Megohmmeter Reads Good but Compressor Won't Start on Load Decision Tree

Why this matters

A clean insulation-resistance reading proves the windings are not grounded, but it says nothing about whether the compressor can produce starting torque, so a "megger is good" result that still will not start under load is a torque or start-component problem, not a ground fault. Techs who stop at the megohmmeter assume the compressor is fine and chase the controls; techs who skip the megger condemn the compressor without proving the windings. The combination here, good insulation resistance plus failure to start against a refrigerant load, points to a short list: a weak or failed start or run capacitor, a failed or missing start relay or PTC device, a mechanically locked or tight rotor, unequalized system pressures making the compressor start against a head, low supply voltage that cannot break the rotor free, or an open internal overload that has not reset. Each leaves a distinct electrical signature, and walking them in order keeps you from replacing a good compressor over a failed start capacitor.

Symptom presentation

The compressor hums or buzzes for a couple of seconds then trips its internal overload, or it draws locked-rotor amperage (LRA, several times RLA) and does not turn, or it makes no attempt at all while the contactor is clearly pulled in and voltage is present at the terminals. The megohmmeter reads in the megohm range to ground (well above the condemnation threshold). On a single-phase unit, the start winding may show normal resistance but the compressor still will not break free. The condenser fan and controls operate normally, isolating the fault to the compressor and its start circuit.

Quick checks

Confirm voltage at the compressor terminals while it attempts to start; a sag below the manufacturer minimum during the inrush prevents starting. Test the run capacitor microfarads and, on PSC or hard-start systems, the start capacitor and relay. Read winding resistances common-to-start and common-to-run and compare to spec; an open or high-resistance winding will not start. Check whether the internal overload is open by reading continuity across the windings after the compressor has fully cooled. Note system pressures: a large high-to-low differential left from a short off cycle can stall a compressor that lacks a hard-start kit.

Isolation tree

Branch 1, capacitor or start components. The most common cause of a good-megger no-start. A run capacitor that has drifted low robs running torque; a failed start capacitor or relay (or a degraded PTC) removes the starting boost a PSC or CSIR/CSCR compressor needs. Test capacitance against the rated microfarads and replace anything out of tolerance. A hard-start kit can confirm a marginal start by getting the compressor turning.

Branch 2, locked or tight rotor (mechanical). If voltage is solid, capacitors and relay test good, and windings read correct, but the compressor draws LRA and will not turn, the rotor is mechanically seized or hydraulically locked by liquid. A brief reverse-rotation or a hard-start attempt may free a tight (not seized) rotor; a truly seized rotor is a compressor replacement.

Branch 3, low or unstable supply voltage. Measure under inrush. A supply that collapses during LRA cannot break the rotor free even with good components. Trace the voltage drop to a loose lug, undersized conductor, weak contactor, or utility issue.

Branch 4, open internal overload not reset. A hot compressor whose overload is open reads open across the windings until it cools. Let it cool fully and re-test continuity before condemning; an overload that resets and then allows a start clears this branch.

Branch 5, starting against unequalized pressure. After a short off cycle, a fixed-orifice or hard-start-less system may try to start against a pressure differential. Allow pressures to equalize (or fit a hard-start kit where the design expects one) and retry.

Confirming diagnosis

Confirm a capacitor or relay fault by measuring microfarads and replacing the out-of-spec part; the compressor starting afterward closes it. Confirm a mechanical lock by the persistence of LRA with proven good voltage, capacitors, relay, and windings; nothing electrical fixes a seized rotor. Confirm a voltage fault by the under-inrush terminal reading. Confirm an open overload by continuity that returns once the compressor cools. The branches separate cleanly: electrical faults restore starting when the component is replaced, while a seized rotor draws LRA regardless of what you change around it.

Remediation

Replace a failed run or start capacitor, start relay, or PTC with the rated part and confirm a clean start and normal RLA. For a tight rotor, attempt a hard-start kit per the manufacturer's guidance; a seized rotor requires compressor replacement with a new drier and proper evacuation per EPA 608. For a voltage fault, repair the loose connection, upsize the conductor, replace the contactor, or escalate to the utility. For an open overload, allow cooldown and address the overheating cause that opened it. Re-verify start behavior, inrush, and steady-state RLA after the repair.

References

• Copeland Application Engineering Bulletin AE4-1300 (compressor starting, electrical troubleshooting, hard-start application)
• EPA Section 608 Technician Certification, 40 CFR Part 82 Subpart F
• UL 60335-2-34 (motor-compressor safety, internal overload protection)
• AHRI Standard 540 (compressor performance and locked-rotor data)
• NFPA 70 National Electrical Code (supply conductor and voltage-drop requirements)