There are special combination instruments for cars (testers or multimeters), designed for measurements on parts of automobile electrical equipment. With the help of such devices it is possible to measure the number of revolutions of the engine, the angle of the closed state of contacts and voltage of up to 20 V. When measuring resistance, the kilo-Ohm range is usually used, i.e. from 1 to 1000 kOhm.
In addition, there are devices for taking measurements on electrical and electronic components. They allow for precise measurements of both small (Ohm) and large (megaOhm) resistances, as well as voltages, which is often necessary for components of electronic systems.
If you only need to check whether there is voltage or not, then there is a simple test lamp (A). This is used only for those purposes in which there are no electronic parts. Electronic parts react very sensitively to large currents. In some cases, they can fail even when the test lamp is connected.
Attention: For checking electronic parts (transistors, diodes, control units) a high-resistance voltage tester (V) is required. It works in the same way as a test lamp, but does not damage electronic components and is suitable for all testing tasks.
Measurement technique
Voltage measurement
Voltage can be checked with a simple test lamp or voltage tester, but in this case you can only find out whether there is voltage at all or not. To determine the voltage value, you need a voltmeter.
First of all, you need to determine the range of the measured voltage. Voltages in a car usually do not exceed 14 V. The exception is the ignition system: the voltage in it can reach 30,000 V. Such voltages are measured only by special devices.
When using a tester that is not specifically designed for on-vehicle measurements, you must switch it to the DC voltage measurement range (is denoted by DCV and ACV stands for alternating voltage). Then the measurement limit is selected. Since the voltage in the car does not exceed 14 V (excluding ignition system), then you can confidently select the measurement limit with an upper limit from 15 to 20 (25) V. If it is known for sure that the measured voltage will be less (for example, about 2 V), then you can switch to a lower measurement limit, which will increase the accuracy of the measurements. If the voltage measured is greater than the measurement limit allows, then the measuring device can be damaged.
Connect the device wires in accordance with the drawing in parallel to the current consumer. In this case, the red wire is connected to the wire coming from the positive (plus) contact of the storage battery, and the black one is connected to the "ground" wire or to the ground on the body or on the engine.
Example of measurements
If the engine does not start because the starter turns too slowly, then you need to check the battery voltage when the starter is running. To do this, connect the red wire of the voltmeter (tester) to the positive terminal of the battery. And the black wire to the "ground" of the car. With the starter on, measure the voltage. If it is less than 10 V (at a battery temperature of about +20°C), then you need to check the battery and, if necessary, recharge it.
Measuring current
The article is reprinted from the portal [mazbook.ru]
This measurement is rarely required in a car, but sometimes it is necessary, for example, in the case of self-discharge of the battery. For this, an ammeter or a tester is needed. Before measuring the current, you need to select the measurement limit. If the expected current value is unknown, then you need to set the maximum limit and then, as necessary, connect lower measurement limits.
To measure the current strength, you need to break the current circuit and connect the ammeter (tester) to this break in the circuit. To do this, for example, disconnect the plug and connect the red wire of the ammeter (+) to the current-carrying wire. Connect the black wire (-) to the contact to which the disconnected current-carrying wire usually goes. The ground contact between the plug and the consumer is provided with an additional wire.
Caution: Never measure the current in the wires going to the starter with a regular ammeter (current consumption is about 150 A) or to the glow plugs of a diesel engine (current about 60 A). Such strong currents will immediately disable a conventional device. In workshops, ammeters with contactless clamps operating on the principle of induction are used to measure such currents.
Measuring resistance
Before measuring resistance, make sure that the part to which the ohmmeter will be connected is not under voltage. Always turn off the ignition, disconnect the wire or remove the entire unit or disconnect the battery before disconnecting the plug. Otherwise, you can damage the ohmmeter (tester).
The ohmmeter is connected to two contacts of the current consumer or to two ends of the wire. The polarity of the ohmmeter connection (+/-) to the contacts does not matter.
Measuring resistance in a car is mainly done in two cases:
- 1. Checking the resistance or other part included in the current supply circuit.
- 2. Checking for contact in the electrical wire, switch or heating coil.
This checks whether the electric wire in the car is broken, and because of this, some electrical device does not work. For measurements, the ohmmeter is connected to the ends of the wire or part being checked. If the resistance is about 0 Ohm, then there is contact, i.e. the wire is OK. If there is a break, the ohmmeter will show infinity.