Before starting this section, ensure that the warnings and suggestions in the general alternator voltage fault finding section and your alternator manual have been followed.
This section applies to machines OFF LOAD. If you have a different problem go back to the alternator voltage fault page.
One phase Low Voltage.
This usually indicates that the phase has a low resistance to ground - it probably needs a rewind or replacement. In a machine with an AVR that uses single phase sensing, this will happen if one of the good phases are being used for sensing.
Low Voltage on One Phase, Other Phases Normal
A low voltage on one phase with the other two normal, this indicates a main stator fault. In this instance the AVR will be sensing from one of the phases that are at normal voltage. The phase that has a problem, will have low insulation resistance to or shorted to ground. You should perform an insulation test, according to the manufacturer to ensure the unit meets its required level. If it fails, you will need to rewind the alternator, or fit a new machine.
Low Voltage on One Phase, Other Phases High
This again indicates a main stator fault. In this instance the AVR will be sensing from the phase reading low, boosting the excitation system to increase the voltage, leading to an increase on the phases that are OK and little or no change on the phase that has a low resistance or shorted to earth.