What you'll see

You hear the outdoor unit, but nothing's turning right. Two very different sounds get called "humming," so your first job is to tell them apart:

  • A buzzing/chattering contactor — a rapid mechanical rattle from the contactor itself, usually a control-voltage problem.
  • A stalled motor hum — a deep electrical hum from the compressor or fan motor that's getting power but won't spin, often ending in a breaker trip or a thermal cutout a few seconds later.

Nail down which one you're hearing before you pull a single part. They send you in opposite directions.

Walk it in order

  1. Listen and look. Is the condenser fan spinning? Is the compressor humming and then kicking off on internal overload? Is the noise the contactor itself buzzing? Pop the panel (power off first) and look for pitted, chattering, or welded contacts.
  1. Branch A — buzzing contactor. A chattering contactor usually means weak or unstable 24V on the coil, dirty/pitted contacts, or a marginal transformer. Measure 24V across the contactor coil while it's calling. Low or fluctuating voltage → walk the 24V back (thermostat, Y wire, safeties in series, transformer load). Solid 24V but it still chatters → the contactor is failing. Replace it.
  1. Branch B — motor hums, won't spin. The contactor pulled in fine, line voltage is present at the load, but the motor just hums. This is almost always a capacitor or a stalled/locked motor. Kill power, bleed the cap, and test it.
  1. Test the run capacitor. Read microfarads against the nameplate (printed on the can). A dual cap has HERM, FAN, and C terminals — test each section. Out of spec (commonly more than ~6–10% low, or open) or bulged/leaking = replace it. A weak cap is the #1 cause of a humming motor that won't start, and it's cheap.
  1. Spin test the fan (carefully). If the fan hums, power off and flick the blade by hand, then restore power. If it takes off after a nudge, that's a dead capacitor or a tired motor with worn bearings. If it won't budge by hand, the bearings are seized.
  1. If the compressor still won't start with a known-good cap → measure inrush with a clamp meter on a start attempt. Pulling locked-rotor amps (LRA on the nameplate) and tripping = the compressor is trying but can't get over the hump. A hard-start kit can rescue a tired-but-good compressor; a mechanically locked rotor won't spin no matter what you throw at it and means the compressor is done.

What "normal" looks like

  • Control voltage at the contactor coil: 24–28 VAC, steady. Flickering/low = your buzz.
  • Run capacitor: within roughly ±6% of the printed µF rating. A 45/5 dual should read close to 45 on HERM-C and 5 on FAN-C.
  • Line voltage at the loads: ~240V with the contactor pulled in.
  • Compressor current: running RLA on the nameplate when up and running; a brief LRA spike at start that drops within a second or two. Sustained LRA = it's stalling.

Common faults & what they mean

  • Weak/dead run capacitor: motor hums and won't start, or fan needs a hand-spin to go. Cheapest, most common fix — always test it first on a humming motor.
  • Chattering contactor / low 24V: buzzing at the contactor, intermittent starts. Could be the contactor itself or a control-voltage supply problem.
  • Tired compressor (high starting load): starts hard, sometimes trips on a hot day or with high head pressure. A hard-start kit buys time; it's a band-aid, not a cure.
  • Locked rotor / seized compressor: deep hum, immediate trip or thermal cutout, won't start with a good cap or a hard-start. Confirm with an amp draw at LRA. The compressor is mechanically stuck.
  • Seized fan-motor bearings: fan hums, won't spin by hand, may smell hot. Replace the motor.

Tech tips & gotchas

  • Always bleed the capacitor before touching terminals. A charged run cap holds enough to hurt you. Discharge it through a resistor or an insulated screwdriver across the terminals, carefully.
  • Test the cap before condemning any motor or compressor. Replacing a $400 fan motor when a $15 cap was the problem is the classic rookie mistake.
  • A hard-start kit is a diagnostic, not just a part. If it makes a hard-starting compressor run, the compressor's still alive but weakening. If it does nothing, the compressor's mechanically locked.
  • High head pressure can fake a hard start. A dirty condenser or overcharge raises starting load. Don't slap a hard-start on a compressor that's only struggling because the head pressure is sky-high — fix the head pressure.
  • Single vs three-phase matters. On three-phase gear there's no run cap; a humming three-phase compressor that won't start points at a lost leg, a bad contactor pole, or a phase issue.

Safety / code notes

  • Lock out and verify zero voltage before opening the condenser; line-voltage terminals stay live until you confirm otherwise.
  • Discharge capacitors before handling — treat every cap as charged.
  • Inrush testing means the unit is energized with the panel off — keep hands and tools clear of the line-voltage section and wear eye protection.
  • Confirm the outdoor disconnect is present and in sight of the unit per NEC §440.