ELECTRICAL CHECKS

Check field windings

A cheap multimeter is fine for these tests. First, measure the resistance of the field windings on the rotor. These should be in the region of 1 to 5Ω. If wildly different, replace the rotor or get it rewound.

Test negative diodes

Switch the meter to its diode test setting. Connect the red probe to the stator’s neutral wire terminal and black to the casing. It should read ‘1’. Reverse the connections. It should read around 0.7V.

Test positive diodes

Connect the red probe to the main output terminal and black to the stator’s neutral terminal. Again, there should be a ‘1’ and a number if the probes are reversed. The number’s not too critical.

Rectification

The diodes are pressed into a frame. To test single diodes: red probe to terminal, black to casing. It should read 0.5-0.8V one way and ‘1’ with the probes reversed. Corrosion can lead to problems.

Test stator

Disconnect the stator coils and test using the meter on its resistance setting. You should measure a minimal resistance between any two terminals of the stator – typically ranging from 0.2 to 0.3Ω.

Identify shorts

Any ‘1’ reading means a broken winding. Replace the stator. Short-circuits are impossible to detect without special equipment – but burned or discoloured windings are a good telltale.