Using Multimeters, Part 1

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?

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.

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:

• Maximum
• Minimum
• Average

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.

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....

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.

Excited by the idea of buying a multimeter and getting into all these measurements? Wanna see how they can help you modify your car? Then have a look at Using a Multimeter to Make Your Car Go Harder.

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