How to Test a Single-Phase Compressor (Without a Wiring Diagram)

Posted on July 26, 2025August 18, 2025 by Paradigm Cooling

3 min read

Last updated on August 18th, 2025 at 03:34 pm

By Paradigm Cooling

Still guessing which compressor terminal is which? Don’t worry — whether you’re an HVACR technician or a hands-on DIYer, this step-by-step guide will show you how to test a single-phase domestic refrigeration compressor — the right way — even if you don’t have a wiring diagram.

Step 1: Safety First

Before performing any tests:

Unplug the unit completely
Confirm it is de-energized
Use a properly rated digital multimeter

Safety is not optional — it’s essential.

Step 2: Understanding Compressor Terminal Layouts

Most domestic single-phase compressors have three terminals arranged in a triangle configuration beneath the terminal cover. These terminals connect to:

Start winding (S)
Run winding (R)
Common (C)

But without a wiring diagram, how do you know which terminal is which? That’s where resistance testing comes in.

Step 3: Identifying Terminals Using Resistance Measurements

What You’ll Need:

A digital multimeter set to the Ohms (Ω) setting

Instructions:

Label the three compressor terminals as A, B, and C
Measure resistance between each pair:
- A to B = 59.2 Ω
- B to C = 79.5 Ω
- A to C = 21.4 Ω

How to Interpret the Results:

Highest resistance = Start to Run (S-R)
Lowest resistance = Common to Run (C-R)
Middle value = Common to Start (C-S)

Use this method to confidently assign S, R, and C to the terminals — without guessing or relying on a missing wiring diagram.

Step 4: Confirming Winding Continuity

To verify that the compressor’s windings and internal protector are in good shape, do the following:

Add:

C to S resistance
C to R resistance

Your result should equal the resistance between the S and R terminals.

If your calculation is correct within 5%, the compressor windings are likely in good condition.
If not, the compressor may be faulty.

Step 5: Testing for Ground Faults (Insulation Resistance)

Now test whether any winding is shorted to the compressor body.

Instructions:

Set your multimeter to Mega-Ohms (MΩ) or the highest Ohms range
Measure resistance between each terminal and the compressor shell

Result:

Open line (OL) on all terminals = compressor passed
Any continuity to ground = winding is shorted

In the example test, the compressor passed, showing no leakage to ground.

Bonus: Testing a Faulty Compressor

We also tested a condemned compressor that was tripping the earth leakage circuit breaker. Here’s what we found:

Resistance Readings:

6.3 Ω
28.1 Ω
27.8 Ω

These values failed the continuity test. The two lower values didn’t add up to the third, indicating internal winding issues.

Ground Test Results:

Each terminal showed resistance to ground, confirming an insulation leak. This is what caused the circuit breaker to trip — and why the compressor was replaced.

Final Recap: Compressor Testing Checklist

To test a single-phase compressor correctly, follow these steps:

Identify terminals using resistance values
Confirm winding continuity through calculations
Test for ground faults using insulation resistance

Completing these steps allows you to diagnose compressor faults accurately and make informed repair or replacement decisions.

Tools We Used

Looking to get the same tools we used in this guide? We’ve included affiliate links below to recommended products — including multimeters and HVAC toolkits. When you buy through these links, you help support Paradigm Cooling at no extra cost.

Got Questions?

Drop your compressor testing questions in the comments below — we answer as many as we can. And don’t forget to subscribe to the Paradigm Cooling YouTube channel for more hands-on HVACR tutorials.

Until next time,
Stay cool, and stay curious.