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?
Measuring Duty Cycle and Pulse Width
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.