Contents: General description ⬇ Search for a broken chain ⬇ Finding the location of a short…⬇ Detection of short circuit to "mass" ⬇
Note: Before you begin, refer to the precautions given in Chapter of this Section. The following tests apply to main electrical circuits only and should not be used on ELECTRONIC circuits.
General description
1. An electrical circuit consists of electrical components, switches, relays, motors, fuses, reverse current relays, wiring. To find the exact fault in an electrical circuit, the electrical connection diagrams are given at the end of this Manuals.
2. Before diagnosing an electrical circuit, first study the electrical wiring diagram. If several components fail at the same time, the problem may be related to a fuse or ground.
3. Electrical faults are usually caused by simple things like corroded connections, faulty grounding, a blown fuse or a faulty relay (refer to Chapter 11). Visually inspect the condition of all fuses, wires, and connections in the application circuit before testing components. Use the wiring diagrams to accurately locate the fault.
4. The basic tools needed to troubleshoot an electrical problem are a circuit tester or voltmeter, a stroboscope, an ohmmeter, a battery, and a set of load resistors. Before troubleshooting with test instruments, use a wiring diagram to determine where to connect to the circuit.
5. To find the source of an intermittent wiring fault (usually due to a loose connection or damage to the wiring insulation), the method of "wiggling" the electrical wiring is used. In this way, it is possible to narrow the area of the fault to a specific circuit. This testing method can be used in combination with any of the tests described in the following subchapters.
6. Besides problems caused by a weak connection, there are two main types of faults that can occur in an electrical circuit: an open circuit or a short circuit.
7. As a result of a break in the circuit, the component powered by it stops working, but this is not related to the fuse.
8. When a circuit is shorted, the current begins to flow through a different path, usually to ground. A short circuit is usually caused by insulation failure, causing the supply wire to touch either another wire or a grounded component, such as the body. A short circuit usually causes the associated fuse to blow.
Search for a broken chain
9. To check for an open circuit, connect one lead of a tester or voltmeter to the negative battery terminal or a good ground.
10. Connect the other wire to the connector of the circuit being tested, preferably closer to the battery or fuse.
11. Turn on the circuit, remembering that some of them are only energized when the key in the ignition switch is turned to a certain position.
12. If voltage is present (indicated by a tester or voltmeter), this means that the part of the circuit between this connector and the battery is OK.
13. Continue checking the remaining parts of the chain in the same manner.
14. When a point is found where no voltage is present, the fault is between that point and the previous voltage point tested. Most faults can be caused by broken, corroded or loose connections.
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Finding the location of a short circuit
15. Before you begin searching for a short circuit, disconnect the electrical equipment from the circuit.
16. Remove the appropriate fuse from the circuit and connect a tester or voltmeter to the fuse connection.
17. Turn on the circuit, remembering that some of them are only energized when the key in the ignition switch is turned to a certain position.
18. If there is voltage (indicated by a tester or voltmeter), this means that part of the circuit between this connector and the battery is shorted.
19. If there is no voltage, but the fuse still blows with the load connected, this indicates a fault in the load itself.
Detection of short circuit to "mass"
20. The negative battery terminal is connected to ground This means that the electrical component's mount and the car body form part of the circuit. Loose or corroded mounts can cause shorts in the electrical circuit. In particular, lights may glow dimly (especially when another circuit connected to the same ground point is on), electric motors may spin slowly (for example, windshield wiper or radiator fan motors), and the operation of one circuit can affect the operation of another. Note that many vehicles use ground straps between certain components, such as the engine/transmission block to the body, where there is no metal-to-metal contact between components due to flexible rubber pads, etc.
21. To check the reliability of the component's connection to the "ground", disconnect the negative cable from the battery and connect one lead of the ohmmeter to a good "ground". Connect the other lead to the ground contact being tested. The resistance should be zero; if this is not the case, check the connection as follows.
22. If a ground contact defect is suspected, disassemble the connection and clean the body and the wire terminal or contact surface of the ground contact to bare metal. Then tighten the connection fasteners securely; if installing a wire terminal, use serrated washers between the terminal and the body to ensure a secure electrical connection. To prevent future corrosion, coat the connection with petroleum jelly or silicone grease.