failures about diesel generator set

There are just 4 failures about the diesel generator and with the processing methods
1. The diesel generator can not generate power,
This is because of the brush is losing the magnet
2.The voltage is not stability lower or higher,
This is because the speed not come the rated speed or there is over speed. or the AVR is damaged.
3.The voltage reduce to lower when there is loading,
This is because of the negative connection on the rectifier bridge or the speed is not stability
4.The diesel generator is overheating
This is because of the overloading, and lower speed , and the ventilation is blocked.