Before diagnosing an electrical circuit, first study the wiring diagram. If several elements fail at the same time, the problem may be with a fuse or ground.
Electrical faults are usually caused by simple causes such as corroded connections, a faulty ground connection, a blown fuse, or a defective relay (follow Section General description of fuses and relays). Visually inspect the condition of all fuses, wires, and connections in the applied circuit before testing components. Use wiring diagrams to pinpoint the exact location of the problem.
Basic tools required for electrical troubleshooting, circuit tester or voltmeter; stroboscope; ohmmeter; battery and a set of load resistors. Before troubleshooting instrumentation, use the wiring diagram to determine where to make connections.
Move the wiring to locate the break in the wiring. This test method can be used in conjunction with other tests described in the following subsections.
Other than broken connections, the two main types of electrical circuit damage are open circuits or short circuits.
Circuit damage caused by an open prevents current from flowing to the cells.
A short circuit is usually caused by damage to the insulation of the electrical wiring, which allows the power wire to touch either another wire or a grounded body element. A short circuit will blow the fuse.
Finding an open circuit
1. To test for an open circuit, connect one wire with a circuit tester or voltmeter to the negative battery post.
2. Connect other wiring of the circuit under test, preferably close to the battery or fuse.
Turn on the ignition.
3. If voltage is present, then the circuit between the connector and the battery is good.
4. Continue checking the rest of the chain in the same way.
5. When a no-voltage point is found, the fault is between that point and the previous test point with voltage. Most failures can be caused by broken, corroded or loose connections.
Search for a short circuit
1. Before starting the search for a short circuit, disconnect the electrical equipment from the circuit.
2. Remove the appropriate fuse from the circuit and connect a tester or voltmeter to the fuse connection.
3. Turn on the ignition.
4. If voltage is present, there is a short in the circuit.
5. If there is no voltage, this indicates internal damage to the electrical equipment.
Finding a short to ground
1. The negative terminal of the battery is connected to the car body. This means that the engine, transmission and body form part of the chain. Open or corroded components can cause a range of short circuits in an electrical circuit, up to and including complete failure of the circuit.
2. Check if the component is well grounded. Connect the battery cable terminal with an ohmmeter to a known good ground point. Connect other wiring to the earth to be tested. The ohmmeter reading should be zero; if not, check the connection as follows.
3. If grounding is faulty, clean ground contact surfaces to bare metal, securely tighten connections when reassembling; if a wire clamp is being installed, use serrated washers to ensure a good connection. Lubricate the connection with petroleum jelly or other lubricant.