Testing an AC capacitor with a multimeter

The run capacitor gives your compressor and outdoor fan motor the extra kick they need to start up and keep running. When it goes weak, you get the classic symptoms: the outdoor unit hums, the fan doesn't turn or turns real slow, or the compressor tries to start and trips the breaker.

1. Kill the power, then prove it's dead

Turn the thermostat off. Pull the disconnect near the outdoor unit and flip off its breaker. Take off the service panel, usually the corner cover where the power lines enter. Use your multimeter to confirm there's no voltage at the contactor before you touch anything else.

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 set your meter to DC volts and verify it reads zero.

3. Look before you test

A bulging or domed top, a split case, or oily residue means it's done. Swap it out and skip the meter entirely.

4. Photograph, then disconnect

Snap 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). Touch the leads to C and HERM, then to C and FAN. Compare each reading to the ratings on the label, like 45/5 µF ±6%. If a reading falls outside that tolerance, the capacitor's weak and needs replacing.

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 put the panel back on and restore power.