Electrical circuits

PROCEDURE

1. A typical electrical circuit consists of electrical components, switches, relays, motors, fuses, fusible links, and wires and connectors that join all components together, to the battery and to the vehicle "ground".
2. The cause of a malfunction can be found much faster if you determine which components of this circuit operate normally. If several components or circuits fail at the same time, the problem is probably a blown fuse or a poor ground connection, since one fuse often protects several circuits.
3. Problems in the electrical system are usually caused by simple faults, such as corroded or loose contacts, a blown fuse, a blown fusible link, or a faulty relay. Visually inspect the condition of all fuses, wires, and connectors in the faulty circuit before checking other components of this circuit.
4. The main tools required to locate a fault in a circuit are:

– tester or voltmeter (or 12 V test lamp with connecting wires);
– test lamp with power source (or continuity tester);
– ohmmeter (for measuring resistance);
– battery;
– probes with wires;
– jumper wire, preferably with a circuit breaker or fuse, which can be used to check wires or electrical components.

5. To locate a loose connection or a short circuit (usually caused by a poor or dirty connection, or damaged insulation), wiggle the wire by hand to see whether the circuit fails when the wire moves. In this way, locate the point with a loose connector or the point where the short circuit occurs.
6. In addition to problems caused by a loose connection, an electrical circuit can have two other main faults: an open circuit or a short circuit.
7. An open circuit may be caused by a broken wire or a missing connection in the circuit, which prevents current from flowing. An open circuit will cause an electrical component to malfunction but will not blow the fuse protecting this circuit.
8. A short circuit fault is caused by a contact in the circuit that causes the current flowing through the circuit to flow through another circuit, most often to "ground". A short circuit is usually caused by damaged insulation that allows the supply wire to touch either another wire or a grounded component, such as the body. A short circuit will blow the fuse protecting the affected circuit.
9. Before searching for the source of a fault or performing repairs on the electrical system, note that different types of wires have different colors.
10. To check for an open circuit, connect one probe of the test lamp to the negative battery terminal or to vehicle "ground".
11. Connect the second probe to a connection in the circuit being tested, preferably as close as possible to the battery or fuse.
12. Apply voltage to the circuit. Remember that in some circuits voltage is supplied only when the ignition key is turned to a specific position.
13. If voltage is present (indicated by the test lamp lighting or by the voltmeter reading), the part of the circuit between the connection and the battery is good.
14. Continue checking the rest of the circuit in the same way.
15. When a point is found where there is no voltage, the fault lies between this point and the previous point where voltage was present. Most problems are caused by a poor connection.
16. The negative battery terminal is connected to vehicle "ground" – the engine/transmission metal and the vehicle body – and most electrical systems are designed so that only one supply wire goes to each electrical component, and current returns through the vehicle body metal. This means that the mounting of an electrical component and the vehicle body are part of the electrical circuit. Therefore, a poor or corroded mounting may cause many faults in the electrical system, ranging from complete circuit failure to unreliable operation. In particular, lamps may glow dimly (especially if another circuit using the same ground point is switched on), motors (for example, wiper motors or the radiator fan) may run slowly, and switching on one circuit may affect the operation of another circuit. Note that many vehicles use bonding straps between various vehicle components, for example between the engine/transmission and the body, i.e. usually where there is no metal-to-metal contact between components due to the use of rubber mounts, etc.
17. To check the ground connection, disconnect the battery and connect one probe of an ohmmeter to vehicle "ground". Connect the second probe to the wire or ground point to be checked. The resistance shown on the ohmmeter should be zero; if not, check the connection as follows.
18. If you suspect the connection is faulty, disassemble the connection and clean the contact surface and wire terminal or the grounded component surface down to bare metal. Remove all dirt and corrosion, then use a knife to remove the layer of paint to obtain a reliable metal-to-metal contact. During assembly, secure the connection firmly; when installing the wire terminal, use toothed lock washers between the terminal and the body. After connecting, apply a layer of petroleum jelly or silicone grease to the connection to prevent corrosion.

FUSES

Fuses are used to interrupt a circuit when the current reaches a specified value, in order to protect components and wires that could be damaged by excessive current. A sudden increase in current is usually caused by a fault in the circuit, most often a short circuit.
The circuits protected by fuses are identified on the cover of the fuse box.

Before removing a fuse, switch off the corresponding circuit (or the ignition), then remove the fuse from its contacts. The wire inside the fuse should be visible; if the fuse has blown, the wire will be broken or melted.

Always use fuses of the specified rating. Never use a fuse of a different rating or replace it with anything else. Do not replace a fuse more than once without eliminating the source of the fault. The current rating of the fuse is marked on top of the fuse; note that fuses have different colors for easy identification.

If a fuse blows immediately after replacement, do not replace it again until the cause of the malfunction has been found and corrected. In most cases, the cause is a short circuit caused by poor insulation. If a fuse protects several circuits, try to locate the circuit causing the fuse to blow by connecting each circuit in turn (if possible). Always keep a set of spare fuses of different ratings in the vehicle. They should be secured at the base of the fuse block.