How an alternator is built and works, the two key diagnostic parameters, and common winding, regulator and diode bridge faults.
The alternator charges the battery and powers the vehicle’s electrical system while the engine runs. When the ignition is switched on, current flows through the brushes to the field winding; the rotor, driven by the crankshaft, induces AC voltage in the stator, and the diode bridge converts it to DC.
Two parameters matter in diagnostics: the speed at which the alternator starts producing rated voltage (about 14.5 V for a 12 V unit) and the current at high speed. Common faults include:
- Shorts in the field or stator winding
- One phase shorted to the housing
- Faulty regulator relay or diode bridge
A ripple component above 10 per cent of the load current indicates a fault in the alternator.