Capacitor Tests Good but Motor Won't Start: Misleading Reading Decision Tree

Why this matters

A capacitance meter reading inside tolerance is a useful but narrow test. A technician pulls the run capacitor, reads it within plus or minus 6 percent of its microfarad rating, declares it good, and is left staring at a motor that hums and trips. The reading is real, but capacitance measured out of circuit at low voltage does not prove the capacitor holds up under operating voltage and load, and it says nothing about the rest of the start circuit: wiring, contactor, windings, bearings, supply voltage, or a start component that the capacitance test never touches.

A motor that will not start while the capacitor reads good is one of the easiest places to replace a part that was not the problem. This tree explains why the reading misleads and how to find the real fault.

Symptom presentation

The capacitor measures within tolerance on a meter. Yet the motor (condenser fan, blower, or compressor) hums, draws locked-rotor amps, trips on its overload, or fails to start. The contactor may pull in cleanly while the motor sits stalled.

Why the reading misleads

A capacitance meter charges the capacitor at low voltage. A capacitor can read correct microfarads cold and still break down under line voltage, lose capacitance when hot, or have high internal resistance (high ESR) that starves the start phase. Out-of-circuit capacitance also cannot reveal a connection problem, a weak contactor, low supply voltage, a grounded or open winding, or mechanical binding. The capacitor is one component in a start circuit that includes the supply, the contactor, the wiring, the windings, and the mechanical load.

Quick checks

• Supply voltage under load: measure voltage at the motor terminals at the moment of the start attempt. Low or sagging voltage stalls a motor a good capacitor cannot rescue.
• Connections: inspect and tighten every terminal in the start circuit; a high-resistance lug or a backed-out spade looks fine but drops voltage under current.
• Contactor: check the contacts for pitting and measure voltage drop across them while energized. Pitted contacts drop voltage to the motor.
• Mechanical: try to spin the fan or check for a seized compressor. A bound bearing or a seized rotor mimics an electrical no-start.
• Capacitor under load: where possible, measure the capacitor's microfarads by the voltage-and-current method while the motor runs, not just out of circuit. A capacitor that reads good cold may read low hot.

Isolation tree

1. Voltage branch. Measure line voltage at the unit and at the motor terminals during the start attempt. A large drop from the disconnect to the motor terminals points to a supply, contactor, or wiring problem, not the capacitor. Correct the voltage source before anything else.

2. Contactor branch. With voltage good at the line side, measure across the closed contactor. More than a small drop across the contacts means pitted contacts starving the motor. Replace the contactor and retest.

3. Connection branch. Inspect every lug and spade in the start path. Re-terminate any discolored, loose, or corroded connection. A high-resistance joint behaves like low supply voltage at the motor.

4. Mechanical branch. Confirm the load turns freely. A seized condenser-fan bearing, a jammed blower wheel, or a mechanically seized compressor will not start regardless of a perfect capacitor. For a compressor, an open internal overload that has not reset (hot compressor) also reads as a no-start; let it cool and retest.

5. Winding branch. Ohm the motor windings and check winding-to-ground. A grounded, open, or shorted winding will not start. On a compressor, compare common-to-start and common-to-run resistances to spec; an out-of-range reading or a ground fault condemns the compressor.

6. Capacitor-under-load branch. If all of the above check out, test the capacitor under operating conditions. A run capacitor that reads correct cold but loses microfarads hot, or a hard-start kit (PTCR/start capacitor and relay) that has failed, prevents start even though the static capacitance reads fine. On a hard-starting compressor with a marginal but in-spec capacitor, a properly matched start kit may be the fix.

Confirming diagnosis

You have the real fault when correcting it produces a clean start with run amps settling to nameplate FLA and start (LRA) current present only momentarily. The decisive tests are motor-terminal voltage under start, contactor voltage drop, mechanical freedom, and winding integrity, not the cold capacitance reading. If every electrical and mechanical test passes and the motor still will not start, recheck the capacitor under load and the start components before condemning the motor.

Remediation

• Low supply or wiring: correct the source, re-terminate connections, replace undersized or damaged conductors.
• Pitted contactor: replace the contactor, verify voltage to the motor.
• Seized bearing or wheel: replace the bearing/motor, free or replace the blower wheel.
• Seized or grounded compressor: confirm with winding and ground tests; replace if failed.
• Failed start component: replace the run capacitor with the exact microfarad/voltage rating, or fit a correctly matched hard-start kit on a verified hard-starting compressor.

Always replace a capacitor with the exact uF rating and equal or higher voltage rating.

References

• UL 60730, automatic electrical controls for motor circuits.
• AHRI Standard 210/240, unitary equipment electrical performance.
• NFPA 70 (National Electrical Code), motor circuit conductors and overcurrent protection.
• ASHRAE Handbook, HVAC Systems and Equipment, motor and compressor fundamentals.
• ANSI/AHRI Standard 1230 and motor manufacturer service data for winding resistance specs.