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Using a Multimeter

Measuring the right stuff the right way!

by Julian Edgar

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If you watch any expert working on the electronics of a car, the chances are that he'll have a multimeter close to hand. He'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 he'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.

Multimeter 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 cars use 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 $20, it pays in the long run to dig deeper to get a meter with more functions.

When selecting a meter, firstly make sure that the current measuring ability of the meter stretches to 20 amps - at least for intermittent measurement. 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 high-power amplifier. Many meters measure up to only 10 amps, which is a bit low. Secondly, in addition to the 'normal' measurements, it helps a heap if the meter can also measure:

  • duty cycle
  • pulse width
  • rpm
  • temperature
  • continuity

Duty cycle indicates the proportion of time which a pulsed actuator is being excited for. Using an injector as an example, it shows what proportion of the time the injector is being held open. 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. 100 per cent duty cycle means that the injectors are constantly open.

Pulse width is a measurement which shows in milliseconds how long the injector (or any other pulsed actuator) is open for. This is useful if measurements are needed on one car to help set up programmable injection on another car, or to compare against manufacturers' specs - but generally the actual length of time which the device is open for doesn't matter very much.

Engine revs has obvious application in measurement - from accurately setting the idle speed to checking the accuracy of the tacho.

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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 an internal buzzer will sound when the meter's probes are connected together. This also means that the buzzer will sound if the probes are connected to either end of an unbroken wire - which is slightly more useful!

Multimeters are available in either digital or analog forms. While the upmarket meters (with duty cycle and temp facilities) all are digital, the humble analog meter does have some application when measuring a variable which is changing very rapidly. This is because the digital meters sample at a relatively slow rate - for example, 3 times a second - while analog meters are constantly measuring. If all you're looking for is a swing of a needle - and not the actual value of the measurement - then an analog meter has got some pluses. Note, though, that all meters - analog or digital - which are being used with engine management systems must have a very high input impedance, or else the circuit being measured may become loaded-down by the current draw of the meter itself. In almost all applications, a digital meter will work fine - and it's also easier and more accurate to read.

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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 autoranging meter looks much simpler to use - just set the knob to 'volts' and the meter does the rest - the meter can be much slower to read the measured value, because it needs to first 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, some autoranging meters also allow you the option of fixing the range, to speed up readings.

One very useful function of a multimeter is a 'peak hold' button. As the name suggests, the meter will retain on its display the max value measured. Especially when working by yourself on the road, this allows balls-to-the-wall testing without having to constantly glance down at the meter. All of the following have application when there's a peak hold facility:

  • max rpm used
  • peak output of any engine management sensor - for example, the airflow meter
  • max temps, including oils, induction air and post intercooler air
  • max duty cycle of injectors

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.

Multimeter Designs

In addition to the range of measurement features and analog/digital designs available, multimeters also vary in other ways. Available from some suppliers is a talking multimeter, which - while it initially sounds like a total wank - could be pretty useful if you're hanging upside down under the dash with the meter probes filling your hands and eyes!

For automotive use, look for a design which comes in a brightly-coloured rubber holster - which 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.

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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 very important. Both sharp probes and also alligator clips should be able to be used, with the better meters having screw-on alligator clips which attach over the probes. The leads themselves should be heavily insulated, and should feel thick and durable.

Accessing the Wiring

When using the meter on engine management wiring, take care that the carefully-manufactured insulation coverings over the loom wiring and plugs aren't destroyed! Two approaches can be taken to avoid this happening. The first is to sharpen the multimeter probes with a file or sandpaper so that they're needle-sharp. While this can cause some personal danger - it gives 'pricks' a whole new meaning - it also means that it's easy to insert the probe through insulation without destroying the insulation cover. The other method is to add a spiral of thin solid copper wire to the probes, allowing them to back-probe connectors. Most EFI connectors have a rubber cap which can be peeled back, which means that a suitably thin probe can be inserted from behind for measurement while the connector is still attached.

When using the meter to check on sensors or measure ECU outputs, consult the car's factory manual, which usually has extensive instructions on how to do this. Be very careful that you are in fact probing the wires mentioned in the manual; it's easy to be one pin across in a connector or confuse the colour code in a loom. It's simple stuff, I know - but when there're 76 pins in an ECU connector, it's easy to lose track of the right one!

Making Measurements

So much for the preliminaries - but how do you go about measuring volts, amps, ohms and all the rest?

When measuring volts the meter should be connected in parallel with the voltage source. Most commonly in a car you're trying to find a 12 volt supply for an accessory, or measuring the voltage output of a sensor. In either of these cases, the meter would be set to its 20 volt (or 40 volt or autoranging DC volts scale, depending on the meter), and the meter probes inserted into the connected wiring. If the polarity is wrong (you've used the negative probe to the positive supply line) then no damage will be done - the meter will simply show negative volts instead of positive volts. When measuring voltage the circuit does not need to be broken - the meter is inserted in parallel.

Measuring current (amps) requires that the circuit be broken and the meter placed into it, so that all the current flows through the meter. If you're measuring currents above milliamps (thousandths of an amp), the meter will often require that the positive probe plug be inserted into another socket. Failure to do this will result at best in the blowing of an internal multimeter fuse, and at worst in damaging the meter. That means you always need to be very careful when measuring amps!

Breaking the circuit to measure current flow can sometimes most easily be done by removing a car fuse, and inserting the meter at that point. Obviously, though, if that circuit has a short circuit then the original fuse protection will be lost and the meter's fuse could be blown instead.

Resistance measurements require that the device is isolated from its normal circuit, otherwise the measurement could be false. In the case of an engine management sensor (like throttle position) this means that it needs to be unplugged. If you try to measure resistance with the sensor still plugged into its circuit the reading will usually be wrong!

When a multimeter is set to its resistance function it passes a small current through the device being measured. This won't damage the device, but it does mean that the multimeter battery is being flattened more quickly. Don't measure resistances for a long period, or leave the meter set to this position. Always check that the multimeter indicates zero resistance when its leads are touched together; if the meter doesn't show zero resistance, what chance does it have of measuring a real resistance accurately?

Both duty cycle and pulse width should be measured with the meter paralleled with the device being measured, while the thing's actually working. Some meters don't like measuring the output of the ECU injector driver unless the injector is actually connected, for example. Injector operation should be measured in real operating conditions, with the best way of doing this being on the road with the multimeter located inside the cabin. With injectors, it is the max duty cycle which is often of greatest relevance, and the 'peak hold' button can be used to measure this.

RPM is usually measured by means of an inductive pick up which attaches to one of the plug leads. If the car runs multi-coil direct-fire ignition you can temporarily place the coil a little way from the plug and connect it to the plug by a normal spark plug lead. The inductive pick up can then be placed on that.

Temperature is most often measured using a bead or probe-type thermocouple. The bead unit has very little mass, and so reacts to temp changes quickly - but is fragile and hard to handle. The probe type has a slower reaction time but is easier to handle and more robust. Because of its speed of reaction, the bead type is more usually used. Using the thermocouple feature of a multimeter is as easy as selecting that function and plugging in the probe. Some meters also have an internal sensor, which allows measurement of the ambient temp of the day - useful when comparing different test day temps.

Conclusion

Multimeters are very cheap tools for the number of functions that they provide. It's generally worthwhile buying one that has all of the functions you need - and then looking after it!

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