You can fix and diagnose a problem with an alternator with careful diagnosis, using a multi-meter and / or an insulation tester, like a Megger.
In order to perform theses tests you should ensure you are electrically competent to do so, ideally having passed a current electrical testing qualification in your country. We have instructions on how to diagnose and fix different common problems. These instructions are generic and should always be undertaken in accordance with your alternators operation manual.
Important: Don't reply on the voltage readings from your control panel to diagnose an alternator failure. You should read the voltages OFF load from a multimeter, at the generator terminals.
380-415V, 3 Phase, High / Low Voltage Off Load