Before you start working on troubleshooting any electrical circuit, carefully study the corresponding diagram in order to understand its functional purpose as clearly as possible. The scope of troubleshooting is usually narrowed by gradually identifying and eliminating normally functioning elements of the same circuit. If several elements or circuits fail at the same time, the most likely cause of failure is a blown fuse or a violation of contact with the "mass" (different circuits in many cases can be shorted to one fuse or ground terminal).
Electrical equipment failures are often due to simple causes such as corroded connector contacts, a blown fuse, a blown fuse link, or a damaged relay. Visually inspect all fuses, wiring, and connectors in a circuit before attempting a more detailed inspection of the components.
When using diagnostic tools to troubleshoot a problem, plan carefully (in accordance with the attached electrical diagrams), at what points of the circuit and in what sequence the device should be connected for the most effective detection of the fault.
Basic diagnostic tools include an electrical circuit tester or voltmeter (you can also use a 12-volt test lamp with a set of connecting wires), a circuit break indicator (test probe) that includes a lamp, its own power source and a set of connecting wires. In addition, you should always have a set of wires in the car to start the engine from an external source (another car's battery), equipped with crocodile clips and, preferably, a circuit breaker. They can be used for shunting and connecting various elements of electrical equipment when diagnosing a circuit. As already mentioned, before you start checking a circuit with diagnostic equipment, determine its connection points from the diagrams.
Checks for the presence of supply voltage are carried out in the event of a fault in the electrical circuit. Connect one of the leads of the electrical circuit tester to either the negative terminal of the battery or ensure good contact with the car body. Connect the other lead of the tester to the terminal of the connector of the circuit being tested, preferably the one closest to the battery or fuse. If the tester indicator lamp lights up, there is supply voltage on this section of the circuit, which confirms the integrity of the circuit between this point of the circuit and the battery. Proceed in the same manner to examine the rest of the circuit. Detection of a supply voltage fault indicates the presence of a fault between this point of the circuit and the last of the previously checked (where was the supply voltage). In most cases, the cause of failure is the loosening of the contact connectors and damage to the contacts themselves (oxidation).
Finding a Short Circuit. One method of finding a short circuit is to remove the fuse and connect a test lamp or voltmeter instead. There should be no voltage in the circuit. Pull the wiring while watching the test lamp. If the lamp starts to blink, there is a short to ground somewhere in the wiring harness, possibly caused by chafing of the wire insulation. A similar check can be made for each component of the electrical circuit by turning on the appropriate switches.
Checking the reliability of the contact with the "ground". Disconnect the battery and connect one of the wires of the test lamp, which has an independent power source, to a point with a known good contact with the "ground". Connect the other wire of the lamp to the wiring harness or connector contact being checked. If the lamp lights up, the contact with the "ground" is OK (and vice versa).
Checking for a break. The check is carried out to detect breaks in the electric circuit. After disconnecting the power supply to the circuit, check it using a test lamp with an independent power source. Connect the test leads to both ends of the circuit. If the test lamp lights up, there is no break in the circuit. If the lamp does not light up, this indicates that there is a break in the circuit. In a similar way, you can check the serviceability of the switch by connecting the test lamp to its contacts. When you move the switch to the "On" position, the test lamp should light up.
Localization of the break point. When diagnosing a section of an electrical circuit suspected of having a break, it is quite difficult to visually detect the cause of the malfunction, since it is difficult to visually check the terminals for corrosion or damage to the quality of their contacts due to limited access to them (usually the terminals are covered by the housing of the contact connector). A sharp jerk of the body of the wiring harness connector on the sensor or the wiring harness itself in many cases leads to the restoration of contact. Do not forget about this when trying to localize the cause of failure of a suspected open circuit. Unstable failures may be the result of oxidation of the terminals or poor contact quality.
Diagnosing faults in electrical circuits is not a difficult task, provided that there is a clear understanding that electrical current is flowing to all consumers (lamp, electric motor, etc.) from the battery via wires through switches, relays, fuses, fuse links, and then returns to the battery through the "ground" (body) of the car. Any problems associated with the failure of electrical equipment can be caused by the cessation of the supply of electric current to them from the battery or the return of current to the battery.