Warning! Read the precautions in section 6 and in section 14 before starting work.
The following tests are designed to test basic electrical circuits and should not be used to test sensitive electrical circuits (such as engine management system or anti-lock brake system), especially if there is an electronic control unit in the circuit.
A typical electrical circuit consists of a piece of electrical equipment, switches, relays, motors, fuses, blown jumpers or circuit breakers, and wires and connections that connect all the elements together and connect them to the battery and vehicle ground.
Before starting to test a faulty circuit, study the circuit diagram of this circuit in order to understand what elements it consists of. The source of the malfunction can be found faster if you determine which of the elements of this circuit are working properly. If several elements or circuits fail at once, the problem is probably a blown fuse or poor grounding, since often one fuse is responsible for several circuits.
Problems with the operation of an electrical system are usually caused by simple causes such as oxidized or unreliable contacts, a blown fuse, a blown blown jumper, or a faulty relay (a description of the relay test operation is given in subsection 14.3). Visually inspect the condition of all fuses, wires, and connections in the faulty circuit before checking other elements in the circuit.
If you are going to use test equipment, use the circuit diagrams to determine which connections need to be checked to find a problem.
The main instruments needed to find a fault in the electrical circuit are:
- circuit tester or voltmeter (or a 12 volt light bulb with connecting wires);
- control lamp with power supply (or a continuity tester);
- ohmmeter (for measuring resistance);
- battery;
- probes with wires;
- loop wire, preferably with a circuit breaker or fuse, which can be used to test wires or electrical equipment.
Before troubleshooting using test equipment, study circuit diagrams to determine connection points.
To find a loose connection or short circuit point (usually due to a bad or dirty connection or damaged insulation) the wires can be shaken by hand to see if the circuit breaks when the wire is moved. In this way, you can find a point with an unreliable connection or a point where a short circuit occurs. This verification method can be used along with the rest of the tests described below.
In addition to the problems associated with a bad connection, the electrical circuit can have two other main faults - the presence of an open circuit or a short circuit.
An open in the circuit can be caused by a break in some wire, or by a lack of connection somewhere in the circuit, preventing current from flowing. An open in the circuit will cause the failure of any piece of electrical equipment in operation, but will not blow the fuse of this circuit.
Short circuit faults are caused by a short somewhere in the circuit, which causes the current flowing in the circuit to flow in another circuit and, most often, to ground. A short circuit is usually caused by a broken insulation that allows the power wire to touch either another wire or a grounded item such as a body. A short circuit will blow the fuse of the corresponding circuit.
Before locating the source of a fault or when repairing an electrical system, remember that different types of wires have different colors.
Finding the break in the chain
1. To find an open circuit, connect one of the test light probes to the negative terminal of the battery or vehicle ground.
2. Connect the second probe to a connection in the circuit under test, preferably as close as possible to the battery or fuse.
3. Apply voltage to the circuit. Do not forget that in some circuits voltage is present only if the key in the ignition switch is turned to a certain position.
4. If voltage is present (what will be indicated by a lit control light or voltmeter readings), this means that the part of the circuit between the connection and the battery is good.
5. Continue checking the rest of the chain in the same way.
6. When you find the point where there is no voltage, it means that the source of the fault lies between that point and the last point where voltage was present. Most problems are caused by a bad connection.
Finding a short circuit
Loads are items of electrical equipment that consume electricity, such as light bulbs, motors, heating elements, etc.
1. To test the circuit for a short circuit, first disconnect the circuit load.
2. Remove the fuse for the appropriate circuit and connect a test light or voltmeter to the fuse terminals.
3. Apply voltage to the circuit. Do not forget that in some circuits voltage is present only if the key in the ignition switch is turned to a certain position.
4. If voltage is present (what will be indicated by a lit control light or voltmeter readings), this means that there is a short circuit in the circuit.
5. If there is no voltage, but the fuse still blows when the load elements are connected, then one of these elements is faulty.
Checking the reliability of grounding
The negative battery terminal is connected to the vehicle ground—the metal of the engine/gearbox and the vehicle body—and most electrical systems are designed to have only one lead wire going to the electrical item, with the current returning through the metal of the vehicle body. This means that the fastening of the electrical equipment element and the car body are part of the electrical circuit. Therefore, poor or oxidized fastening can cause a large number of problems in the electrical system, from complete failure of the circuit to unreliable operation. In particular, light bulbs can burn dimly (especially if another circuit is switched on using the same ground), engines (e.g. wiper motors or radiator cooling fan) may operate slowly, and turning on one circuit may affect the operation of another circuit. Note that many vehicles use bonding ground strips between various parts of the vehicle, such as between the engine/transmission and the body, usually when there is no metallic contact between the components due to the use of rubber mounts, etc.
1. To check the reliability of grounding, disconnect the battery and connect one of the ohmmeter probes to the vehicle ground.
2. Connect the second probe to the wire or ground point you want to test.
3. The resistance recorded by the ohmmeter must be zero; if not, check the connection as follows.
4. If you think the connection is not good, disassemble the connection and clean to bare metal the contact surface and the wire terminal or the surface of the grounding element.
5. Remove all dirt and corrosion, then use a knife to remove a layer of paint in order to obtain a reliable metal-to-metal bond.
6. When assembling, fix the connection securely; When installing the wire terminal, use toothed washers between the terminal and the body.
7. After connection, apply a layer of Vaseline or silicone grease to the connection to prevent corrosion.
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