Voltage measurement
You can verify the presence of voltage using a simple test lamp or voltage tester. Thus, first of all, find out whether there is a voltage at all. To measure the value of the available voltage, you must connect a voltmeter (voltage measuring instrument).
First of all, at the voltmeter it is necessary to set the measurement range in which the measured voltage is supposed to be. The voltage in the car does not exceed, as a rule, about 14 volts. The exception is the ignition system: here the ignition voltage can be up to 30,000 volts. This high voltage can only be measured with a special meter or oscilloscope.
If when using measuring instruments designed specifically for cars, you only need to turn on the voltmeter on the control panel, then when using a universal instrument, a number of decisions must first be made. First of all, a constant voltage area is set on the control panel (DCV instead of ACV - area corresponding to alternating voltage). Then the measurement range is selected. Since in the car, except in the ignition system, there is no voltage of more than 14 volts, the upper value of the set measuring range should be slightly larger (about 15 to 20 volts). If it is known for certain that the measured voltage is significantly lower, for example, about two volts, the upper value of the settable measuring range can be reduced in order to obtain a higher measurement accuracy. If the value of the measured voltage exceeds the upper value of the measurement range of the device, the device may fail.
Connect the wires of the measuring device, as shown in the figure, in parallel with the power consumer. In this case, the red wire of the device is connected to the conductor going to the positive pole of the battery, and the black wire is connected to the ground wire or to the mass of the car, for example, the engine block.
Measurement example: When the engine is difficult to start due to the starter turning too slowly, it is advisable to check the battery voltage while the starter is running. To do this, connect the red wire (+) voltmeter to the positive pole of the battery and black wire to the vehicle ground (-). After that, ask the assistant to turn on the starter and read the voltage value. If the voltage is below about 10 volts (at battery temperature +20°C), the battery must be checked and, if possible, charged before the next attempt to start the engine.
Current measurement
In automobiles, it is relatively rare to need to measure current strength. For example, see chapter "Spontaneous battery discharge". This requires an ammeter built into the multimeter.
Before measuring the current, the device is adjusted to the measurement range, in which the value of the measured current strength is supposed to be. If the current strength is not known in advance, the device is adjusted to the maximum measurement range. If at the same time the device does not show any value, successively switch the device to smaller measuring ranges.
To measure current, the electrical target must be disconnected and the meter (ammeter) connected to break the circuit.
To do this, for example, the plug is disconnected, and the red wire (+) ammeter is connected to a current-carrying conductor. black wire (-) connected to the contact to which the conductor is normally connected. The ground contacts between the power consumer and the plug must be connected with a jumper.
Attention: In no case should you measure the current in the wire to the starter with a conventional ammeter (about 150 A) or to diesel engine glow plugs (up to 60 A). The resulting high currents can damage the measuring device. In the workshop, an ammeter with a DC clamp is used for such measurements. In this case, the clamp is connected through an insulated wire, and the current value is measured through induction.
Resistance measurement
Before checking the resistance, it is imperative to make sure that no voltage is applied to the part to which the ohmmeter is connected. That is, you must first always disconnect the plug, switch off the ignition, remove the conductor or assembly, or disconnect the battery. Otherwise, the device may be damaged.
An ohmmeter is connected to two consumer connectors or to two ends of an electrical conductor. It does not matter which wire (+/-) measuring device to which pin is connected.
Measuring resistance in a car can be one of two types:
- 1. Control of the resistance or part included in the electrical circuit.
- 2. Checking the conductivity of an electrical conductor, switch or electric heating element. This checks whether there is a break in the electrical conductor in the vehicle, in connection with which the connected electrical device cannot function. For measurement, an ohmmeter is connected to both ends of the corresponding conductor. If the conductor conducts electricity, then the meter will show a resistance value of 0 ohms. This means that the conductor is in order. If the conductor breaks, the device will show the resistance value ∞ (infinity) Ohm.
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