Checking electrical circuits

To measure electrical parameters, a digital or analog (arrow) tester is used - a voltmeter, ohmmeter and other devices combined in one housing

Digital instruments have low inertia, they are insensitive to vibrations and body position during measurements, but pointer instruments more clearly show the dynamics of changes in the measured parameters.

In addition, the liquid crystal display of digital devices is sensitive to lighting and temperature changes.

Diagram of the control system of a Gazelle car with a ZMZ-40524 engine

Before work, we calibrate the ohmmeter. At the selected measurement limit (for most circuits up to 200 Ohms), we close the tips of the probes. On an analog device, use the “0” setting knob to set the arrow to the zero division.

There is no such regulator in household digital devices.

Therefore, before measuring small values (up to 1–2 ohms), by shorting the probes, we determine the internal resistance of the ohmmeter and its wires, which is 0.03–0.06 ohms.

This value must be subtracted from the resulting resistance value.

To check the circuit, we disconnect at least one end of it (otherwise the current will go around, through other sections of the circuit, and the readings will be incorrect).

It is better to put an alligator clip on one of the probes and connect it to ground.

When checking devices with one-way conductivity (for example, a generator rectifier unit), take into account the polarity of the device.

To check the windings of the starter, generator, high-voltage wires, etc. switch the device to the lower measurement limit.

In practice, the accuracy of conventional autotesters is not enough to test 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 needle from the zero division, and after the measurement we check the calibration of the instruments again.

We check the short circuits of the winding to the “housing” and the interturn with a megohmmeter (range “M”).

For many testers (pointer) when operating in this range, an additional DC source is required.

If it is absent, being careful, we check the circuit with a lamp powered by a voltage of 220 V.

Checking live circuits

We check live circuits with a voltmeter and ammeter. We connect the voltmeter in parallel to the device or circuit section being tested. Measurement limit 0–15 or 0–25 V DC.

We connect the negative wire (probe) to ground, the positive wire to consumers or current sources.

By the voltage drop, you can determine a malfunction of the supply circuit (break, 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, an AMH12-3 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 being tested. We connect the “plus” of the device to the current source, and the “minus” to the consumer.

We measure the current consumption and compare it with the rated one specified in the technical specifications of the device being tested.

Since the actual voltage in the on-board network differs from the rated one (in the reference data the rated current corresponds to the rated voltage, i.e. 12 V), the resulting value may differ slightly from the specified one.

If the current is less than required, then the electrical circuit is faulty, and if it is more, a short circuit has occurred in the consumer.