Lesson 3/Learning Event 1
Automotive electrical systems are not as sensitive and are designed for more variation in voltage.
Therefore, built-in voltmeters are not needed on vehicles. If the voltage varies more than is
allowed or if the voltage is thought to be wrong, portable voltmeters are available to measure the
voltage. Some voltmeters are in separate cases while others are part of a combination-type meter.
Two good examples of combination meters are the low-voltage circuit tester and multimeter.
Voltmeters vary depending on their model and use. For example, some voltmeters have only one
scale and others have two or more scales. All portable voltmeters include flexible test leads, lengths
of wire with a male plug on the meter end and a test probe or an alligator clip on the other end.
The test leads are usually color-coded red and black. The red lead is connected to the positive
terminal on the meter and the positive side of the circuit to be tested. The black lead is used to
connect the negative terminal of the meter to the negative side of the circuit.
There are two ways to connect the two test leads to the meter so that all ranges can be used. In
one method, the test leads are always connected to the same two terminal sockets of the meter,
and a selector switch is used to select the proper range. The second method uses one common
terminal for the negative or positive lead, and the other lead is connected to a different terminal for
each range.
Why do we need more than one range on a voltmeter? Why not have one range from 0 to 500
volts? That is not practical. Electrical test meters are most accurate in the center of the scale or
range. This means the 0- to 500-volt scale would be most accurate around 250 volts. The battery-
generator voltage of most military vehicles is in the 24- to 28-volt range. You will therefore be
using the voltmeter to test voltages ranging from 5 volts to a fraction of 1 volt.
Suppose you were issued a voltmeter like the one in Figure 21. Do you think you could use it?
Notice that the meter dial contains three ranges or scales, but the selector switch indicates there are
actually four ranges. The ranges on this voltmeter are 0 to 1 volt, 0 to 10 volts, 0 to 20 volts, and 0
to 50 volts.
The range or scale for 0 to 10 volts can also be accurately used when the selector switch is in the 0-
to 1-volt position. The numbers on the 0- to 10-volt scale then become tenths of a volt. For
example, 2 becomes 0.2 (two tenths) of a volt, 3 becomes 0.3 volt and 10 becomes 1.0 volt.
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