AC Compressor Not Starting: Humming, Clicking or Nothing at All

Safety: Lock out and tag out power, verify it's dead with a meter, and discharge the capacitor before touching terminals. A compressor that's failed to ground can be dangerous. Never energize one with the terminal cover off.

If it hums but doesn't start

The compressor is getting voltage but can't get turning.

1. Run capacitor: test it under load or with the meter's capacitance setting. A weak or open HERM side is the #1 cause. See how to test a run capacitor.
2. Voltage at the compressor while it's trying to start: a heavy drop points to undersized wire, a bad connection, a pitted contactor or utility supply problems.
3. Contactor: check for voltage drop across the closed contacts. See how to test a contactor.
4. Locked rotor: if voltage and the capacitor are good, current draw jumps to around the locked-rotor amps (LRA) on the nameplate and the overload trips. A hard-start kit can sometimes get a stuck compressor going, but treat it as a diagnostic step or a stopgap, not a cure for a failing compressor.

If it clicks on and off

That's usually the internal overload opening and resetting. The compressor is too hot.

• Let it cool. A hot compressor shell can take an hour or more to reset. A cool compressor with an open overload reads open between C and both S and R, while S to R still shows continuity.
• Find out why it overheated: low charge (not enough cool suction gas), a failed condenser fan, a dirty condenser coil, low voltage, or a weak capacitor.

If there's no sound at all

The compressor isn't getting voltage.

1. Is the contactor pulled in? If not, check for 24 V at the coil.
2. No 24 V at the coil: look upstream for an open high- or low-pressure switch, a defrost board or compressor-protection lockout, a blown low-voltage fuse, or a thermostat or wiring problem.
3. Contactor pulled in, but no voltage on the load side: the contacts are burned open.
4. Voltage at the compressor terminals but no hum: suspect an open internal overload or an open winding.

Check the windings

With power off and wires removed from the terminals:

• Measure C–S, C–R and S–R. The largest reading is S–R, and C–S + C–R should equal S–R (within a small tolerance).
• An infinite reading on any pair is an open winding, or an open internal overload if the compressor is hot.
• Check each terminal to ground (the copper suction line or a clean spot on the shell). Any continuity to ground is a grounded compressor. Use a megohmmeter for a proper insulation test.

FAQ

How do I tell if the compressor or the capacitor is bad?

Test the capacitor first; it's cheap and fails often. If the capacitor tests within its tolerance, the voltage holds up during the start attempt, and the windings read correctly, but the compressor still hums and trips, the compressor is likely locked.

Is a hard-start kit a fix?

It can get a compressor that starts hard running again, and some manufacturers specify one in certain applications. On a compressor that's mechanically failing, it only buys time. Tell the customer that.

Why does the compressor trip after running a few minutes?

Look at what's heating it up or overloading it: condenser fan, dirty condenser, overcharge, non-condensables, low charge or low voltage.

Sources

• HVAC School: technical tips and diagnostics