Before attempting to determine the source of the fault, first study the relevant electrical diagram to obtain an understanding of the components installed in that circuit. The range of possible sources of the fault can be narrowed down by checking the operation of other components included in the circuit. If several components or circuits fail simultaneously, the problem is likely to be in the fuse common to these circuits or components, or in the contact with the body.
Electrical problems are usually caused by simple problems such as loose or corroded connectors, poor contact with the body, blown fuses, a melted fusible link, or a faulty relay. Visually inspect all fuses, wires, and connectors in the faulty circuit before checking the rest of the components. Use the wiring diagrams to determine which terminals need to be checked to locate the source of the problem.
The main tools needed to locate the source of the fault are a tester or voltmeter (a 12 volt light bulb and a couple of wires with probes on the ends can also be used to perform some tests); ohmmeter; a battery and a set of wires with probes, a jumper, preferably with a circuit breaker or fuse, which is used to bypass the wires or elements being tested.
To detect the cause of unreliable operation of any of the elements (usually due to poor connection or dirty contacts, or damaged insulation), you can perform a wire shake test. You need to shake the wire with your hand to check if the fault appears when the wire moves. This method can narrow down the possible sources of the fault to a particular wire.
Besides problems due to poor wiring connections, two main types of faults can occur in an electrical system - an open circuit or a short circuit.
Open circuit problems occur when there is a break in the electrical circuit that interrupts the flow of current. An open circuit will cause the electrical component to shut off.
Short circuit problems are caused by a short in a section of a circuit, which causes current to leak from another circuit, usually directly to the body. Short circuits are usually caused by a break in the insulation of the wires, allowing the wire to touch another wire or some grounded element, such as the body. Short circuits usually cause the associated fuse to blow.
Detecting a broken chain
To check for continuity, connect a circuit tester or the negative probe of a voltmeter to the negative battery terminal or a grounded component.
Connect the second probe to a connection in the circuit being tested, preferably the one closest to the battery or fuse. This section of the circuit should be live from the battery, unless the battery connector is dead or the fuse is blown (at the same time, do not forget that some electrical circuits are turned on only when the key in the ignition switch 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 voltage is present (what does the control lamp or voltmeter reading indicate), this means that the section of the circuit between the corresponding connection and the switch has no breaks.
Continue testing the circuit in the same manner, and if you find a section where there is no voltage, then the open circuit has occurred between that point and the point where there was voltage in the previous test. Most open circuit problems are caused by a broken or loose connector.
Detecting the source of a short circuit
To locate the source of the short circuit, first disconnect the load elements of the circuit (load elements of a circuit are elements that consume electrical current, such as light bulbs, electric motors, heating elements, etc.).
Remove the appropriate fuse and connect the tester or voltmeter probes to the fuse terminals.
Turn on the power to the circuits, remembering that some electrical circuits are only turned on when the key in the ignition switch is turned to a certain position.
If voltage is present (what does the control lamp or voltmeter reading indicate), this means that there is a short circuit in the circuit.
If there is no voltage when testing, but the fuse still blows when the circuit load is connected, this indicates that the load element has failed.
Detecting poor grounding
The negative terminal of the battery is connected to the "ground" - the metal of the car body, engine or gearbox, while many electrical components are connected in such a way that only the positive wire goes to them, and the current returns to the battery through the metal of the car body. This means that the fastening of the electrical component and the car body are part of the electrical circuit. As a result, a poor or rusty fastening can cause the component to fail or lead to its unstable or poor operation. In particular, the bulbs may burn with a dim light (especially if the grounding point of this light bulb is used to ground some other electrical equipment that is switched on), electric motors can run slowly, and the operation of one circuit can have subtle effects on the operation of another circuit. Remember that many vehicles use ground wires between certain components, such as the engine/transmission and the body, 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, it is necessary to disconnect the battery and connect one of the ohmmeter probes to a reliably grounded element. Connect the other probe to the wire or connection to the body that needs to be checked. The resistance shown by the ohmmeter should be zero; if not, check the connection as follows.
If you suspect a missing ground, disassemble the connection and clean the body area and the wire terminal (or the element's grounding surface) to bare metal. Carefully remove all traces of dirt, then use a knife to remove all paint so that reliable contact between the two metal surfaces is achieved. When reassembling, tighten the connector securely; when connecting the wire terminal, install a serrated washer between the terminal and the body surface to ensure a secure connection. When connecting, prevent future corrosion by applying a layer of Vaseline or silicone grease.