Sponsored by 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. www.classicsmonthly.com 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 Sponsored by WORKSHOP WORKSHOP Multimeter Masterclass Multimeter Masterclass 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. www.classicsmonthly.com 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. DECEMBER 2010 CLASSICS MONTHLY 71