Table of contents: Measuring instruments ↓ Voltage measurement ↓ Example of verification ↓ Measuring current ↓ Measuring resistance ↓
When checking electrical equipment, a motorist, referring to technical documentation, constantly encounters such concepts as voltage, current, resistance.
Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ohm). In automotive engineering, voltage is usually understood to mean the voltage of the battery. In this case, we are talking about direct current voltage equal to approximately 12 V. The value of the battery voltage depends on the degree of its discharge and the ambient temperature. Battery voltage can fluctuate within 10-13 volts. On-board voltage is generated by the generator. At average engine speeds, it is about 14 V.
The concept of current strength in automotive engineering occurs relatively rarely. Current strength is indicated, for example, on fuses. Current strength displays the maximum current that can flow through the fuse without causing it to burn out or break the electrical circuit.
Warning! Before starting work on repairing the electrical system, be sure to disconnect the terminal from the negative pole of the battery. Work on the electrical system is considered the simplest disconnection of an electrical wire.
Wherever current flows, it must overcome resistance. Resistance depends, among other things, on the following factors: the cross-section and length of the conductor, its material, the current consumed, etc. If the resistance is too high, it can cause a malfunction. For example, the resistance of the high-voltage wires of a gasoline engine should not be too high, because in this case the spark plugs will not be able to generate a sufficiently powerful spark that will ignite the air-fuel mixture, thereby starting the engine.
Attention! Wiring harness restoration should only be performed using the AUDI VAS 1978 repair kit.
Measuring instruments
To perform the measurements required for servicing cars, so-called combined control and measuring devices are available for sale. They combine a voltmeter for measuring voltage, an ammeter for measuring current, and an ohmmeter for measuring resistance. The devices available for sale differ from each other mainly in their measurement ranges and accuracy. The measurement range determines within what limits the measured value of voltage or resistance must be so that it can be registered by the device at all.
For car enthusiasts involved in repairs, there are combined measuring devices specially designed for making control measurements on cars. With the help of such a device, it is possible to measure the engine speed, the angle of the closed state of the ignition system breaker contacts, and also voltage up to 20 V. Such a device, as a rule, can measure resistance in the range from 1 to 1000 Ohm.
In addition, devices for testing electrical and electronic components are offered. They allow measuring both small resistances of several Ohms and large ones, in the megaohm (mOhm) range. With these devices, it is possible to measure voltages (in volts) with very high accuracy, which is especially necessary when testing electronic units.
If you just need to make sure whether there is any voltage in the circuit at all, then a regular tester with a control diode lamp will do. This, of course, only applies to circuits that do not contain electronic components, since electronic devices are extremely sensitive to too high currents. Sometimes electronic elements fail already at the moment of connecting such a control lamp.
Attention! For checking voltages in circuits containing electronic components (transistors, diodes, relays) a high-resistance voltage tester is needed. It performs almost the same task as a test lamp, but does not damage electronic circuits and is suitable for performing any tests.
Voltage measurement
The presence or absence of voltage can be checked with a simple test light or tester. Of course, this can only check the voltage when it is present. To check the magnitude of the voltage present, you need to connect a voltmeter.
First, set the voltmeter to the measurement range that corresponds to the expected value of the measured voltage. Voltages in a car, as a rule, do not exceed 14 volts. An exception is the ignition system, where the ignition voltage can reach 30,000 volts. Such voltage can only be measured using a special measuring device or an oscilloscope.
If when using measuring instruments specifically designed for control measurements on vehicles, it is enough to turn on, in this case, the voltmeter by pressing the button, then when using combined control and measuring instruments, you must first make a number of decisions. First, you need to select the DC voltage measurement mode (DCV), as opposed to the AC voltage measurement mode (ACV). Then the measurement range is selected. Since there are no voltages higher than 14 V in the car, except for the ignition system, you can choose an upper measurement limit slightly higher than this value (approximately 15-20 volts). If it is known for sure that the voltage being measured is significantly lower, for example, in the region of 2 volts, then the device can be switched to a lower range, which will allow for greater measurement accuracy. If the voltage to be measured is higher than the one for which the range is selected, then the measuring device may be damaged.
The measuring device wires are connected in parallel to the current consumer. In this case, the red measuring device wire is connected to the electric wire coming from the positive pole of the battery, and the black wire of the device is connected to the wire going to the "ground" (-), or to the car body, for example, to the cylinder block.
Example of verification
If the engine starts with deviations from the norm, because the starter speed is too low, then it is advisable to check the battery voltage with the starter running. To do this, connect the red wire of the voltmeter (+) to the positive pole of the battery, and the black one to the car body (-). Then ask an assistant to work the starter and read the voltmeter reading. If the voltmeter registers a voltage of at least 10 volts (at a battery temperature of +20*C), then you need to check the battery and, perhaps, charge it before making further attempts to start the engine.
Measuring current
It is comparatively rare to need to measure current strength. For this, an ammeter is needed, which can also be a component of combined control and measuring devices.
Before measuring the current strength, the measurement range corresponding to the expected value of the measured current is set on the device. If the current strength is unknown, then the highest limit must be set and, if the device does not show any current, the device must be successively switched to measurement ranges of increasingly smaller currents.
To measure the current, the electrical circuit must be broken and the measuring device (ammeter) must be connected to the break in the circuit. For this, for example, the plug is disconnected and the red wire (+) of the ammeter is connected to the supply wire. The black wire (-) of the ammeter is connected to the contact to which the disconnected wire was connected. The ground contact (-) between the plug and the current consumer should be closed with an auxiliary wire.
Warning! Never measure the current on the starter wire with a regular ammeter (about 150 A) or to glow plugs of a diesel engine (up to 60 A), because the high currents flowing in this case can cause the device to fail. In workshops, current clamps are used for such measurements. The clamps are connected to an insulated wire and the current is measured using the phenomenon of induction.
Measuring resistance
Before taking measurements, you must make sure that the ohmmeter has a non-discharged battery and that the contacts to which the ohmmeter is connected are not under voltage. This means that you must first disconnect the battery before taking measurements. Otherwise, you may damage the measuring device or the electrical circuit.
The ohmmeter is connected to two terminals of the consumer or to two ends of the electric wire. It does not matter which wire of the ohmmeter (+) or (-) is connected to one or another contact.
Resistance measurement in the automotive industry is limited mainly to two areas:
1. Checking the resistance or any other part included in the electrical circuit.
2. "Ringing" of an electric wire, switch, or heating element coil. This checks whether any wire is broken and whether this is the cause of failure of the device connected to it. To measure, an ohmmeter is connected to both ends of the wire being checked. If the resistance is 0 Ohm, the wire is OK. If the wire is broken, the device shows ∞ Ohm (infinite resistance).
(The original text of the material can be found on the website: «AUDImanual»)