Finding the source of a fault in an electrical system (Opel Omega B)

Warning! Read the safety precautions in section 6 and in section 14 before starting work.

The following tests are intended to test basic electrical circuits and should not be used to test sensitive electrical circuits (such as engine management systems or anti-lock braking systems), 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, fuse links or circuit breakers, and the wires and connections that connect all of the components together and to the battery and vehicle ground.

Before you begin testing a faulty circuit, study the circuit diagram to understand what components it consists of. You can find the source of the fault faster if you determine which components of the circuit are working properly. If several components or circuits fail at once, the problem is likely a blown fuse or poor grounding, since one fuse often controls several circuits.

Problems with the operation of the electrical system are usually caused by simple reasons such as corroded or unreliable contacts, a blown fuse, a burnt-out jumper, or a faulty relay (description of the relay test procedure is given in subsection 14.3). Visually inspect the condition of all fuses, wires and connections in the faulty circuit before testing other components of the circuit.



If you are going to use test equipment, use the schematic diagrams to determine which connections need to be tested to locate the fault.

The main devices required to find faults 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 device for checking the integrity of circuits);
• ohmmeter (for measuring resistance);
• battery;
• probes with wires;
• a jumper cable, preferably with a circuit breaker or fuse, that can be used to test wires or electrical components.

Before you begin troubleshooting using test equipment, study the circuit diagrams to determine the connection points.

To find a loose connection or short circuit point (usually due to a poor or dirty connection or damaged insulation) the wires can be shaken by hand to see if the circuit breaks when the wire is moved. This can help find a point with a poor connection or a point where a short circuit occurs. This test method can be used along with the other tests described below.

Besides problems associated with a poor connection, an electrical circuit can have two other main faults - an open circuit or a short circuit.

An open circuit can be caused by a broken wire or a loose connection somewhere in the circuit that prevents current from flowing. An open circuit will cause a piece of electrical equipment to fail to operate, but will not cause the fuse for that circuit to blow.

Short circuit faults are caused by a short somewhere in the circuit, causing the current flowing in the circuit to flow through another circuit, most often to ground. A short circuit is usually caused by a break in the insulation, allowing the supply wire to touch either another wire or a grounded element, such as the body. The short circuit causes the fuse for the corresponding circuit to blow.



Before troubleshooting or repairing an electrical system, remember that different types of wires have different colors.

Finding a break in the chain

1. To find an open circuit, connect one of the test light probes to the negative battery terminal or vehicle ground.

2. Connect the second probe to a connection in the circuit being tested, preferably located as close to the battery or fuse as possible.

3. Apply voltage to the circuit. Remember that some circuits only have voltage when the ignition key is turned to a certain position.

4. If voltage is present (what will the illuminated control lamp or voltmeter readings indicate?), this means that the part of the circuit between the connection and the battery is OK.

5. Continue checking the rest of the chain in the same manner.

6. When you find a point where there is no voltage, it means that the source of the problem lies between this point and the last point where voltage was present. Most problems are caused by a bad connection.

Finding a short circuit

Loads are the electrical equipment components that consume electrical energy, such as light bulbs, motors, heating elements, etc.

1. To test a 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 connection terminals.

3. Apply voltage to the circuit. Remember that some circuits only have voltage when the ignition key is turned to a certain position.

4. If voltage is present (what will the illuminated control lamp or voltmeter readings indicate?), this means that there is a short circuit in the circuit.



5. If there is no voltage, but the fuse still blows when connecting the load elements, then one of these elements is faulty.

Checking the reliability of grounding

The negative terminal of the battery is connected to the vehicle chassis – the metal of the engine/gearbox and the car body – and most electrical systems are designed so that only one supply wire goes to the component and the current returns through the metal of the car body. This means that the component's mount and the car body are part of the electrical circuit. Therefore, a poor or corroded mount can cause a wide range of problems in the electrical system, from complete failure of the circuit to unreliable operation. In particular, light bulbs may glow dimly (especially if another circuit is connected that uses the same ground), engines (for example, windshield wiper motors or radiator cooling fan) may operate slowly and the operation of one circuit may affect the operation of another circuit. Note that many vehicles use grounding straps between various components of the vehicle, such as between the engine/gearbox and the body, usually where there is no metal-to-metal contact between the components due to the use of rubber mounts etc.

1. To check for proper 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 registered by the ohmmeter should be zero; if not, check the connection as follows.



4. If you think the connection is not OK, disassemble the connection and clean the contact surface and the wire terminal or the surface of the grounded element down to bare metal.

5. Remove all dirt and corrosion, then use a knife to remove the paint layer to create a secure metal-to-metal bond.

6. When assembling, securely fix the connection; when installing the wire terminal, use toothed washers between the terminal and the body.

7. After connecting, apply a layer of Vaseline or silicone grease to the connection to prevent corrosion.