How to Test an AC Compressor: A Field Guide for HVAC Contractors

When I first started in HVAC service, I treated compressor testing like a single test: check resistance, and if it's not shorted, move on. Took me three callbacks and a replacement unit that failed within a month to realize how wrong I was. A proper compressor check isn't one test. It's a sequence. Miss a step, and you risk misdiagnosing a bad compressor—or worse, condemning a good one.

This guide covers a 5-step checklist I use on every suspect compressor call. It works for standard scrolls, digital scrolls, and the AAON units I see most often in commercial settings.

Before You Start: Tools & Safety

Get these ready first:

• Digital Multimeter (DMM) with capacitance function
• Megohmmeter (Megger) for insulation testing
• Amperage Clamp Meter
• PT Chart / App for refrigerant pressures
• Owner's Manual for the specific unit. Seriously, the manufacturer's data is step zero.

Step 1: Visual & Contextual Check (Don't Skip This)

Before you touch a test lead, look for the obvious. On any AAON unit HVAC I approach, I check for the common AAON fan POF failure—Proof of Flow switch issues.

Check these three things:

1. Breaker / Fuse at the unit: Is it tripped? A tripped breaker usually points to a short or locked rotor, not a dead compressor.
2. Contactor: Is it pulling in? If the contactor isn't closing, you have a low-voltage issue. Check the thermostat, transformer, and safeties.
3. Run Capacitor: See any bulging or leaking? That alone will kill compressor starting.

I once spent 45 minutes testing a scroll compressor on a rooftop unit. Gave it a clean bill of health. Then noticed the high-pressure switch was open. Reset it, compressor started right up. A lesson learned the hard way.

Step 2: Electrical Test - Winding Resistance

With the unit completely disconnected from power (verify with your meter), test resistance across the windings.

• Common to Run (C-R): Should be small, typically 1-5 ohms.
• Common to Start (C-S): Higher, typically 3-10 ohms.
• Start to Run (S-R): Should equal C-R + C-S.

The numbers aren't the main story. A short between windings (open or infinite reading) is a bad compressor. On AAON scrolls, I see this less frequently, but when it happens, it's sudden.

Most common test error: Not zeroing your meter leads. A meter with 0.5 ohms in the leads will make a good 1.5 ohm winding look like an open.

Step 3: Insulation Test (This is the One Most Miss)

Here's where my old method failed. A standard multimeter only applies 9V. You need a Megger at 500V or 1000V (check manufacturer specs).

An insulation breakdown that only shows under high voltage will pass a standard meter test. But run the compressor for 10 minutes under load, and that weak spot heats up, arcs, and you get a ground fault.

To test:

1. Disconnect all compressor terminals from contactor and capacitor.
2. Clip one lead to a terminal (C, R, or S).
3. Clip the other to a clean, bare copper ground point on the compressor body.
4. Apply voltage for 60 seconds.

A reading above 10 megohms is generally acceptable. Below 1 megohm? The compressor is likely grounded internally. On AAON units with scroll compressors, I've seen insulation fail due to moisture ingress in outdoor units that weren't properly sealed after service.

Step 4: Run Test with Amperage

Now the risky part—powering up. If Steps 2 & 3 pass, you can start the compressor and check its running amps.

Use your amp clamp on the common wire (C).

• Compare the measured running amps (RLA) against the Rated Load Amps on the nameplate.
• High amps: Hard starting, tight bearings, or high head pressure.
• Low amps: Bad valves (unloaded) or low refrigerant charge.
• Locked Rotor Amps (LRA): If it pulls LRA for more than a few seconds without starting, the compressor is seized.

Not ideal, but this is where specific experience matters. For example, on a pool heater application, the compressor runs under very high head pressure. Slightly elevated amps aren't automatically a condemnation. On an incense burner line? Unlikely. But the principle holds: compare to the nameplate, not a guess.

Step 5: Performance Check (The Functional Test)

This is the final cross-check: does it actually move heat? Connect your gauges and compare the suction and discharge pressures to a PT chart for the refrigerant.

• A good compressor will show a clear pressure differential.
• A system that equalizes too quickly after shutoff suggests bad valves.
• A system that won't build head pressure means the compressor isn't compressing.

This step confirms the electrical tests. I've seen compressors that pass all electrical checks but have broken internal reeds. The performance test catches it.

Common Mistakes & Red Flags

• Skipping the Megger test: As I mentioned, this is the number one reason for misdiagnosis.
• Ignoring the POF switch on AAON units: An AAON fan POF failure will lock out the compressor. If the airflow switch doesn't close, the compressor can't run. Test this before condemning the compressor.
• Not checking the capacitor before testing: A weak capacitor can make a good compressor appear bad.
• Testing a hot compressor: A compressor that just tripped on internal overload will test as open. Wait 30 minutes for it to cool. Or connect a garden hose to cool it (carefully).