Single-phase compressor motors need a capacitor to create the phase shift for starting torque. When the capacitor fails, the motor hums loudly but won't spin. Common on older compressors and after electrical surges.

What you'll see

You flip the switch and the motor makes a loud humming or buzzing sound but doesn't rotate. It just sits there humming and getting hot. After a few seconds, the thermal overload trips and everything goes quiet. If you were to spin the motor shaft by hand and then flip the switch, it might start -- but it shouldn't need help. On some compressors, you'll see a cylindrical canister mounted on top of or near the motor -- that's the capacitor. It may look swollen, have leaked fluid, or show scorch marks.
Before you assume this is the problem

If the motor doesn't hum at all -- just silence -- the problem is more likely the pressure switch, wiring, or power supply. The capacitor only matters when the motor gets power but can't start spinning. If the motor starts but runs rough or weak, it could be a run capacitor instead (different from the start capacitor). See: Pressure Switch Failed.

See all causes of compressor won't start →

How to diagnose

  1. Identify the capacitor

    The start capacitor is a cylindrical canister, typically black or silver, about the size of a soda can or smaller. It's mounted on or near the motor and has two wire terminals. Some compressors have both a start capacitor and a run capacitor -- the start capacitor is usually the larger one. Check for obvious signs of failure: swelling, leaking oil, scorch marks, or a blown-out top.
    Result: Physical damage visible = replace. Looks OK = test with meter.

  2. Test with a multimeter

    Disconnect power, discharge the capacitor, then disconnect one wire. Set your multimeter to capacitance (microfarads) if it has that function. The reading should be within 10% of the value printed on the capacitor label. If it reads zero, open, or way below spec, it's dead. If you don't have a capacitance meter, set the multimeter to resistance (ohms) -- touch the probes to the terminals. You should see the resistance rise steadily from low to high as the capacitor charges from the meter's battery. If it stays at zero or infinity, the capacitor is bad.
    Result: Within 10% of rated microfarads = OK. Zero, open, or far below spec = replace.

How to fix it

  1. Replace the start capacitor

    Match the replacement to the original: same microfarad rating (printed on the label, e.g., 88-108 MFD) and same or higher voltage rating (e.g., 250V). The voltage rating can be higher but not lower. The microfarad rating must match. Disconnect the old wires (note which terminal they go to -- usually it doesn't matter for start caps as they're not polarized), mount the new one, reconnect.

  2. Check the centrifugal switch while you're in there

    Single-phase motors have a centrifugal switch that disconnects the start capacitor once the motor reaches about 75% speed. If this switch is stuck open, the capacitor never engages. If it's stuck closed, the capacitor stays in the circuit too long and burns out -- which is why capacitors fail repeatedly on some compressors. Check that the switch contacts are clean and the mechanism moves freely.

Common mistakes

Don't replace the start capacitor with a different microfarad value -- too high and you'll damage the start winding, too low and the motor won't start reliably. The voltage can be higher than original but never lower. Also: if capacitors keep failing, check the centrifugal switch. A stuck centrifugal switch will burn out a new capacitor within days. One more thing -- don't confuse the start capacitor with the run capacitor. The start capacitor gives the initial push; the run capacitor improves efficiency during operation. They have different values and purposes.

Parts & tools

Replacement start capacitor (match microfarad rating, same or higher voltage). Insulated screwdriver for discharging. Multimeter with capacitance function. Wrenches or nut drivers for mounting.

Review safety precautions before starting →

Safety

Capacitors store electrical charge even when the power is off. Disconnect power and wait a minute before touching the capacitor. Discharge it by shorting across the terminals with an insulated screwdriver. A charged capacitor can deliver a painful shock.

This issue can also cause

  • Overload Relay Trips Thermal overload or motor breaker keeps tripping: either an electrical issue or the motor is working too hard.