TESTING ELECTRICS
Electrical problems usually start with the battery. Each component in the charging circuit should be checked in turn with an appropriate multi-meter, starting with the battery.
BATTERY: Before looking elsewhere for electrical or ignition faults, the first rule is to establish the battery is fit and healthy. A 12V system such as on Jimmy’s converted G80 should give a battery reading of 12.5V or higher (6.4V minimum on a six Volt system).
RECTIFIER: Is the regulator/rectifier sending a charge to the battery? To check the charging system is functioning correctly on a 12V set-up, a reading of around 14.0V - 14.4V should be recorded across the battery with the bike running at a fast idle (7.0V - 7.2V for a 6V system). Less than 13V suggests there is a problem, more than 16V could point to rectifier failure.
ALTERNATOR: An alternator generates alternating current, which is converted into direct current (or charge) by the rectifier, before being stored in the battery.
To test the alternator’s condition, disconnect the alternator wires from the bike’s wiring. Find a working headlamp bulb and attach it to a pair of wires so that you can connect it to one pair of alternator wires in turn. With the engine running at a modest tickover the bulb should light up enough to see that revving the engine might blow it! Now connect the bulb to another pair of wires and a similar result should be observed. Don’t worry about which wires to use – just make sure that all the wires are tested, and give output by lighting the bulb. If the bulb does not light up, or is very dim, then you might have a problem with the rotor magnets.
Finally, connect one end of the lamp to the engine (earth) and the other end to any one wire, there should not be any bulb lighting. This would indicate a good stator.
On a three-lead RM20 type alternator, take on the bike reading tests by joining both the green/yellow and green/black output wires to one probe and the white/green to the other probe of your voltmeter (use the rectifier as a guide – your voltmeter simply replaces it to read the alternator output). A healthy alternating voltage should vary between 13-19V as you rev the engine. Similarly, take a reading between the white/green and green/yellow wires on a two-wire output, R21 type alternator.
RECTIFIER DIODE: If the alternator checks out OK then the fault is probably with the rectifier. The diode in the rectifier should conduct from positive to negative, but not in the other direction. It can be checked by measuring resistance between the alternator connections to +ve and -ve alternately. They should read a low resistance in one direction but a very high resistance in the other direction.