Contents: Basic information ↡ Detecting chain breaks ↡ Short circuit detection ↡ Ground fault detection ↡
Note: Before starting work, read the warnings given in the section "Safety first!" and in section 1 of this chapter. The procedures described in this chapter apply only to the main electrical circuits and should not be used to check electronic circuits, especially those related to the electronic control unit.
Basic information
1. A typical electrical circuit consists of electrical components: switches, relays, motors, fuses or circuit breakers, wiring, and connectors that link the components together and to the battery and chassis. To help you troubleshoot electrical circuits, wiring diagrams are provided at the end of this manual.
2. Before you begin troubleshooting an electrical circuit, first study the relevant wiring diagram and gain a thorough understanding of the components in the circuit. Finding possible sources of trouble is easier if you make sure that other components in the circuit are working properly. If several components or circuits fail at the same time, the problem is likely to be a fuse or ground wire.
3. Electrical circuit faults usually result from simple causes such as poor connections in connectors, poor grounding, blown fuses, or faulty relays. Before you begin checking circuit parts, inspect the condition of all fuses, wires, and connectors in the faulty circuit. Use wiring diagrams to determine the order in which to check the circuit.
4. Basic tools for troubleshooting electrical circuits include a tester or voltmeter (a 12-volt test light may also be used for some tests); ohmmeter (to check the circuit for broken wires); battery; jumper wire, preferably fused, that can be used to bypass the wires or electrical components being tested. Before operating the instruments, study the wiring diagram to determine the points at which any measurements should be made.
5. To find the source of an "intermittent" electrical wiring fault (usually due to a poor connection or damaged insulation of the electrical wiring), you can simply move the wires. If the contact in the circuit disappears and then appears when moving the wires, then the source of the fault is in this place of the electrical wiring. This method of checking can be used together with any others described in the following subsections.
6. Besides poor contact, there are two other main types of faults: open circuit and short circuit.
7. When the circuit is broken, the component is disconnected from the power source or from the ground. In either case, it will not work.
8. Short circuits in a circuit are usually caused by a failure of the insulation in the wiring that allows a wire to touch either another wire or the housing. A short circuit usually blows the fuse for the circuit in question.
Detecting chain breaks
9. To locate an open circuit, connect one terminal of the tester or the negative terminal of the voltmeter to the negative terminal of the battery or to a good ground.
10. Connect the second terminal of the tester to the terminal in the circuit being tested, preferably the one closest to the battery or fuse. Battery voltage should be present at this point unless the battery or fuse cable itself is damaged (it should be noted that some circuits are closed when the ignition switch is in a certain position).
11. Turn on the circuit, then connect the tester terminal to the location closest to the area to be tested.
12. If voltage is present (or the control light is on), this means that the circuit section between the corresponding connector and the switch is OK.
13. Continue checking the circuit in the same manner.
14. When the point where there is no voltage is determined, the fault will be between that point and the previous point tested. Test that section of the circuit to locate the fault.
Short circuit detection
15. Before testing a circuit for a short circuit, first disconnect the load from the circuit (load - components that consume power, types of light bulbs, motors, heating elements, etc.).
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16. Remove the appropriate circuit fuse and connect the tester or voltmeter leads to the fuse terminals.
17. Turn on the circuit, keeping in mind that some circuits are closed when the ignition switch is in a certain position.
18. If voltage is present (or the control light is on), this means that there is a short circuit in the circuit.
19. If there is no voltage during this test, but the fuse blows when the load is connected, this indicates an internal fault in the load.
Ground fault detection
20. The negative terminal of the battery is connected to the "ground" - the engine-gearbox-body block. Most consumers of electrical energy are connected only to the positive wire, and the negative wire is the metal of the body. This means that the body mass is part of the circuit. In this regard, loose connections can cause a partial or complete break in the circuit. In particular, the lighting devices may burn dimly (especially if another circuit is connected that uses the same ground point), engines (such as windshield wiper motors or cooling fan motors) may rotate slowly and the action of one circuit may affect the operation of another. Note that many vehicles use flexible grounding bars to connect to the bodywork those units that do not have direct metallic contact with it, such as the engine-transmission unit mounted on rubber cushions
21. To check if a component is well grounded, disconnect the battery and connect one lead of the ohmmeter to a point with a good ground. Connect the other lead of the ohmmeter to the point of the component being tested. The ohmmeter should read zero resistance; if not, check the ground connection as follows.
22. If there is a suspicion that the ground connection is broken, disassemble this connection, clean both the contact point on the body and the wire terminal (or the mounting surface of the unit body) to the metal. Remove all traces of dirt and corrosion, scrape off the paint with a knife to ensure a clean metal-to-metal connection. Assemble the joint and tighten the fasteners carefully; if the wires have terminals on the ends, install serrated washers between the terminal and the housing to ensure a secure connection. To prevent corrosion, lubricate the outside of the connection with petroleum jelly or silicone grease.