Alternator / stator

How it works

The alternator makes the bike's electricity while the engine runs. On most motorcycles it is a stator (fixed windings) under a crankcase cover, with a magnetised rotor on the end of the crankshaft spinning around it. The alternating current produced goes through the regulator-rectifier to charge the battery and power the lights, ignition and injection. The stator often sits in oil, which cools it. With heat and age, the insulation on its windings can break down: one phase burns out and charging drops, so the battery slowly drains while riding. Power-hungry accessories (heated grips, extra lights) load the alternator further, and its output is limited. The stator is checked with a multimeter: resistance between phases, insulation to earth and output voltage with the engine running, against the manual's values. Replacing it means removing the alternator cover, with a new gasket.

Maintenance tips

• ✓If the battery drains while riding, measure the charging voltage before replacing the battery.
• ✓Check the stator with a multimeter (resistance between phases, insulation to earth) against the manual.
• ✓Inspect the connector between stator and regulator: it is often the part that overheats and melts.
• ✓Don't overload the circuit with accessories that draw more than the alternator's spare capacity.
• ✓Replacing the stator needs a new cover gasket and sometimes a rotor puller.