Digital instruments have a 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 dead circuits
Before work, calibrate the ohmmeter. At the selected measurement limit (for most circuits up to 200 ohms) close the tips of the probes. On an analog instrument with a setting knob «0» set the pointer to zero. There is no such regulator in household digital appliances. Therefore, to measure small quantities (up to 1–2 ohm), having shorted 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 resulting resistance value.
To test the circuit, disconnect at least one of its ends (otherwise, the current will bypass, in other parts of the circuit, and the readings will be incorrect). On one of the probes of the device it is better to put on a clamp of the type «crocodile» and connect it to «mass». When checking devices with one-way conduction (e.g. generator rectifier) note the polarity of the device.
To check the windings of the starter, generator, high voltage wires, etc. switch the device to the lower limit of measurements. In practice, the accuracy of conventional autotesters is not enough to check circuit sections where even the slightest increase in resistance is unacceptable, for example, due to poor contact. Therefore, we pay attention to slight deviations of the arrow from zero division, and after the measurement we check the calibration of the instruments again.
Winding shorts on «frame» and interturn check with a megger (range «M»). For many testers (turnouts) operating in this range requires an additional DC source. In its absence, being careful, we check the circuit with a lamp powered by 220 V.
Checking Live Circuits
We check the voltage circuits with a voltmeter and an ammeter.
We connect the voltmeter in parallel with the device under test or section of the circuit. Measuring range 0-15 or 0-25 V DC. negative wire (probe) connect with «weight», positive - with consumers or current sources. By the voltage drop, you can determine the malfunction of the supply circuit (breakage, oxidation of contacts, etc.), as well as a short circuit in the consumer.
To test live circuits, you can use a test lamp with a power of no more than 3-4 W, rated for 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 direct current, as well as overload protection. We connect the ammeter in series with the device under test. «Plus» device is connected to a power source, and «minus» to the consumer. We measure the consumed current and compare it with the rated current indicated in the technical characteristics of the device under test. Since the actual voltage in the on-board network differs from the nominal (in the reference data, the rated current corresponds to the rated voltage, i.e. 12 V), the value obtained may differ slightly from the specified value. If the current is less than required, then the electrical circuit is faulty, and if more, a short circuit has occurred in the consumer.