I've just bought a small 6KW diesel generator, it's only 2 years old but has about 2500 hours on it. Anyway, the engine seems fine, oil is clean and it starts right up no smoke like a dream. However 5 minutes after starting the engine bogs down and the voltage drops, then the unit cut's off due to low speed (has a digital controller). If I disconnect the generator capacitor it will run fine for hours, so I think the actual generator is faulty however it generates the required voltage fine before bogging down and the windings ohm out fine. The generator is the brushless type without an AVR but uses a capacitor. I've come across generators that have shorted windings and bog the engine as soon as it starts but, in this case, it works fine for 5 minutes I can even put a load on it and works fine, then it'll slow down like I'm overloading it and cut out even with nothing plugged in. What could this be, bad alternator, capacitor? Has anyone come across this??
if you remove the capacitor and short it with a screwdriver. Look for bulging or leaking. If so it needs replacement. With an ohmmeter you can set the meter to ohms and attach the leads and watch the ohmmeter change as it charges with the tiny current from the ohmmeter
DVMs that measure a capacitor are becoming more common.
If you want to start checking out the actual generator you will need a 4 wire ohmmeter and the troubleshooting specs for the generator head. Typically very low resistance like .5 ohms.