Voltage measurement
The presence of voltage in the circuit can be checked with a simple test light or probe, but a voltmeter is needed to determine the voltage.
First of all, you need to determine the measurement range. Typically, the voltage in the electrical system of a car does not exceed 14 V, with the exception of the ignition system, where the voltage reaches 30,000 V. This high voltage is measured by special instruments. For normal measurements, the upper range of the measured voltage should not exceed 15-20 V. When measuring lower voltages, for reliability, you need to switch the voltmeter to lower measurement limits in turn to obtain the necessary accuracy of the results.
The voltmeter wires must be connected in parallel to the consumer as shown in the figure, observing the polarity of the connection. Black (negative) the wire of the device is connected to the ground of the car on the cylinder block or on the body, and the red (positive) wire - to the wire coming from the positive contact of the battery.
Example: if the starter cranks the engine too slowly when starting, then you need to check the voltage of the battery. To do this, you need to connect the wires of the voltmeter to the battery. Then the assistant should turn on the starter. If the battery voltage is less than 10 V (temperature around 20°C), then the battery needs to be checked and recharged.
Current measurement
Such measurements on a car are carried out quite rarely, for example, when searching for the cause of self-discharge of the battery. This requires an ammeter, which is included in the multimeter (tester).
Before taking measurements, you need to set the range of measured currents. If the estimated value of the current strength is unknown, it is better to switch the measurement limits from a larger to a smaller one.
To measure the current, you need to break the current circuit and turn on the ammeter in the open circuit. To do this, for example, disconnect the plug and connect the red (positive) ammeter wire to live wire. Black (negative) the wire must be attached to the contact from which the wire is disconnected. The ground connection between the consumer and the plug must be made using an additional wire.
Attention: It is impossible to measure large currents with a conventional ampere meter, which pass, for example, in the starter circuit (up to 150 A) or in the glow plug circuit of a diesel engine. High currents can damage a conventional ammeter. For such measurements, a special inductive ammeter is needed.
Resistance measurement
Before measuring resistance with an ohmmeter, make sure that there is no voltage in the circuit and disconnect the plug. Turn off the ignition, disconnect the wire or part, or disconnect the wires from the battery. If you do not do this, you can disable the ohmmeter.
An ohmmeter is connected to two consumer contacts or to two ends of an electrical wire. In this case, the polarity of the ohmmeter connection does not matter.
Resistance measurements on a car can be divided into 2 types:
1. Checking a resistor or part in a circuit.
2. Checking the integrity of the electrical wire, the presence of a switch contact or a heating coil. At the same time, it is checked whether there is a wire break, due to which the corresponding device does not work. To test, connect an ohmmeter to both ends of the wire to be tested. If the resistance is close to 0 ohms, then there is a contact, and the wire is in order. If the wire is broken, then the resistance will be infinite.