Voltage measurement
The presence of voltage can be proven with a simple test light or voltage indicator. However, in this case it is only determined whether voltage is being applied at all. In order to determine the magnitude of the applied voltage, it is necessary to connect a voltmeter. First of all, when using a voltmeter, it is necessary to set the measurement range in which the measured voltage is presumably located. As a rule, the voltage in a car does not exceed 14 V. The ignition system is an exception: here the secondary voltage of the ignition system can reach up to 30,000 V. This high voltage can only be measured with a special measuring device - an oscilloscope. First of all, the DCV voltage range is set using the selector switch, as opposed to ACV - alternating current voltage. Then the measurement range is selected. Since the car does not have a voltage higher than 14 volts except for the ignition system, the upper limit of the set measurement range should be set somewhat higher (approximately 15-20 V). If the measured voltage is obviously much lower, for example, it is in the range of two volts, then you can switch to a narrower range to achieve greater accuracy of reading. If higher voltages are applied than determined by the measuring range of the device, then the latter may be damaged.
In accordance with the figure, connect the measuring device wires in parallel to the consumer. In this case, the red measuring wire is connected to the wire coming from the positive pole of the storage battery, and the black measuring wire is connected to the wire connecting the storage battery and the car body, or directly to the car ground, for example, to the engine cylinder block.
Example of control: if the engine does not start properly because the starter rotates too slowly, it is advisable to check the voltage of the battery while pressing the starter. To do this, connect the red wire (+) of the voltmeter to the red terminal of the positive pole of the battery, and the black wire to the vehicle ground (-). Then have an assistant press the starter and read the voltage. If the voltage is below about 10 V (at a battery temperature of +20°C), then it is necessary to check the battery and, possibly, charge it before the next attempts to start the engine.
Measuring current
In a car, it is relatively rare to need to check the current strength. For example, see the section "Self-discharge of the storage battery". This requires an ammeter, which is also built into the combined measuring device.
Before measuring the current, set the measuring device to the range in which the current to be measured is approximately located. If this is not known, set the highest measuring range and, if there is no indication, successively switch the device to lower ranges.
To measure the current, it is necessary to disconnect the electric circuit and connect a measuring device (ammeter) to the break in the circuit. To do this, tighten 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 broken line is connected. In this case, the ground contacts of the consumer and the plug must be connected using an auxiliary wire.
Warning! Never measure the current with a regular ammeter in the wire going to the starter (approximately 150 A) or in the glow plugs of diesel engines (up to 60 A). The high currents generated in this case can burn out the measuring device. In auto repair shops, an ammeter equipped with a DC clamp is used for such measurements. In this case, the current clamp clamps the insulated electrical wire, and the current value is measured using induction.
Measuring resistance
Before measuring resistance, you must be sure that the element of the structure to which the ohmmeter is connected is not under voltage. Therefore, each time, pull off the plug in advance, turn off the ignition, disconnect the wire, for example, from the unit or disconnect the battery. Otherwise, the measuring device may be damaged.
The ohmmeter is connected to two terminals of the consumer (load) or to two ends of the electric line. It does not matter which wire (±) of the measuring device is connected to the terminals.
Resistance measurement in a vehicle is divided into two main areas:
1. Checking a resistor or structural element built into an electrical target.
2. "Current flow test" in an electrical line, switch or heater coil. This checks whether the electrical circuit in the vehicle is broken and therefore the connected electrical device cannot function. To measure, an ohmmeter is connected to both ends of the corresponding electrical line. If the resistance is 0, then the electrical current flows, i.e. the electrical line is OK. In the case of a broken line, the measuring device shows infinity (in Ohms)