Body Electrical System Troubleshooting - General Information

GENERAL INFORMATION

A typical electrical circuit may include an electrical component, various switches, relays, electric motors, fuses, fusible links or circuit breakers associated with the component, as well as wiring and electrical connectors used to connect the component to the battery and body ground. To assist in troubleshooting electrical circuits, wiring diagrams are provided at the end of this manual. Before starting work to troubleshoot any electrical circuit, carefully study the corresponding wiring diagram in order to clearly understand its function. Narrowing down a fault is usually done by gradually identifying and eliminating normally functioning components of the same circuit. When several components or circuits fail simultaneously, the most likely cause of the failure is a blown fuse or a ground fault (different circuits in many cases may share one fuse or one ground terminal).

Electrical failures are often caused by simple reasons such as corroded terminals, a blown fuse, a burnt fusible link, or a faulty relay. Perform a visual inspection of all fuses, wiring, and electrical connectors of the circuit before proceeding to more specific testing of its components.

When using diagnostic instruments to locate a fault, carefully plan, according to the attached wiring diagrams, at which points of the circuit and in what sequence the instrument should be connected in order to identify the fault most efficiently.

Basic diagnostic tools include a circuit tester or voltmeter (a 12 V test lamp with a set of jumper wires may also be used) and a circuit continuity tester (probe) consisting of a bulb, its own power source, and a set of jumper wires. In addition, always keep in the vehicle a set of jump-start cables with alligator clips and, preferably, a circuit breaker, which can be used to bypass and connect various electrical components during circuit diagnosis. As mentioned above, before testing a circuit with diagnostic equipment, determine from the wiring diagrams where to connect it.

Voltage Check

Voltage checks are performed when a circuit malfunctions. Connect one lead of the circuit tester either to the negative battery terminal or to a well-grounded point on the vehicle body. Connect the other lead of the tester to the circuit connector terminal, preferably the one nearest to the battery or fuse. If the test lamp lights, voltage is present in this section of the circuit, confirming that the circuit between this terminal and the battery is intact. Continue in the same manner to test the remainder of the circuit. If no voltage is detected, there is a fault between this point and the last point tested (where voltage was present). In most cases, the cause of failure is loose electrical connections and poor contact quality.

CAUTION

Note that power is supplied to some electrical circuits only when the ignition key is in the "ACC" (Accessory) or "RUN" positions.

Short Circuit Search

One method of locating a short circuit is to remove the fuse and connect a test lamp or voltmeter in its place. There should be no voltage in the circuit. Flex the wiring while observing the test lamp. If the lamp flickers, there is a short to ground somewhere in this harness, possibly caused by chafed wire insulation. A similar check can be performed for each component of the circuit, including switches.

Ground Circuit Check

This check is performed to determine the integrity of a component ground. Disconnect the battery and connect one lead of the self-powered test lamp to a known good ground point. Connect the other lead of the lamp to the harness or terminal being checked. If the lamp lights, the ground is good (and vice versa).

Continuity Check

This check is performed to detect open circuits. After disconnecting power to the circuit, check it with a self-powered test lamp. Connect the probe leads to both ends of the circuit (or to the “power” end (+) and a well-grounded body point). If the test lamp lights, there is no open in the circuit. If the lamp does not light, the circuit has lost continuity. Similarly, a switch can be checked by connecting the probe to its terminals. When the switch is set to ON, the test lamp should light.

Locating an Open Circuit

When diagnosing a suspected open circuit, it is difficult to visually find the cause of the fault, since inspection of terminals for corrosion or poor contact quality is hindered by limited access (terminals are usually enclosed in the connector housing). Sharply flexing the sensor connector housing or its wiring harness in many cases restores continuity. Do not forget this when attempting to locate the cause of a suspected open circuit. Intermittent faults may be caused by terminal oxidation or poor contact quality.

Diagnosing electrical circuit faults is not a difficult task, provided there is a clear understanding that current flows from the battery through wires, switches, relays, fuses, and fusible links to all electrical loads (lamp, electric motor, etc.), and then returns to the battery through the vehicle ground. Any problem with electrical equipment failure can only be caused by an interruption of the current supply from the battery to the load, or of its return to the battery.