Electrical Component Failure Diagnosis - Ground Fault Diagnosis

The negative battery terminal is grounded to the vehicle "body ground", which consists of the metal of the engine, chassis and body components. The electrical circuits of most electrical equipment are constructed so that wiring is used only to supply power to the load from the positive battery terminal, while the return current to the battery flows through the body ground metal. This means that the mounting fasteners of electrical loads form the return part of the electrical circuit. Due to this situation, loosening of the mounting or corrosion of the supporting elements of a working circuit component leads to malfunction of the circuit (from complete failure of the circuit to partial failure of various sections of the circuit). In particular, as a result of loose fasteners, the brightness of lighting devices may decrease (especially when there is a common ground with another circuit), or the speed of an electric motor (for example, a windshield wiper drive or cooling fan) may decrease. In addition, failure of one circuit may cause malfunction of another circuit that appears to be unrelated to the failed one. Note that on many vehicles, certain units are connected to each other by special ground straps. Such straps are used where there is no direct metal-to-metal contact between the units because the mountings are fitted with flexible rubber bushings (such as the engine mounts to the vehicle chassis).

To check the ground of a component, disconnect the battery and connect one ohmmeter lead to a known good ground point on the vehicle. Connect the second meter lead to the ground point of the component being tested. The meter should indicate zero resistance; otherwise, check the connection for continuity.

If poor quality of the terminal connection is suspected, disassemble the ground terminal assembly and clean the mating terminal surfaces down to bare metal. Remove all traces of corrosion and dirt, then scrape off paint with a knife to ensure reliable metal-to-metal contact. When assembling, make sure the fasteners are tightened securely. To guarantee a good electrical connection, install serrated washers between the wiring terminals and the ground contacts. To prevent corrosion in the future, coat mated terminal connections with acid-free petroleum jelly or silicone grease. Ignition system sealing compound aerosol and moisture-repellent grease are also good products.