AC Compressor Wiring and Failure Diagnosis
The compressor is the heart of the air conditioner — and also the most expensive component. Before condemning a compressor, confirm that every wiring and component issue upstream has been eliminated. A failed capacitor, burned contactor, or grounded compressor wire causes the same symptom: compressor won't run. Correct diagnosis saves $800–$2,000 in unnecessary parts or a premature system replacement. This guide covers compressor terminal testing (C, S, R — Common, Start, Run), wiring checks at the contactor and outdoor board, and how to identify a locked-rotor or grounded compressor using a multimeter.
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Common Symptoms
- Outdoor unit hums loudly but compressor won't start
- Compressor trips thermal overload and shuts off within 1–2 minutes
- Breaker trips immediately when system starts
- Fan runs but compressor never starts (capacitor or wiring issue)
- Buzzing or clicking at outdoor unit with no cooling
- 240V present at contactor but compressor doesn't run
Most Likely Causes
- 1
Failed Run Capacitor (Most Common)
The HERM side of the dual-run capacitor provides start and run assistance to the compressor. A weak or failed capacitor causes the compressor to draw locked-rotor current, hum without starting, and eventually trip its thermal overload. Test capacitor MFD with a multimeter in capacitance mode — more than 10% low = replace.
- 2
Burned or Pitted Contactor Contacts
The contactor passes L1 and L2 (240V) to the compressor and fan motor. Pitted or welded contacts cause voltage drop under load, compressor overheating, and breaker trips. With power off, inspect the contact surfaces — they should be smooth and silver-colored. Black pitting or carbon deposits = replace the contactor.
- 3
Locked-Rotor / Seized Compressor
A compressor with seized internal components draws full locked-rotor amperage (LRA) — often 4–6x normal running amps — which trips the breaker. Test: with power off, measure resistance between C-S, C-R, and S-R terminals on the compressor. All three should read continuity (not OL). C-to-ground should read OL — a ground fault means the compressor windings have shorted to the shell.
- 4
Open Compressor Winding
An open winding means the compressor motor coil has broken internally. Measure resistance between C-S, C-R, and S-R: one pair reading OL (open) indicates a blown winding. The compressor must be replaced — there is no field repair for this condition.
- 5
Shorted Compressor Winding to Ground
A ground fault trips the breaker instantly. With power off and wiring disconnected from the compressor terminals, measure from each terminal (C, S, R) to the compressor housing (chassis ground) with a multimeter set to the highest resistance range. Any reading other than OL (open) indicates a ground fault — the compressor is failed.
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Quick DIY Checks
Compressors operate at 240V AC (L1 and L2) and the capacitor retains a lethal charge after power is removed. Always turn off the disconnect AND the breaker, wait 5 minutes, and discharge the capacitor with a 1k ohm resistor before touching any compressor wiring.
A compressor with a grounded winding will trip the breaker instantly and poses an electrocution hazard if the equipment ground wire is missing. Confirm the outdoor unit is properly grounded before troubleshooting.
Compressor replacement requires refrigerant recovery, installation, and recharge — EPA 608 certification required. Do not attempt refrigerant work without proper certification and equipment.
- 1Turn off the outdoor unit disconnect and the breaker. Open the outdoor unit access panel. Locate the compressor terminal block — usually a plastic cover on the side of the compressor with three terminals labeled C (common), S (start), and R (run). These are connected to the run capacitor and contactor via wires.
- 2Discharge the run capacitor: touch a 1k ohm resistor between HERM-C and FAN-C terminals for 5 seconds each. Then disconnect all wires from the compressor terminal block. Set your multimeter to resistance (ohms) mode.
- 3Measure resistance between each terminal pair: C-to-S, C-to-R, S-to-R. Record all three values. On a good compressor, C-to-R + C-to-S should approximately equal S-to-R (Kirchhoff's law for motor windings). Typical values: C-to-R: 2–8 ohms; C-to-S: 3–12 ohms; S-to-R: 5–20 ohms. Any OL reading = open winding, compressor failed.
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Try Pro — $7.99/mo- 4With wires still disconnected, test each terminal to the compressor housing (ground). Use the highest resistance range or a megohmmeter. Good compressor: all terminals read OL to ground. Any resistance reading = ground fault. Breaker will trip every time. Compressor must be replaced.
- 5If the compressor terminals check out (correct resistance, no ground fault), inspect the contactor: look for burned contacts or weak coil pull-in. Then test the run capacitor HERM value with a multimeter in capacitance mode. Replace the contactor or capacitor as indicated, then retest. If the compressor still doesn't start after confirming good capacitor and contactor, try a hard-start kit (Supco SPP6E) before condemning the compressor.
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Repair vs Replace
Always replace the capacitor and contactor first — they're cheap and often the real culprit. If the compressor itself is confirmed failed, weigh the compressor replacement cost against system age: a unit under 10 years old with a failed compressor under warranty is worth repairing. Over 12–15 years, compressor replacement cost often justifies system replacement, especially if other components are near end of life.
Est. Repair Cost
$15–$45 (capacitor); $40–$80 (contactor); $800–$1,800 (compressor replacement)
Est. Replacement Cost
$3,000–$7,000 for new system
Recommended Tools & Parts
- Buy on Amazon →
Dual Run Capacitor (match MFD and voltage to nameplate)
Replace the HERM side capacitor before condemning the compressor. Match the MFD rating (±5%) and voltage rating (equal or higher) from the unit's data plate.
$12–$30
- Buy on Amazon →
Hard-Start Kit (Supco SPP6E / SPP6)
Adds additional start capacitance and a current relay to help aging or marginal compressors start reliably. Often eliminates locked-rotor trips on compressors that test electrically good but struggle to start.
$25–$50
- Buy on Amazon →
HVAC Contactor (2-pole, 240V, 30–40 amp)
Replacement contactor for outdoor unit. Match the amperage rating and coil voltage (24V) to your unit's existing contactor. Burned contacts cause voltage drop and compressor damage.
$15–$35
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Frequently Asked Questions
- How do I test a compressor with a multimeter?
- Disconnect all wires from the compressor terminals (C, S, R). Set your multimeter to ohms. Measure C-to-R, C-to-S, and S-to-R. All three should show resistance (not OL). The S-to-R reading should roughly equal C-to-R plus C-to-S. Then test each terminal to the compressor housing — all should read OL (no continuity). Any ground fault means the compressor is failed.
- Why does my AC breaker trip immediately when the system starts?
- Immediate breaker trip on startup usually means a grounded compressor winding, a shorted capacitor, or a shorted contactor. With power off, disconnect the compressor wires and test each terminal to ground with a multimeter. If any terminal shows continuity to the compressor housing, the compressor has a grounded winding and must be replaced. Also pull and inspect the capacitor for visible bulging or burn marks.
- What is a hard-start kit and does my AC need one?
- A hard-start kit (like the Supco SPP6E) adds a start capacitor and current relay that boosts starting torque during the first fraction of a second of compressor startup. This helps compressors that are mechanically healthy but struggle to start due to high head pressure, low voltage, or age-related wear. If your compressor hums for 1–3 seconds before starting (or fails to start), a hard-start kit is worth trying before condemning the compressor.
- Can I replace just the compressor without replacing the whole system?
- Technically yes, but it's expensive. Compressor replacement requires refrigerant recovery, compressor removal and installation, new filter-drier, evacuation, and recharge — typically $800–$1,800 in labor and parts even before the compressor cost. On units under 10 years old with manufacturer warranty on the compressor, it makes financial sense. On units over 12–15 years old, the total repair cost often approaches new system cost.