Digital instruments have low inertia, they are insensitive to vibrations and the position of the body during measurements, but pointer instruments clearly show the dynamics of changes in the measured parameters. In addition, the liquid crystal display of digital instruments is sensitive to lighting and temperature changes.
Checking de-energized circuits
Before work, calibrate the ohmmeter. At the selected measurement limit (for most circuits up to 200 Ohm) we short-circuit the probe tips. On an analog device, we set the arrow to zero division using the "0" setting regulator. There is no such regulator in household digital devices. Therefore, to measure small values (up to 1–2 Ohm), by short-circuiting the probes, we determine the internal resistance of the ohmmeter and its wires, which is 0.03–0.06 Ohm. This value must be subtracted from the resistance value obtained.
To check the chain, disconnect at least one end of it (otherwise the current will go around, through other parts of the circuit, and the readings will be incorrect). It is better to put a crocodile clip on one of the device probes and connect it to the ground. When checking devices with one-way conductivity (for example, a generator rectifier block) take into account the polarity of the device.
To check the windings of the starter, generator, high-voltage wires, etc., we switch the device to the lower measurement limit. In practice, the accuracy of conventional autotesters is not enough to check sections of the circuit where even the slightest increase in resistance is unacceptable, for example, due to poor contact. Therefore, we pay attention to minor deviations of the arrow from the zero division, and after measuring, we check the calibration of the devices again.
We check the winding short circuits to the "housing" and interturn short circuits with a megohmmeter (range "M"). For many testers (switch) when working in this range, an additional source of direct current is required. If it is not available, we carefully check the circuit with a lamp powered by 220 V.
Testing live circuits
We check the live circuits with a voltmeter and ammeter.
We connect the voltmeter in parallel to the device or section of the circuit being tested. The measurement range is 0–15 or 0–25 V DC. We connect the negative wire (probe) to the "ground", the positive wire - to consumers or current sources. The voltage drop can be used to determine the malfunction of the power supply circuit (breakage, oxidation of contacts, etc.), as well as a short circuit in the consumer.
To check live circuits, you can use a test lamp with a power of no more than 3-4 W, designed for a voltage of 12 V (for example, the AMN12-3 lamp used in the instrument cluster).
The ammeter must have an upper measurement limit of 10 A or more of direct current, as well as overload protection. The ammeter is connected in series with the device being tested. The "plus" of the device is connected to the current source, and the "minus" to the consumer. We measure the consumed current and compare it with the nominal one specified in the technical characteristics of the device being tested. Since the actual voltage in the on-board network differs from the nominal one (in the reference data the rated current corresponds to the rated voltage, i.e. 12 V), the obtained value may differ slightly from the specified one. If the current is less than required, the electrical circuit is faulty, and if it is more, there is a short circuit in the consumer.