Before attempting to determine the source of a fault, study the relevant electrical circuit diagram to gain an understanding of the components in that circuit. The number of possible sources of failure can be reduced by checking the operation of other components in the circuit. If several components or circuits fail at the same time, the fuse common to these circuits or components may be faulty, or the connection to the body - the "ground" - may be broken.
The causes of the malfunction are 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 faulty circuit before proceeding to check the remaining components. Use the electrical schematics to identify the terminals that need to be tested to locate 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 wires with probes, a jumper, preferably with a circuit breaker or fuse, which is used to bypass the wires or components being tested.
In addition to poor wiring connections, there are two other main types of faults that can occur in an electrical system: an open circuit or a short circuit.
The circuit is opened as a result of a break in the electrical circuit, which interrupts the current, causing the electrical component to switch off.
To check the integrity of a circuit, connect a circuit tester or voltmeter: one lead to the negative battery terminal or a grounded element, and the other lead to a terminal in the circuit being tested, preferably closest to the battery or fuse. In this case, the section of the circuit being tested must be under voltage from the battery, unless the battery connection connector does not conduct current or the fuse has 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 item being tested.
If there is tension (what does the control lamp or voltmeter reading indicate?), this means that there are no breaks in the section of the circuit between the corresponding connection and the switch.
Material copied from the resource peugeotbook
If a section is found where there is no voltage, then the circuit is broken between this point and the point of the previous test where there was voltage. A break in the circuit is caused by a damaged or loose connector.
To detect the source of the short circuit, check the power 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 circuits, keeping in mind that some electrical circuits are only turned on when the key in the ignition switch is turned to a certain position.
If there is voltage in the circuit (what does the control lamp or voltmeter reading indicate?), this means that there was a short circuit in it.
If there is no voltage during the test, but 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 gearbox. A poor or corroded fastening may cause the component to fail or malfunction. Keep in mind that many vehicles use ground wires to connect certain components (such as engine, transmission, body), where there is no direct contact between metal elements 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 leads to a reliably grounded element. Connect the other lead to the wire or connection to the body that needs to be tested. The resistance shown by the ohmmeter should be zero, if not, check the connection as follows.
If there is any doubt about the reliability of the ground contact, disassemble the connection, remove any dirt and clean the contacts. When reassembling, tighten the connector fastener by applying a layer of petroleum jelly or silicone grease to prevent corrosion.