ELECTRICAL SYSTEM
10
10.8 - DIAGNOSIS INSTRUMENT
Using a multimeter to check the electrical systems
Introduction
This instrument allows you to measure resistances, voltages, and current values. Multimeters can be divided
into two basic types: those with an analogue display and those with a digital display Units of the first type are
equipped with a dial and needle. The dial is marked with the scales to be used for measurement of the
various parameters. Digital units are equipped with a dial that displays numbers corresponding to the values
of the measured parameters. The type of measurement to be carried out (voltage, current or resistance) is
set by means of a selector or by means of several different sockets in which to insert the two test probe
connector terminals. In certain cases it is essential to set the full scale value before proceeding. For
example, in order to measure a 12 V signal, you need to set a full scale that is close to this value (e.g. 15 V
or 20 V). It would be illogical to set a full scale value of 10 V; the same applies when setting current (Amps)
or resistance (Ohms) full scale values. Sometimes the instrument can set the required full scale value
automatically. Never exceed the maximum value allowed by the tester when measuring voltage or current
signals.
Voltage measurement
Voltage measurements must be carried out by connecting the terminals of the tester in parallel to the load
(e.g. to the wires feeding a light bulb or a relay, the two battery terminals, or the two wires supplying power to
a control unit). Voltages can be constant over time (DC voltage) or variable over time (AC voltage). In the
first case, it is important to consider the negative and positive polarity of the application. It is therefore
necessary to select on the multimeter the type of voltage you intend to measure. (DC voltage is shown by
the symbol = while AC voltage is denoted by ~).
Current measurement
Current measurements must be made by connecting the multimeter terminals in series with the load (e.g.
disconnect one of the wires feeding power to a light bulb and connect one terminal of the multimeter to the
free end of wire and the other terminal to the light bulb. When the lights switch is set to ON, the bulb will
illuminate normally and the tester will show the absorbed current, i.e. the amount of current passing through
the wire). Warning: connections in series must be made and removed only when the power is switched off.
Never attempt to make or break a series connection when a device is powered. Always make sure that the
connection in series of the tester terminals on the electrical device is made is a safe manner in such a way
that it cannot be broken accidentally.
Sub 10-40
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