The 30-second answer: On capacitor-excited generators (common on smaller brushless units), a dead run capacitor means no output. Discharge it first with an insulated screwdriver across the terminals — capacitors hold a charge that can bite you. Then test it with a multimeter’s capacitance setting and compare to the microfarad (µF) rating printed on it. If it reads far off or zero, replace it with the same µF and voltage rating — usually $10–$25.
Some generators — especially smaller brushless models — use a capacitor instead of an AVR and brushes to excite the alternator. When that capacitor dies, the generator runs perfectly but produces little or no power at the outlets. It is a cheap part and an easy swap, but a charged capacitor can shock you, so the safety step here is not optional.
How to tell if your generator uses a capacitor
If your unit has no brushes and no obvious AVR board, and the spec sheet or the side of the alternator mentions a capacitor (you will see a cylindrical can with a microfarad rating like “20 µF 450V”), it is capacitor-excited. These are common on inexpensive open-frame generators. The capacitor lives near the alternator, often behind the rear cover.
Safety first: discharge it
A run capacitor can hold a charge for a long time after the engine stops. Before you touch the terminals, shut the engine off and discharge the capacitor: lay the metal shaft of an insulated screwdriver across both terminals at once for a few seconds (hold the insulated handle only). A small pop is normal. Better yet, bleed it through a resistor if you have one. Skip this step and you can take a real jolt — I have seen it happen to people who assumed “it’s off, so it’s safe.”
Testing the capacitor
Once discharged, disconnect the leads and set your multimeter to the capacitance (µF) range. Read across the terminals and compare to the rating printed on the can. A healthy capacitor reads within about 10–15% of its marked value. A reading of zero, near-zero, or wildly off means it is dead. No capacitance setting on your meter? A bulged or leaking can is a dead giveaway, and a basic resistance test will at least show a hard short or a fully open unit.
Replacing it
Match the replacement exactly on two numbers: the capacitance in microfarads (µF) and the voltage rating. You can go up in voltage rating but never down, and the µF must match. A correct generator run capacitor is usually $10–$25. Discharge the old one, note which lead goes where, swap it in, and reassemble. Start the generator and check your output — it should jump right back to 120 V.
If voltage is still missing
If a new, correctly-rated capacitor does not bring the power back, the issue is in the windings or, on some units, residual magnetism. Try flashing the field to restore residual magnetism first — it costs nothing. If that fails, you are looking at stator or rotor testing, which is alternator-level work. Be honest with yourself about the value of the machine before you sink more time and parts into it.