How to Troubleshoot and Trace an Automotive Harness
- 1). Identify the problem circuits. If there is only one symptom or affected circuit, don't chase every other wire in the harness. If there are multiple symptoms, the harness itself may be at fault. Do a visual inspection. Follow the wires of the problem circuit. A good wiring diagram should give you the physical route of the wires as well, or at least the location of main components and connectors.
- 2). Check vulnerable points along the route. Most harness damage happens wherever the harness is disturbed. Check near the door hinges, the hood and the trunk. The movement of theses parts causes stretch or fatigue breaks in harnesses. Check the engine compartment, if any wires are close to the engine exhaust they may have melted together or burnt apart. Any wires that run underneath the battery may be melted through due to battery acid drip. Check bulkhead connectors and auxiliary connections; these may have pulled apart. Small screwdrivers and picks are useful to open and check theses connectors.
- 3). Check for voltage at the power input terminal of the affected component. There should be either 12 or 5 volts, depending on the system. If no voltage is present and it is switched on at its control, there is most likely an open circuit, or break in the wire. If voltage is present test the ground side of the component for any resistance. Do this by using the ohm-meter part of your multimeter. Resistance should be zero, if there is some present, the problem is in the grounding wire.
- 4). Pull the fuse for the circuit at the fuse box. test for power on one side -- there should be system voltage present. If not, there is a power distribution problem before the fuse. Insert one end of the paper clip into the terminal without power. Using alligator clips, attach a long ground lead. Test along the wire for continuity and resistance. High resistance or loss of continuity indicates a problem.
