are available in auto-ranging or manual-range types. An
auto-ranging meter has much fewer selection positions on its main knob - just
Amps, Volts, Ohms and Temp, for example. When the probes of the meter are
connected to whatever is being measured, the meter will automatically select the
right range to show the measurement.
Meters with manual selection must be set to the right range first. On a
manual meter, the 'Volts' settings might include 200 mV, 2 V, 20 V, 200 V and
500 V. When measuring battery voltage in a car, the correct setting would be '20
V', with anything up to 20 volts then able to be measured.
While an auto-ranging meter looks much simpler to use - just set the knob to
'volts' and the meter does the rest - the meter can be slower to read the
measured value. This is because it first needs to work out what range to operate
in. If the number dances around for a long time before settling on the right one
it can be a pain in the butt for quick measurements, and very difficult if the
factor being measured is changing at the same time as well! However, to speed up
readings, some auto-ranging meters also allow you the option of fixing the
range. Note that expensive multimeters will very quickly get the right reading,
even if they are auto-ranging.
Talking about how long it takes for the reading to settle, the higher the
sampling speed of the multimeter, the better. No multimeter will update the LCD
faster than about 3 times a second (any faster and you can’t read it) but many
also provide an additional bar-graph that can show rapid changes. Bar graph
meters all have fast sampling.
Some multimeters have a PC interface facility. If you have a lap-top PC this
means that it's fairly easy to set up a single channel data logging system.
However, check that the meter samples fast enough to make the information
useful.
Secondhand?
If you are looking at buying a multimeter under about AUD$100, it’s not worth
going secondhand. You may as well pay new and usually get more features and have
a meter that is more likely to be in calibration and have good condition leads.
However, if you’re looking at the top end of the market (eg a Fluke meter), some
good secondhand bargains are to be had. Let eBay be your friend....
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A backlight function is very useful when working with cars – it allows night
on-road testing and also makes things easier when working in darkened
footwells.
Some meters have two displays, although they still have only one pair of
input leads. The two displays are used to simultaneously show two
characteristics of the one signal that’s being measured. To do this, the two
different signals have to be on the same input – you can’t show temperature and
voltage for example. But if you are measuring (say) a pulsed solenoid that
controls boost, you can simultaneously measure both its duty cycle and frequency
– one display shows duty cycle and the other, frequency. But in most car
applications this isn’t all that advantageous – you usually only want the one
parameter measured at a time.
For automotive use, look for a meter design which comes in a
brightly-coloured rubber holster - it helps protects the meter from damage as
well as making it easier to find - and one which is protected against the
entrance of moisture. Good meters use 'O'-rings to seal the case and jacks.
In some meters, the probes can be attached to the base of the holster,
allowing easy one-hand probing of circuits. Other designs of multimeter even
incorporate one of the probes into the body of the meter itself, making
one-handed operation very easy.
The probes and leads are important. Both sharp probes and also alligator
clips should be able to be used, with the better meters having alligator clips
which attach over the probes. Other probes are available that are
insulation-piercing, extend small hooks that can hang on exposed conductors, or
can work in confined spaces. The leads themselves should be heavily insulated -
it’s best if they use silicone insulation - and should feel thick and durable.
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