The home of automatic scootering

Home Workshop: Electrical fault finding (pt2)

Voltage checks

A voltage check can determine whether power is reaching a component. Use a multimeter set to the DC (direct current) voltage scale to check for power from the battery or regulator/rectifier, or set to the AC (alternating current) voltage scale to check for power from the alternator. A test light can be used to check for dc voltage. The test light is the cheaper component, but the meter has the advantage of being able to give a voltage reading.

Connect the meter or test light in parallel (ie across the load).

Identify the relevant wiring circuit by referring to the wiring diagram at the end of this manual. If other electrical components share the same wiring circuit, take note whether they are working correctly – this is useful information in deciding where to start checking the circuit.

If using a meter, check first that the meter leads are plugged into the correct terminals on the meter (red to positive (+), black to negative (-). Set the meter to the appropriate volts function (dc or ac), where necessary at a range suitable for the battery voltage – 0 to 20 VDC. Connect the meter red probe (+) to the power supply wire and the black probe to a good metal earth (ground) on the scooter’s frame or directly to the battery negative terminal. Battery voltage, or the specified voltage, should be shown on the meter with the ignition switch, and if necessary any other relevant switch, ON.

If using a test light, connect its positive (+) probe to the power supply terminal and its negative (-) probe to a good earth (ground) on the scooter’s frame. With the switch, and if necessary any other relevant switch, ON, the test light should illuminate.

If no voltage is indicated, work back towards the power source continuing to check for voltage. When you reach a point where there is voltage, you know the problem lies between that point and your last check point.

Earth (ground) checks

Earth connections are made either directly to the engine or frame (such as the starter motor or ignition coil which only have a positive feed) or by a separate wire into the earth circuit of the wiring harness. Alternatively a short earth wire is sometimes run from the component directly to the scooter’s frame.

Corrosion is a common cause of a poor earth connection, as is a loose earth terminal fastener.

If total or multiple component failure is experienced, check the security of the main earth lead from the negative (-) terminal of the battery, the earth lead bolted to the engine, and the main earth point(s) on the frame. If corroded, dismantle the connection and clean all surfaces back to bare metal. Remake the connection and prevent further corrosion from forming by smearing battery terminal grease over the connection.

To check the earthing of a component, use an insulated jumper wire to temporarily bypass its earth connection – connect one end of the jumper wire to the earth terminal or metal body of the component and the other end to the scooter’s frame. If the circuit works with the jumper wire installed, the earth circuit is faulty.

To check an earth wire first check for corroded or loose connections, then check the wiring for continuity between each connector in the circuit in turn, and then to its earth point, to locate the break.

WARNING: Remember that all electrical circuits are designed to conduct electricity from the battery, through the wires, switches, relays, etc. to the electrical component (light bulb, starter motor, etc). From there it is directed to the frame (earth) where it is passed back to the battery. Electrical problems are basically an interruption in the flow of electricity from the battery or back to it.

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