Diagnosis - Alternator/Charging Circuit

The charging system consists of an alternator, a rectifier/regulator assembly and the battery.

The alternator is made up of two parts, the stator, which is mounted to the crankcase or the engine cover, and the rotor, mounted to the end of the crankshaft.

The stator is an assembly of 18 coils, arranged into 3 phases. The rotor is a series of magnets mounted in the engine flywheel, which are arranged so as to be positioned around the outside of the stator coils.

As the engine rotates the alternator produces an AC (alternating current) voltage in each of the three phases of the alternator, typically of around 35 to 40 volts AC at 4,000 - 5,000 rpm, although this figure varies between models.

As the battery requires DC (direct current) voltage for correct charging, this AC voltage must be first rectified to DC current, and then regulated to the correct voltage for the battery of 14.5 +/- 0.5 volts.

This is done by the rectifier/regulator, which uses diodes to convert the alternator output to DC Volts and limits the resulting output to the correct figure required for optimal battery charging.

If the charging circuit does not operate correctly, the following basic checks must be carried out before further diagnosis is performed:

• Check the battery terminals are clean and tight.

• Check the frame and engine earth connections are clean, tight and free from corrosion.

• Ensure the battery is fully charged and in good condition.

• Check that any fuse in the circuit is not blown and is of the correct rating.

Rectify any defects as necessary.

Diagnosis – Charging Circuit