The run capacitor gives the compressor and outdoor fan motor the extra kick they need to start and keep turning. When it weakens, you get the classic symptoms: the outdoor unit hums, the fan doesn't turn or turns slowly, or the compressor tries to start and trips the breaker.
This job involves line voltage and stored energy. The outdoor unit runs on 240 volts, and a capacitor can hold a dangerous charge even with the power off. If you're not comfortable working inside an electrical panel, stop here and call a technician.
1. Kill the power, then prove it's dead
Turn the thermostat off. Pull the disconnect near the outdoor unit and turn off its breaker. Remove the service panel, which is usually the corner cover where the power lines come in. Use your multimeter to confirm there's no voltage at the contactor before you go further.
2. Discharge the capacitor
Use a capacitor discharge tool or a suitable resistor with insulated leads across the terminals. On a dual capacitor, discharge HERM to C and FAN to C. Then check with your meter set to DC volts that it reads zero.
3. Look before you test
A bulging or domed top, a split case, or oily residue means it has failed. Replace it and skip the meter.
4. Photograph, then disconnect
Take a clear picture of which wire goes to which terminal. Most outdoor units use a dual run capacitor with three terminals:
- C: common
- HERM: the compressor (hermetic)
- FAN: the outdoor fan motor
Pull the wires off the terminals you're testing.
5. Measure
Set your meter to capacitance (µF). Put the leads on C and HERM, then on C and FAN. Compare each reading to the ratings printed on the label, for example 45/5 µF ±6%. If a reading falls outside that tolerance, the capacitor is weak and should be replaced.
Replacing it
- Match the µF ratings exactly.
- Match the voltage rating or go higher. A 440-volt capacitor can replace a 370-volt one, but not the other way around.
- Reconnect the wires exactly as in your photo, then restore the panel and power.
If a new capacitor doesn't fix it, the problem may be the contactor, the fan motor or the compressor. Those need a technician to diagnose.