Compressor Won’t Start? Check the Relay, Capacitor & Windings

A compressor that hums, trips its overload, or never turns over is not automatically a bad compressor. The start circuit (contactor, relay, capacitor) and the supply feeding it fail often enough that the quickest way to avoid a wrong-part swap is to prove the motor windings in a fixed order before anything gets pulled. This guide walks through that order, from power at the contactor to winding resistance to the start components, and ties each step to the parts that most often turn out to be the real culprit.

Why does proving the compressor matter more this fall?

Replacement cost is moving, and that changes the economics of a wrong call. Procurement reporting in early September 2026 described HVAC equipment tariffs of 25% in April that were later reduced to 15% in June, with further tariffs on steel, aluminum, and copper proposed in August (Inside Supply Management, Sept 1, 2026). Treat those figures as directional, since they come from secondary trade reporting and the policy is still shifting. The practical point holds either way: when a compressor swap gets more expensive, a misdiagnosed no-start gets more expensive too, and published compressor-manufacturer guidance from late August 2026 makes the same argument that returned compressors are frequently found to be fine and the fault sits in the electrical or system side.

This post stays on the electrical and mechanical side of that diagnosis. System-side root causes (airflow, floodback, contamination) are a separate diagnosis and not covered here.

What should you check before you touch the compressor terminals?

Work from the supply toward the compressor, and stop at the first thing that fails.

1. Is there voltage at the contactor line side, and at the load side when the contactor pulls in? A pitted or worn contactor can show control-circuit action while delivering low or uneven voltage to the compressor. See our HVAC contactor failure guide.
2. Are the safeties closed? Pressure switches and protection modules can hold the compressor out. For protection-module lockouts and what the fault patterns mean, see Copeland Compressor Protection.
3. Is the internal overload open? A compressor that reads open between terminals right after tripping may simply be hot. Let the shell cool fully and retest before drawing conclusions. An internal overload that resets after cooling points you back toward what made it trip (low voltage, a weak start circuit, a locked rotor), not automatically toward a failed motor.
4. Is voltage at the compressor terminals within range while it attempts to start? Measure under load. A voltage that looks fine at rest and sags during the start attempt is a supply or connection problem.

If the compressor has been sitting through cold weather, also review crankcase heater behavior, covered in Crankcase Heater Failure: Diagnosing Migration, Flooding & Slugging.

How do you test the windings to confirm the motor is actually bad?

Disconnect power, lock out, and remove the leads from the compressor terminals. On a single-phase compressor you are measuring three pairs.

• Common to Run (C–R): the lowest reading of the three.
• Common to Start (C–S): higher than C–R.
• Start to Run (S–R): approximately the sum of the other two. If it is not, suspect a measurement error or a winding problem.
• Each terminal to ground (shell): you want no path. Any continuity to the shell on an ohm check is a grounded winding and a confirmed internal failure.

Open between any pair (after the overload has cooled) or a short to ground confirms a failed motor. A winding that checks normal on resistance but trips repeatedly under load still needs the start circuit and supply ruled out first.

Two cautions. First, a low-resistance reading cannot reveal a partial turn-to-turn short on a basic ohm check, so a normal ohm reading does not fully clear the motor on its own. Second, do not apply a high-voltage insulation tester to a hermetic compressor that is under vacuum, and follow the compressor manufacturer's stated insulation-resistance guidance for the model you are testing rather than a rule of thumb.

Which start components fail first, and how do you tell them apart?

If the windings check out and voltage is good, the compressor usually hums and trips, or hums and does nothing. That points at the start circuit.

Current relay. A current relay's coil sits in series with the run winding. Inrush current closes the contacts, which energizes the start winding (and start capacitor, if used); as the motor accelerates and current falls, the contacts drop out. Failure shows up as scorched or welded contacts. A relay that fails closed leaves the start components in the circuit too long. A relay that fails open gives you a hum with no start. Current relays are rated by pick-up and drop-out amperage, so the replacement has to match the original's calibration, not just its footprint. Mount it in the orientation marked on the relay body, because some current relays are position-sensitive.

Potential relay. A voltage-sensing relay that drops the start circuit based on back-EMF across the start winding. It is not interchangeable with a current relay and should not be substituted for one.

Start capacitor. A high-capacitance electrolytic unit meant for intermittent duty (only the first moments of a start). Start capacitors are labeled with an MFD range, not a single value, so match the range and the voltage rating printed on the original.

Verified Copeland start-circuit parts in the GSIstore catalog

Confirm the part number against your compressor's model and serial number before ordering. Browse the full Copeland collection for related components.

Why do start capacitors keep failing?

A start capacitor that vents, bulges, or reads out of range is usually the symptom, not the cause. The usual upstream causes are:

• A relay that fails closed. Contacts that weld or stick leave the start capacitor in the circuit past the start, where it overheats. If a start capacitor has failed, inspect the relay contacts before fitting a new capacitor.
• Short cycling. Repeated start attempts build heat in the capacitor and pit relay contacts. Find out why the system is cycling before replacing parts.
• Low supply voltage. A sagging supply stretches the start, which stretches the time the capacitor is under load.
• A failed or missing bleed resistor. The bleed resistor drains the capacitor after the start so relay contacts do not arc when they re-close. Several of the capacitors above are listed with the resistor built in.

The practical rule: replace the relay and start capacitor together when one has failed from heat or contact damage, and check the run capacitor in the same visit (see our run capacitor guide).

What does a locked rotor look like on the meter?

A locked or mechanically seized compressor hums, draws a very high current for as long as it is allowed to, and trips the overload. If the windings check normal, the start components test good, and the compressor still hums and trips, you are looking at a mechanical problem or a system condition holding the pump from turning.

Limit start attempts. Each attempt pushes locked-rotor current through the windings, and repeated tries can turn a recoverable problem into a burned one. Take a measured amperage reading on the first attempt, compare it to the nameplate locked-rotor figure, and stop. Save any further start attempts until you have a reason to expect a different result.

Symptom, first check, likely culprit

Frequently asked questions

Can I test a start relay without removing it?
Remove power, discharge any capacitor in the circuit, and remove the relay for the check. A current relay's contacts should be open at rest and its coil should show low resistance. Inspect the contacts for pitting or welding, and check that the relay's calibration matches the original.

What happens if I swap in a start capacitor with a different MFD?
A value outside the original range changes the phase shift available to the start winding. Too low can leave the motor with weak starting torque, and too high can stress the windings. Match the printed range and the voltage rating.

Is a compressor that reads open between terminals bad?
Not necessarily. An internal overload that has just tripped can read open until the shell cools. Retest after a full cool-down before condemning the motor.

Why did my new start capacitor fail within days?
Look upstream. A relay whose contacts stick closed, short cycling, or low supply voltage will take out a new capacitor quickly. Replacing the capacitor alone treats the symptom.

Can a current relay replace a potential relay?
No. They sense different things (motor current versus voltage across the start winding) and are not interchangeable. Replace like for like, confirmed against the compressor model.

A note on compatibility

Part numbers listed here are for reference, and a part may not fit your particular compressor or unit. Always confirm part-number-to-model-number compatibility before purchase.