Before attempting to determine the source of the fault, first study the relevant electrical diagram to obtain an idea 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 pair 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 shake test on the wires. 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.