Electrical Malfunction Diagnosis - General Information

The procedures described below allow general diagnosis of the main electrical circuits. They must not be used to check sensitive electrical systems (such as ABS), especially those including electronic control modules (PCM).

A typical electrical circuit consists of a current-consuming device (working component), a set of switches, relays, actuator electric motors, fuses, fusible links/circuit breakers related to the operation of this component, as well as connecting wiring, its terminal contacts and connectors.

To simplify diagnostic procedures, the last Section of this Chapter contains wiring diagrams for the various electrical systems of the vehicle.

Before beginning to search for the cause of a failed current-consuming device, carefully study the relevant wiring diagram and try to clearly understand how the components of the suspected circuit operate. The list of possible causes of failure can be minimized by eliminating components that are functioning properly and are related to the operation of the circuit being tested. If several components fail at the same time, the most likely cause is a blown fuse/fusible link common to the relevant circuits, or a loss of ground.

Most electrical failures are caused by simple reasons, such as corrosion damage, or loose terminal connections, a blown fuse or fusible link, relay failure, etc. Before searching for internal faults in the failed component itself, carefully check the condition of all fuses, connectors and connecting wires related to its operation. To determine which connections and terminals must be checked, study the relevant wiring diagram.

Diagnostic equipment required for locating electrical faults includes a circuit tester/voltmeter (for some checks, a 12-volt test lamp with a set of jumper wires can also be used), a self-powered test light (sometimes called a continuity tester), an ohmmeter, a power source with a set of jumper wires, and a set of jumper wires fitted with various types of terminal clips and, preferably, a built-in circuit breaker or fuse (for bypassing suspect circuit sections or electrical components). Before using diagnostic equipment, carefully study the wiring diagram of the components in the relevant circuit.

To locate the cause of an intermittent fault (such faults are usually related to oxidation of terminal contacts or loosening of wiring connections), a simple circuit check can be performed by moving various sections of the wiring of the relevant circuit, which helps to localize the defective section of the circuit. This check can be performed together with any of the methods listed below in the corresponding subsections.

In addition to problems related to poor electrical connections, the most likely and frequently occurring faults in electrical circuits are open circuits and short circuits.

An open circuit is usually caused by mechanical damage to the conductors or disconnection of terminal contacts, which interrupts the electrical circuit and stops the flow of current. As a result of an open circuit, its working component stops functioning, but the corresponding fuses/fusible links do not blow.

A short circuit is an unintended electrical connection in the circuit wiring. In this case, current begins to flow along the shortest path, usually to ground. Short circuits are most often related to damaged wire insulation and always cause the corresponding fuses/fusible links to blow.