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Home Workshop: Electrical fault finding (pt1)

A typical electrical circuit consists of an electrical component, the switches, relays, etc, related to that component and the wiring and connectors that link the component to the battery and the frame. Before tackling any troublesome electrical circuit, first study the wiring diagram thoroughly to get a complete picture of what makes up that individual circuit.

Trouble spots can often be narrowed down by noting if other components related to that circuit are operating properly or not. If several components or circuits fail at one time, chances are the fault lies either in the fuse or in the common earth (ground) connection, as several circuits are often routed through the same fuse and earth (ground) connections.

Electrical problems often stem from simple causes, such as loose or corroded connections or a blown fuse. Prior to any electrical fault finding, always visually check the condition of the fuse, wires and connections in the problem circuit. Intermittent failures can be especially frustrating, since you can't always duplicate the failure when it's convenient to test. In such situations, a good practice is to clean all connections in the affected circuit, whether or not they appear to be good. All of the connections and wires should also be wiggled to check for looseness which can cause intermittent failure.

If you don’t have a multimeter it’s highly advisable to obtain one – they are not expensive and will enable a full range of electrical tests to be made. Go for a modern digital one with LCD display as they are easier to use. A continuity tester and/or test light are useful for certain electrical checks as an alternative, though are limited in their usefulness compared to a multimeter.

Continuity checks

The term continuity describes the uninterrupted flow of electricity through an electrical circuit. Continuity can be checked with a multimeter set either to its continuity function (a beep is emitted when continuity is found), or to the resistance (ohms/Ω) function, or with a dedicated continuity tester. Both instruments are powered by an internal battery; therefore the checks are made with the ignition OFF. As a safety precaution, always disconnect the battery negative (-) lead before making continuity checks, particularly if ignition system checks are being made.

If using a multimeter, select the continuity function if it has one, or the resistance (ohms) function. Touch the meter probes together and check that a beep is emitted or the meter reads zero, which indicates continuity. If there’s no continuity there will be no beep or the meter will show infinite resistance. After using the meter, always switch it OFF to conserve its battery.

A continuity tester can be used in the same way – its light should come on or it should beep to indicate continuity in the switch ON position, but should be off or silent in the OFF position.

Note that the polarity of the test probes doesn’t matter for continuity checks, although care should be taken to follow specific test procedures if a diode or solid-state component is being checked.

Switch continuity checks

If a switch is at fault, trace its wiring to the wiring connectors. Separate the connectors and inspect them for security and condition. A build-up of dirt or corrosion here will most likely be the cause of the problem – clean up and apply water dispersant, or alternatively, use a dedicated contact cleaner and protection spray.

If using a multimeter, select the continuity function if it has one, or the resistance (ohms) function, and connect its probes to the terminals in the connector. Simple ON/OFF type switches, such as brake light switches, only have two wires whereas combination switches, like the handlebar switches, have many wires. Study the wiring diagram to ensure that you are connecting to the correct pair of wires. Continuity should be indicated with the switch ON and no continuity with it OFF.

Wiring continuity checks

Many electrical faults are caused by damaged wiring, often due to incorrect routing or chafing on frame components. Loose, wet or corroded wire connectors can also be the cause of electrical problems.

A continuity check can be made on a single length of wire by disconnecting it at each end and connecting the meter or continuity tester probes to each end of the wire. Continuity should be indicated if the wire is good. If no continuity is shown, suspect a broken wire. To check for continuity to earth in any earth wire connect one probe of your meter or tester to the earth wire terminal in the connector and the other to the frame, engine, or battery earth (-) terminal. Continuity should be indicated if the wire is good. If no continuity is shown, suspect a broken wire or corroded or loose earth point.

• These tips continue in the next issue, with advice on voltage and earth checks.

This workshop routine is taken from the Haynes manual – ‘Chinese scooters, 50-200cc’ (Haynes 4768). Visit www.haynes.co.uk for more information

WARNING: To prevent the risk of short circuits, the ignition switch must always be OFF and the battery negative (-ve) terminal should be disconnected before any of the scooter’s other electrical components are disturbed. Don’t forget to reconnect the terminal securely once work is finished or if battery power is needed for circuit testing.