Voltage is measured in Volts (IN), current strength - in Amperes (A), and the resistance is in ohms (Ohm).
In automotive engineering, voltage is usually understood as the voltage of the battery. In this case, we are talking about a DC voltage of approximately 12 V.
The amount of battery voltage depends on the degree of its discharge and on the ambient temperature. The battery voltage can fluctuate between 10-13 volts.
The onboard voltage is generated by the generator. At medium engine speeds, it is about 14 V.
The concept of current strength in automotive technology occurs relatively rarely. The current strength is affixed, for example, on fuses.
The amperage indicates the maximum current that can flow through a fuse without causing it to blow and without breaking the electrical circuit.
Wherever current flows, it must overcome resistance. The resistance, among other things, depends on the following factors, namely: the cross-section and length of the conductor, its material, the current consumed, etc.
If the resistance is too high, it may cause a malfunction.
For example, the resistance of the high voltage wires of a gasoline engine should not be too high, as in this case, the spark plugs will not be able to produce a strong enough ignition spark to ignite the air/fuel mixture, thereby starting the engine.
Attention! Before starting work on the repair of the electrical system, be sure to disconnect the terminal from the negative pole of the battery, see chapter «Battery - removal and installation». Work on the electrical system is considered to be the simplest disconnection of the electrical wire.
Measuring instruments
To carry out the measurements necessary for servicing vehicles, so-called combined measuring and control instruments are commercially available.
They combine a voltmeter to measure voltage, an ammeter to measure current, and an ohmmeter to measure resistance.
The devices available for sale differ from each other mainly in the ranges and accuracy of measurements.
The measurement range determines the limits within which the measured value of voltage or resistance must lie so that it can be recorded by the device at all.
For motorists involved in repair, there are combined measuring instruments specially designed for making control measurements on cars.
Using such a device, you can measure engine speed, the angle of the closed state of the contacts of the ignition system interrupter, as well as voltage up to 20 V. Such a device, as a rule, can measure resistance in the range up to one kiloohm, i.e. from 1 to 1000 Ohm.
In addition, instruments for checking electrical and electronic parts are available. They allow you to measure both small resistances of a few ohms, and large ones, in the megaohm range (mΩ). These devices can measure voltages (in volts) with very high accuracy, which is especially necessary when checking electronic components.
If you just need to make sure that there is any voltage in the circuit at all, then in this case a conventional tester with a control diode light bulb A is suitable (see illustration 1.0a).
This applies, of course, only to such circuits that do not contain electronic components in their composition, because. electronic devices are extremely sensitive to too high currents. Sometimes electronic elements fail even at the moment such a test lamp is connected.
Attention! For checking voltage in circuits containing electronic parts (transistors, diodes, relays) high-resistance voltage tester required (see illustration 1.0a). It performs almost the same task as a test light, but does not damage electronic circuits and is suitable for any test.
Taking measurements
Voltage measurement
The presence or absence of voltage can be checked with a simple test light or tester. Of course, this way you can check the voltage only when it exists at all. To check the amount of voltage present, you must connect a voltmeter.
First, on the voltmeter, you need to set the measurement range corresponding to the expected value of the measured voltage.
The voltage in the car, as a rule, does not exceed 14 volts. The exception is the ignition system, where the ignition voltage can reach 30,000 volts. Such a voltage can only be measured with a special measuring device or an oscilloscope.
If, when using measuring instruments specially designed for control measurements on vehicles, it is enough to turn on, in this case, a voltmeter by pressing a button, then when using combined instrumentation, a number of decisions must first be made.
First you need to select the DCV voltage measurement mode, as opposed to the ACV mode - AC voltage measurement. Then the measurement range is selected. Since in the car, except for the ignition system, there are no voltages higher than 14 V, you can choose an upper measurement limit that is slightly higher than this value (about 15-20 volts).
If it is known for sure that the measured voltage is significantly lower, for example, lies 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 that for which the range is selected, the meter may be damaged.
Connect the wires of the measuring device, as shown in illustration 1.0b, in parallel with the current consumer. In this case, the red wire of the measuring device is connected to the wire coming from the positive pole of the battery, and the black wire of the device is connected to the wire going to «mass» (-), or the car body, such as a cylinder block.
Check example:
If the engine starts abnormally because the starter speed is too low, it is advisable to check the battery voltage while the starter is running.
To do this, the red wire of the voltmeter (+) need to connect to the positive pole of the battery, and the black one to the car body (-).
Then ask an assistant to operate the starter and read the voltmeter.
If the voltmeter registers a voltage of at least 10 volts (If the battery temperature is +20°C then the battery should be checked and possibly charged before further attempts to start the engine are made.
Current measurement
It is relatively rare to measure the current strength. An example of measurements is given in chapter «Battery self-discharge». This requires an ammeter, which can also be an integral element of combined instrumentation.
Before measuring the current, the instrument sets the measurement range corresponding to the expected value of the measured current. If the current strength is not known, then you need to set the highest limit and, if the device does not show any current, successively switch the device to the measurement ranges of ever smaller currents.
To measure the current strength, the electrical circuit, as shown in illustration 1.0c, must be broken, and the measuring device (ammeter) included in the chain break.
To do this, for example, the plug is disconnected, and the red wire (+) ammeter is connected to the supply wire. black wire (-) ammeter is connected to the contact to which the disconnected wire was connected.
Mass contact (-) between the plug and the current consumer must be closed with an auxiliary wire.
Attention! In no case do not measure the current on the wire to the starter with a conventional ammeter (about 150 A) or to diesel engine glow plugs (up to 60 A), because the high currents flowing in this way can damage the device. In workshops, current clamps are used for such measurements. The clamps are attached to an insulated wire and the current is measured using the phenomenon of induction.
Resistance measurement
Before measuring, it is imperative to make sure that the contacts to which the ohmmeter is connected are not energized. This means that you should always disconnect the battery first. Otherwise, the measuring device may be damaged.
An ohmmeter is connected to two consumer terminals or to both ends of an electrical wire. It does not matter which wire of the ohmmeter (+) or (-), attached to a particular contact (see illustration 1.0g).
The measurement of resistance in the automaker is mainly limited to two areas:
1. Checking the resistance included in the electrical circuit or some other part.
2. «ringing» electric wire, switch, electric heating coil. At the same time, it is checked whether any wire is broken and whether this is the cause of the failure of the device connected to it. For measurement, an ohmmeter is attached to both ends of the wire under test. If the resistance is 0 ohms, then the wire is good. With a broken wire, the device shows ∞ Ohm (infinite resistance).