First, on the voltmeter, you need to set the measurement range corresponding to the expected value of the measured voltage. The voltage in the car, as a rule, does not exceed 14 volts. The exception is the ignition system, where the ignition voltage can reach 30,000 volts. Such a voltage can only be measured with a special measuring device or an oscilloscope.
If, when using measuring instruments specially designed for control measurements on vehicles, it is enough to turn on, in this case, a voltmeter by pressing a button, then when using combined instrumentation, a number of decisions must first be made. First you need to select the DC voltage measurement mode (DCV), in contrast to the mode (ACV) — measurement of alternating current voltage. Then the measurement range is selected. Since in the car, except for the ignition system, there are no voltages higher than 14 V, you can choose an upper measurement limit that is only slightly higher than this value (about 15-20 volts). If it is known for sure that the measured voltage is significantly lower, for example, lies in the region of 2 Volts, then the device can be switched to a lower range, which will allow for greater measurement accuracy. If the voltage to be measured is higher than that for which the range is selected, the meter may be damaged.
Connect the measuring device wires in parallel with the current consumer. In this case, the wire of the measuring device with red insulation is connected to the electric wire coming from the positive pole of the battery, and the wire of the device with black insulation is connected to the wire «masses» (-), or to the car body, for example, to the cylinder block.
Validation example
If the engine starts abnormally because the starter speed is too low, it is advisable to check the battery voltage while the starter is running. To do this, the red wire of the voltmeter (+) need to connect to the positive pole of the battery, and the black one to the car body (-). Then ask an assistant to operate the starter and read the voltmeter. If the voltmeter registers a voltage of at least 10 volts (at battery temperature +20°С), then the battery should be checked and possibly charged before any further attempt is made to start the engine.