⚠ Safety Notice: Compressor diagnosis and replacement must be performed by a licensed HVAC/R technician. This article is for educational and reference purposes only. Always follow manufacturer specifications, applicable codes, and EPA refrigerant handling regulations.
Quick Summary — Bad vs. Locked Up Compressor
- Locked Up: Motor windings are intact — compressor is mechanically seized but may be salvageable
- Bad Compressor: Open, shorted, or grounded windings — electrical failure that cannot be reversed
- Key Test: Ohm test across C, S, and R terminals confirms winding condition
- Best Tool: Copeland Mobile app for exact winding specs, LRA/RLA values, and troubleshooting guides
- Important: Always rule out a failed run capacitor before condemning a compressor
Nothing stops a job in its tracks like a compressor that won't run. Before you write it off and order a replacement, it is worth knowing whether you are dealing with a truly failed compressor — or one that is simply locked up and might still have life in it. The two conditions can look identical from the outside, but the diagnosis and fix are completely different.
This guide covers the step-by-step diagnostic process using field-proven techniques backed by Copeland's own published procedures.
What Is the Difference Between a Bad and a Locked-Up Compressor?
| Condition | What It Means | Salvageable? |
|---|---|---|
| Locked Up (Seized) | Motor windings are electrically intact. Internal mechanical components have seized and cannot rotate. | Sometimes |
| Bad Compressor | Open, shorted, or grounded motor windings. Electrical failure that cannot be reversed. | No — Replace |
Step-by-Step Diagnostic Procedure
Step 1 — Listen and Observe at Startup
When you call for cooling, pay close attention to what happens:
- Hums then trips the overload or breaker — Classic locked rotor. The motor is energized but cannot rotate, drawing massive amperage until the protection device shuts it down.
- No sound at all — Could indicate an open winding or an electrical supply issue upstream.
- Grinding or clanking before shutdown — Likely internal mechanical damage.
- Runs but does not cool — Compressor may be spinning freely but not pumping refrigerant — common with broken valves.
Step 2 — Check the Run Capacitor First
A weak or failed run capacitor is one of the most common reasons a compressor appears locked up when it is not. A bad capacitor cannot deliver the starting torque the compressor needs, and the result looks identical to a mechanical seizure.
- Test the capacitor with a capacitor meter
- If it reads more than ±6% below its rated MFD value, replace it
- A capacitor costs a fraction of a compressor — always rule this out first
Step 3 — Try a Hard Start Kit
If the capacitor checks out, install a hard start kit (compressor saver / start assist). This provides an extra jolt of starting torque that can sometimes break a mild mechanical lock free.
- Wire it in per manufacturer instructions and attempt a restart
- If the compressor starts and runs — it was likely struggling from low voltage, a weak capacitor, or refrigerant migration, not true failure
- If it still trips — continue to the next steps
Step 4 — Check for Refrigerant Migration
If a system has been off for an extended period, liquid refrigerant can migrate into the compressor. When it tries to start, it attempts to compress liquid instead of gas — which it cannot do. This causes a locked rotor condition that can sound like a hard mechanical seizure.
- Apply heat to the compressor body with a heat blanket or heat gun
- Allow time for the refrigerant to boil back into vapor, then attempt restart
- Install a crankcase heater going forward to prevent recurrence
Step 5 — Perform a Winding Resistance Test (Ohm Test)
This is the definitive test. Using a digital multimeter set to ohms, test across the three terminals: C (Common), S (Start), and R (Run).
Copeland Winding Resistance Guidelines
| Terminal Pair | Expected Reading |
|---|---|
| C-to-R (Run winding) | Lowest resistance of the three readings |
| C-to-S (Start winding) | Typically 3 to 5 times higher than C-to-R |
| S-to-R | Should approximately equal C-to-S + C-to-R |
| 3-Phase (U, V, W) | Equal and balanced across all three pairs — typically 0.5 to 2 ohms depending on model |
| Any terminal to shell (ground) | No reading (OL) = good. Any continuity = grounded / failed compressor |
What bad readings look like:
- Open winding (OL on any pair) — Winding is burned or broken. Compressor is bad.
- Near-zero ohms — Windings have shorted together. Compressor is bad.
- Continuity to shell (ground) — Grounded winding. Compressor is bad. System likely has acid contamination.
⚠ Important — Copeland Scroll Compressors: Per Copeland's AE-1294 bulletin, do not use a megohmmeter (megger) to condemn a Copeland Scroll compressor. A scroll can read as low as 0.5 MΩ and still be perfectly fine. Refrigerant in the oil and winding temperature can produce false bad readings. A standard ohmmeter test to ground is the correct method.
Step 6 — Check Amperage Draw
With power applied and a hard start kit installed, use a clamp meter to check amperage at startup:
- High LRA that does not drop — Classic locked rotor. Compressor is seized.
- Amps spike then drop to normal RLA — Compressor freed and is running.
- No amp draw at all — Open winding or upstream electrical issue.
Always compare your readings to the compressor nameplate LRA and RLA values. Use the Copeland Mobile app to look up exact specs for the model you are working on.
Diagnostic Summary — Bad vs. Locked Up
| Symptom / Test | Locked Up | Bad Compressor |
|---|---|---|
| Hums and trips overload | ✅ Common | Possible — open winding won't hum |
| Winding resistance normal | ✅ Yes | ❌ No — open, short, or ground |
| Starts with hard start kit | Sometimes | ❌ No |
| Starts after heating (refrigerant migration) | ✅ Yes | ❌ No |
| Continuity from terminal to shell | ❌ No | ✅ Yes — grounded, replace |
| Runs but suction/discharge equalize | ❌ No | ✅ Yes — failed valves |
| Low megger reading (scroll only) | Inconclusive | ❌ Not conclusive — use ohmmeter per Copeland AE-1294 |
Use the Copeland Mobile App for Exact Specs on the Job
One of the most valuable tools a tech can have on a compressor call is the Copeland Mobile app — and it is free. Rather than guessing whether your ohm readings are in spec, Copeland Mobile lets you scan or enter the compressor's model or serial number and instantly pull up that unit's exact specifications.
What Copeland Mobile Gives You On a Diagnostic Call
- Exact winding resistance specs for the specific model you are testing
- LRA and RLA values to compare against your amp clamp readings
- Electrical wiring diagrams pulled directly from Copeland's database
- Step-by-step troubleshooting guides — enter your symptoms and it walks you through the diagnosis
- Compressor operating envelope to verify the unit is within design parameters
- Replacement cross-reference — if it is time to replace, find the right model immediately
- Scout AI chatbot — Gen AI-powered HVACR assistant built into the app
- "Where to Buy" locator — find the nearest authorized wholesaler carrying the part you need
Available free on the Apple App Store and Google Play. If you are working on Copeland equipment and not using it, you are working harder than you need to.
When It Is Definitely Time to Replace
Once you have confirmed any of the following, the compressor is beyond saving:
- Open, shorted, or grounded windings confirmed by ohmmeter
- Compressor trips repeatedly even after a new capacitor and hard start kit
- Acid contamination in the system — burnt oil smell, discolored oil, positive acid test
- Runs but suction and discharge pressures equalize — failed valves
- Discolored or burnt paint on the compressor shell — sign of chronic overheating
- Physical damage — broken valves or catastrophic internal failure
Important: Before installing a new compressor, identify why the original failed. Installing a new compressor into a faulty system will result in another failure. Check for low refrigerant charge, restriction, acid contamination, electrical issues, and improper airflow before starting up a replacement.
Frequently Asked Questions
Can a locked-up compressor be freed?
Sometimes. Mild mechanical seizure caused by refrigerant migration or extended downtime can sometimes be resolved with a heat blanket and a hard start kit. A compressor that has seized from internal mechanical failure cannot be freed and must be replaced.
What is the most common reason a compressor appears locked up but is not?
A failed run capacitor. This is the first thing to check on any compressor that hums and trips. A weak capacitor cannot provide the starting torque the compressor needs, and the symptom is identical to a locked rotor condition.
Should I use a megger to test a Copeland Scroll compressor?
No. Per Copeland's AE-1294 bulletin, a megohmmeter should not be used to condemn a Copeland Scroll compressor. Use a standard digital multimeter to test for continuity between the terminals and the compressor shell. A scroll compressor can show a low megger reading and still be in perfectly good condition.
Where do I find the exact winding resistance specs for my Copeland compressor?
The free Copeland Mobile app. Scan or enter the model number and you will have the exact published specs for that unit, including winding resistance, LRA, RLA, and wiring diagrams — right on your phone at the job site.
Does iComfort AC Supply carry replacement compressors?
Yes. iComfort AC Supply stocks and sources a wide range of Copeland and aftermarket replacement compressors for residential and light commercial systems, including hard-to-find and special-order models. Contact us with your model number or nameplate photo and we will check availability and pricing before you commit to an order.
Ready to Order or Need Help?
Send us your model number or a photo of your nameplate and we will check availability, pricing, and shipping options before you order.
📞 Phone: (337) 516-6891
📧 Email: sales@icomfortacsupply.com
🌐 Website: icomfortacsupply.com
iComfort AC Supply is operated by iComfort Air Conditioning and Heating LLC, based in Abbeville, Louisiana.
⚠ Important Notice: Compressor diagnosis and replacement must be performed by a licensed HVAC/R technician. Information in this article is provided for educational and reference purposes only and does not constitute professional HVAC advice. Always verify voltage, phase, refrigerant type, oil type, and system condition before ordering or installing any compressor. Follow all manufacturer specifications, applicable codes, and EPA Section 608 refrigerant handling regulations. Improper diagnosis, compressor selection, or installation can result in equipment damage, electrical hazards, or refrigerant issues.