How to test a run capacitor (before you guess the charge)
The compressor hums, kicks, and gives up — and from the driveway it looks exactly like low charge. Guessing gas first is how a weak capacitor turns into an overcharged system with a cooked compressor.
The field sequence
Lock the unit out — breaker or service disconnect off — then prove it dead with a meter at the load side. Never trust the handle position.
Discharge the capacitor across its terminals with a proper discharge tool or resistor. A capacitor can hold a dangerous charge after power is off.
Read the can: rated µF, tolerance band, and voltage. Ratings vary by market — read the nameplate; US 120/240 V vs 230/240/415 V depending on region.
Measure capacitance with a µF-capable meter. On a dual-run can, test C–HERM and C–FAN separately and note which side feeds the compressor.
Know what you are holding: a run capacitor stays in circuit; a start capacitor works with a relay for a few seconds only. The test expectations differ.
Decide from the reading, not the symptom. Outside tolerance means replace like-for-like. Inside tolerance means the no-cool cause is elsewhere — move on to the charge decision.
Common wrong moves
“Just add gas” on a humming compressor.
Lab fail: misdiagnosis — a weak capacitor mimics low charge, and the debrief calls it out.
Swapping the capacitor with the disconnect still closed.
Lab fail: isolation step skipped — an instant safety fail.
Reading only the HERM side of a dual-run can.
Lab fail: incomplete test — a dying fan side passes unnoticed.
Run it, don't just read it
The lab grades the same sequence you just read — isolation, measurement order, and the decision — with a debrief on every call.
Keep going
Related reading
After the labs, you know which exam to book
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