Digital multimeters

Multimeters are designed and mass produced for electronics engineers. Even the simplest and cheapest types may include features which you are not likely to use. Digital meters give an output in numbers, usually on a liquid crystal display.

The diagram below shows a switched range multimeter:

The central knob has lots of positions and you must choose which one is appropriate for the measurement you want to make. If the meter is switched to 20 V DC, for example, then 20 V is the maximum voltage which can be measured, This is sometimes called 20 V fsd, where fsd is short for full scale deflection.

For circuits with power supplies of up to 20 V, which includes all the circuits you are likely to build, the 20 V DC voltage range is the most useful. DC ranges are indicated by on the meter. Sometimes, you will want to measure smaller voltages, and in this case, the 2 V or 200 mV ranges are used.

What does DC mean? DC means direct current. In any circuit which operates from a steady voltage source, such as a battery, current flow is always in the same direction. Every constructional project descirbed in Design Electronics works in this way.

AC means alternating current. In an electric lamp connected to the domestic mains electricity, current flows first one way, then the other. That is, the current reverses, or alternates, in direction. With UK mains, the current reverses 50 times per second.

You are not at all likely to use the AC ranges, indicated by on your multimeter. An alternative style of multimeter is the autoranging multimeter:

The central knob has fewer positions and all you need to do is to switch it to the quantity you want to measure. Once switched to V, the meter automatically adjusts its range to give a meaningful reading, and the display includes the unit of measurement, V or mV. This type of meter is more expensive, but obviously much easier to use.

Where are the two meter probes connected? The black lead is always connected into the socket marked COM, short for COMMON. The red lead is connected into the socket labelled VmA. The 10A socket is very rarely used.

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 that 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.

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 up market 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.

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 auto ranging 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.

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!

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