Before attempting to determine the source of a fault, study the relevant electrical diagram to obtain an understanding of the components of that circuit. The number of possible sources of fault can be reduced by checking the operation of other components in the circuit. If several components or circuits fail simultaneously, the fuse common to those circuits or components may be faulty, or the connection to the body - "ground" may be broken.
Causes of failure include loose or oxidized connectors, poor contact with the body, blown fuses or faulty relays. Visually inspect the condition of all fuses, wires and connectors in the failed circuit before proceeding to check the remaining elements.
Use the electrical diagrams to identify the terminals that need to be tested to determine the source of the fault.
The basic tools needed to locate the source of the fault are a tester or voltmeter, a 12V test lamp, an ohmmeter, a battery, and a set of leads with probes, preferably with a circuit breaker or fuse that is used to bypass the wires or components being tested.
[Content based on information from Mazbook.ru]
In addition to faulty wiring in an electrical system, there are two other main types of faults that can occur: open circuits or short circuits.
The circuit is opened due to a break, as a result of which the current is interrupted, causing the electrical equipment element to switch off.
To determine the integrity of a circuit, connect a circuit tester or voltmeter: one lead to the negative terminal of the battery or a grounded element, and the other to a contact in the circuit being tested, preferably closest to the battery or fuse. The section of the circuit being tested should be live from the battery, unless the battery connector is not conducting current or the fuse is blown (remember that some electrical circuits are only activated when the ignition key is turned to a certain position).
Turn on the circuit, then connect the tester probe to the connection closest to the circuit breaker on the side of the component being tested.
If there is tension (what does the control lamp or voltmeter reading indicate), which means that there are no breaks in the section of the circuit between the corresponding connection and the switch.
If a section is found where there is no voltage, then there is a break in the circuit between this point and the point of the previous test where there was voltage. The break in the circuit is caused by a damaged or loose connector.
To detect the source of the short circuit, turn off electrical consumers - lamps, electric motors, heating elements, etc.
Remove the appropriate fuse and connect the tester or voltmeter leads to the fuse terminals.
Turn on the power to the circuit; remember that some electrical circuits are only activated when the ignition key is turned to a certain position.
If there is voltage in the circuit (what does the control lamp or voltmeter reading indicate), which means there is a short circuit in the circuit.
If there is no voltage when checking, and the fuse still blows when the same load is connected, the load element has failed.
The negative terminal of the battery is connected to the "ground" - the body, engine or transmission. A poor or corroded connection can cause the component to fail or malfunction. Remember that many vehicles use "ground" wires between certain components, such as the engine/transmission and the body, i.e. in places where there is no direct contact between metal components due to soft rubber mounts or a layer of paint.
To check the reliability of the element grounding, disconnect the battery and connect one of the ohmmeter leads to a reliably grounded element. Connect the other lead to the wire or connection to the body that needs to be checked. The resistance shown by the ohmmeter should be zero, otherwise check the connection as described below.
If there is any doubt about the reliability of the contact with the "ground", disassemble the connection, remove dirt and clean the contacts. When reassembling, tighten the connector fastening, for which purpose apply a layer of technical petroleum jelly or silicone grease to prevent corrosion.
A continuity test is performed to ensure that a circuit, section, or element of a circuit is conducting current. Disconnect the wires from the battery terminals and connect the probe of a test lamp with its own power source, such as a continuity tester, to one end of the circuit and the other probe to the other end of the circuit. If the lamp lights, the circuit is intact and conducting current. Switches can be tested in the same way.