AC Compressor Locked Rotor — Diagnose a Hard-Start Failure

Common Symptoms

• Compressor hums for 2 to 5 seconds then shuts off on thermal overload
• Circuit breaker trips immediately when the AC compressor attempts to start
• LRA drawn at startup exceeds what the breaker can sustain for more than a few seconds
• Compressor case is hot to the touch after a failed start attempt
• Single-phase hum from the compressor cabinet with no mechanical rotation sound
• Condenser fan spins normally but the compressor never starts

Most Likely Causes

1. 1

   Failed Run Capacitor — Most Common Cause

   The run capacitor provides the phase shift and additional starting current the compressor motor needs to break away from rest. A capacitor rated at 40 to 60 µF for a typical 3 to 5 ton residential Copeland Scroll compressor must deliver a precise phase relationship to the start winding. When the capacitor fails — typically by losing capacitance to more than 10% below its rated µF — the compressor does not have sufficient starting torque to overcome head pressure. Scroll compressors have very high locked rotor amperage relative to their running amperage, typically 4 to 6 times full-load amps. A weak capacitor cannot supply the needed phase current, the compressor stalls at LRA, and the thermal overload trips.

2. 2

   Mechanical Seizure from Liquid Slugging — Refrigerant Flood Back

   Refrigerant slugging occurs when liquid refrigerant flows back through the suction line into the compressor during the off cycle, typically from system overcharge, loss of crankcase heat, or a failed TXV allowing excess liquid through. Scroll compressors have no clearance for liquid refrigerant — liquid is incompressible, and a slug entering a Copeland Scroll can mechanically seize the scroll set or wash out the lubricating oil film and cause bearing failure. A compressor that has experienced liquid slugging may restart after the liquid boils off, then seize again when another slug arrives. Frost on the suction line at the compressor inlet during the off cycle is a field indicator of active refrigerant flood back.

3. 3

   Oil Breakdown and Bearing Failure

   Reciprocating piston compressors maintain an oil sump for crankshaft and piston bearing lubrication. After liquid slugging events or years of degraded operation, the oil degrades or becomes diluted with refrigerant. Without adequate lubrication, crankshaft bearings seize, mechanically locking the rotor. When the rotor cannot turn, the motor draws LRA indefinitely until the thermal overload trips. Reciprocating compressors with bearing failure cannot be field-repaired — compressor replacement is required.

4. 4

   High Head Pressure — Dirty Condenser Coil or System Overcharge

   Compressor starting torque must overcome the pressure differential between the high side discharge and low side suction of the refrigerant system. High head pressure from a dirty condenser coil that cannot reject heat effectively, or from a refrigerant overcharge that packs excess refrigerant into the high side, raises the pressure the compressor must compress against at startup. Even a fully functioning compressor with a good capacitor will hard-start if head pressure is elevated enough. After a short off cycle before system pressures equalize — typically 5 to 10 minutes — starting immediately creates the same high-differential hard-start condition.

5. 5

   Low Voltage at Compressor Terminals Under Load

   Compressor motors require supply voltage within plus or minus 10% of nameplate, typically 208 to 240V for residential systems. Low voltage at the compressor terminals increases the current draw required to produce a given starting torque. Low voltage is caused by undersized service entrance wiring, long feeder runs with resistive voltage drop, a weak utility transformer during peak demand, or a loose connection in the 240V circuit to the condenser unit. Measure voltage at the compressor terminal block under load — C, R, and S posts — with a multimeter to confirm supply voltage is within spec.

6. 6

   Internal Winding Failure

   When compressor motor windings fail — an open winding where the conductor is broken and the circuit is open, or a shorted winding where turns are shorting together or to the compressor shell — the compressor cannot start or run. An open winding reads infinite resistance (OL) between the affected terminal pair on a multimeter resistance measurement. A shorted winding reads lower than normal resistance between terminals or shows continuity between any terminal and the compressor shell. Winding failures are catastrophic and cannot be repaired in the field — compressor replacement is required.

Quick DIY Checks

1. 1Kill power and discharge the run capacitor before touching anything in the condenser: turn off the 240V disconnect at the condenser unit and the 240V breaker at the main panel. Wait 5 minutes — run capacitors hold charge for hours after power removal. Locate the run capacitor inside the condenser cabinet, typically a silver or gray cylinder 3 to 6 inches tall mounted near the contactor. Using an insulated screwdriver with a 1000V rated handle, briefly short each pair of capacitor terminals: C to HERM and C to FAN. A small spark on a charged capacitor is normal and confirms the discharge worked. Verify 0V at the terminals with a multimeter before handling any components.
2. 2Test the run capacitor in capacitance µF mode — this is the most critical diagnostic step: set your multimeter to CAP or µF mode. Disconnect the capacitor wires and note which wire goes to which terminal — C, HERM, and FAN — before removing them. Touch the multimeter probes to the HERM and C terminals. Read the capacitance and compare to the nameplate rating on the capacitor body, for example 45+5 µF at 440V where the larger number is the compressor HERM rating. A reading more than 10% below the nameplate value confirms capacitor failure. A 45 µF capacitor reading 38 µF is 15.5% low — replace it. For dual-run capacitors with three terminals, test both HERM-C and FAN-C readings. See the multimeter HVAC field guide at /fixes/multimeter-hvac-field-guide for tool recommendations.
3. 3Install a hard-start kit if the capacitor tests good or to supplement a borderline capacitor: if the capacitor measures within 10% of spec but the compressor still hard-starts, or to add starting torque margin, install a hard-start kit. The Supco SPP6 and ICM CompressorSaver are the two most widely used universal kits. They wire in parallel with the existing run capacitor at the HERM and C terminals — two wires, no polarity. The hard-start kit provides a large additional current pulse at startup, effectively multiplying starting torque by 200 to 500% for the critical first half-revolution. Restore power and monitor the first several start cycles. The compressor should start cleanly within one second. If it still hums and trips, proceed to voltage measurement.

1. 4Measure supply voltage at compressor terminals under load: with power restored and the system calling for cooling, use a multimeter on the 240V AC range to measure voltage directly at the compressor terminal block. You should read 208 to 240V AC between the two line voltage terminals. A reading below 190V under load indicates a voltage supply problem. Check the contactor for burnt or pitted contacts that create a voltage drop, inspect for a loose wire at the disconnect or condenser terminal block, and confirm utility voltage at the meter is within spec. Low supply voltage causes the compressor to draw excessive startup current — a hard-start kit helps compensate if voltage cannot be immediately corrected.
2. 5Inspect condenser coil condition — dirty coils raise head pressure and worsen hard-start: examine the condenser coil fins from outside the unit. On a clean coil, you can see through the fins to the interior cabinet. On a dirty coil, the fins are packed with cottonwood seeds, dust, or compressed debris and the coil face is opaque. A dirty condenser raises condensing temperature and head pressure by 20 to 40 degrees above design, significantly increasing the pressure differential at startup. Clean with a garden hose from the inside out. A clean coil restores head pressure to design levels and gives the compressor the best possible starting conditions for the next startup attempt.
3. 6Check for refrigerant slugging — inspect the suction line at the compressor inlet: with the system off for at least 30 minutes, look at the large-diameter suction line where it enters the compressor. On a properly operating system this line is cool to the touch but dry. Frost or ice formation on the suction line at the compressor inlet during or just after the off cycle indicates liquid refrigerant is pooling and flooding back into the compressor. Active liquid slugging requires an HVAC technician for refrigerant system diagnosis — causes include overcharge, TXV failure, or inoperative crankcase heater. Do not continue starting a compressor with active liquid slugging.
4. 7If the compressor still will not start, measure winding resistance to determine repair vs. replace: with power off and capacitor discharged, disconnect all three wires from the compressor terminal block labeled C, R, and S. Set multimeter to ohms on the lowest range. Measure R-to-C, S-to-C, and R-to-S. On a healthy compressor: R-to-C is the lowest resistance, S-to-C is slightly higher, and R-to-S equals approximately R-to-C plus S-to-C. Any measurement reading OL means the winding is open — compressor replacement required. Also probe each terminal to the compressor metal shell: any reading other than OL means a winding is grounded to the shell — compressor replacement required. Good windings with no ground fault on a compressor that still will not start despite new capacitor and hard-start kit indicates mechanical seizure, which requires refrigerant system diagnosis by an HVAC technician.

Repair vs Replace

Recommended Tools & Parts

• Dual-Run Capacitor for AC Compressor (HERM + FAN)

  Replacement run capacitor serving both the compressor and condenser fan motor. Match the µF ratings and voltage rating exactly to the nameplate on your existing capacitor, for example 45+5 µF at 370V or 440V. Common compressor capacitor ratings: 35, 40, 45, 50, 55, 60 µF for the HERM side. A 440V capacitor can replace a 370V original; never use a lower voltage rating than the nameplate value.

  $12–$40

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• Hard-Start Kit (Supco SPP6 or ICM CompressorSaver)

  Universal hard-start capacitor kit that wires in parallel with the existing run capacitor at HERM and C terminals. Provides 200 to 500% more starting torque for the first revolution. The Supco SPP6 and ICM 524 CompressorSaver are the two most widely used universal kits compatible with most residential compressors in the 1.5 to 5 ton range.

  $20–$60

  Buy on Amazon →

• Digital Multimeter with Capacitance µF Mode

  Required for testing run capacitor µF value. Must have a dedicated CAP or µF measurement mode — not all meters include this function. Also needed for voltage measurement at compressor terminals under load and winding resistance testing. Use a meter with CAT III or higher rating for 240V circuit work.

  $25–$80

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• AC Condenser Contactor Replacement

  If voltage testing reveals burnt or pitted contactor contacts causing voltage drop across the closed contactor, replace the contactor. Match the coil voltage — 24VAC for most residential systems — and amperage rating to the existing contactor. Single-pole contactors control one line of the 240V supply; double-pole controls both lines.

  $15–$45

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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