Volkswagen Golf/Jetta 2 manual

Electrical fault-finding - general information
Body electrical systems / Electrical fault-finding - general information

1 A typical electrical circuit consists of an electrical component, any switches, relays, motors, fuses, fusible links or circuit breakers related to that component and the wiring and connectors that link the component to both the battery and the chassis. To help you pinpoint an electrical circuit problem, wiring diagrams are included at the end of this Chapter.

2 Before tackling any troublesome electrical circuit, first study the appropriate wiring diagram to get a complete understanding of what components are included in that individual circuit. Trouble spots, for instance, can be narrowed down by noting if other components related to the circuit are operating properly. If several components or circuits fail at one time, the problem is probably in a shared fuse or earth connection, as more than one circuit can be routed through the same connections.

3 Electrical problems usually stem from simple causes, such as loose or corroded connections, a faulty earth, a blown fuse, a melted fusible link or a faulty relay. Visually inspect the condition of all fuses, wires and connections in a problem circuit before testing the components. Use the diagrams to note which terminal connections will need to be checked in order to pinpoint the trouble spot.

4 The basic tools needed for electrical faultfinding include a circuit tester or voltmeter (a 12-volt bulb with a set of test leads can also be used), a continuity tester, a battery and set of test leads, and a jumper wire, preferably with a circuit breaker incorporated, which can be used to bypass electrical components.

Before attempting to locate a problem with test instruments, use the wiring diagram to decide where to make the connections.

Voltage checks
5 Voltage checks should be performed if a circuit is not functioning properly. Connect one lead of a circuit tester to either the negative battery terminal or a known good earth. Connect the other lead to a connector in the circuit being tested, preferably nearest to the battery or fuse. If the tester bulb lights, voltage is present, this means that the part of the circuit between the connector and the battery is problem-free. Continue checking the rest of the circuit in the same fashion.

When you reach a point at which no voltage is present the problem lies between that point and the last test point with voltage. Most problems can be traced to a loose connection. Bear in mind that some circuits are live only when the ignition switch is switched to a particular position.

Finding a short circuit
6 One method of finding a short circuit is to remove the fuse and connect a test lamp or voltmeter to the fuse terminals with all the relevant electrical components switched off.

There should be no voltage present in the circuit. Move the wiring from side to side while watching the test lamp. If the bulb lights there is a short to earth somewhere in that area, probably where the insulation has rubbed through. The same test can be performed on each component in the circuit, even a switch.

Earth check
7 To check whether a component is properly earthed, disconnect the battery and connect one lead of a self-powered test lamp (sometimes known as a continuity tester) to a known good earth point. Connect the other lead to the wire or earth connection being tested. If the lamp lights, the earth is sound; if not, it must be rectified.

8 The battery negative terminal is connected to earth ( the metal of the vehicle body) and most systems are wired so that they only receive a positive feed, the current returning via the metal of the vehicles body. This means that the component mounting and the body form part of that circuit and loose or corroded mountings, therefore, can cause a range of electrical faults. Note that these may range from total failure of a circuit to a puzzling partial fault. In particular, lamps may shine dimly (especially when another circuit sharing the same earth point is in operation), motors (eg. wiper motors or the radiator cooling fan motor) may run slowly and the operation of one circuit may have an apparently unrelated effect on another. Note that a poor earth may not cause the circuits fuse to blow; in fact it may reduce the load on the fuse.

9 If an earth connection is thought to be faulty, dismantle the connection and clean back to bare metal both the bodyshell and the wire terminal or the components earth connection mating surface. Be careful to remove all traces of dirt and corrosion, then use a knife to trim away any paint, so that a clean metal-to-metal joint is made. On reassembly, tighten the joint fasteners securely; if a wire terminal is being refitted, use serrated washers between the terminal and the bodyshell to ensure a clean and secure connection. When the connection is remade, prevent the onset of corrosion in the future by applying a coat of petroleum jelly or silicone-based grease or by spraying on (at regular intervals) a proprietary ignition sealer or a water dispersant lubricant.

Continuity check
10 A continuity check is necessary to determine if there are any breaks in a circuit.

With the circuit switched off (ie no power in the circuit), a self-powered test lamp (sometimes known as a continuity tester) can be used to check the circuit. Connect the test leads to both ends of the circuit (or to the positive end and a good earth); if the test lamp lights, the circuit is passing current properly. If the lamp does not light, there is a break somewhere in the circuit.

11 The same procedure can be used to test a switch, by connecting the continuity tester to the switch terminals. With the switch in the relevant position, the test lamp should light.

Finding an open circuit
12 When checking for possible open circuits, it is often difficult to locate them by sight because oxidation or terminal misalignment are hidden by the connectors; merely moving a connector on a sensor or in the wiring harness may correct the fault. Remember this if an open circuit is indicated when faultfinding in a circuit. Intermittent problems may also be caused by oxidised or loose connections.

13 Electrical fault-finding is simple if you keep in mind that all electrical circuits are basically electricity flowing from the battery, through the wires, switches, relays, fuses and fusible links to each electrical component (light bulb, motor, etc.) and to earth, from where it is passed back to the battery. Any electrical problem is an interruption in the flow of electricity from the battery.

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