How do I test refrigerator compressor windings using a multimeter?

Testing a compressor’s internal motor windings determines whether the motor is shorted, open, or grounded to its external frame.

Before testing, unplug the refrigerator and disconnect all wiring, relays, and overload protectors attached to the compressor terminals.

1. Locate and Identify the Three Pins

On the side of the compressor, under the terminal cover, you will see three exposed copper/brass pins arranged in a triangle:

• C: Common
• S: Start
• R: Run

       [ C ] (Common)
        / \
       /   \
 (Start)    (Run)
     [ S ]---[ R ]

Note: The orientation varies (e.g., Common can be at the top or bottom). If the pins are unlabeled on the housing, you will identify them using resistance math in Step 2.

2. Test Winding Resistance (Open/Shorted Test)

Set your multimeter to the Lowest Resistance setting (e.g., 200Ω).

Measure resistance across all three pairs of pins and write down the numbers:

1. Common (C) to Start (S) → Intermediate Resistance
2. Common (C) to Run (R) → Lowest Resistance
3. Start (S) to Run (R) → Highest Resistance

The Golden Winding Formula

For a healthy single-phase compressor motor, the combined resistance across the Start and Run pins must equal the sum of the individual readings:

Resistance(S to R)​=Resistance(C to S)​+Resistance(C to R)​

Example Healthy Readings:

• C to R: 5Ω
• C to S: 10Ω
• S to R: 15Ω (5Ω+10Ω=15Ω)

Fault Conditions:

• Open Winding: Any pair reads “OL” or infinity (∞). The internal wire coil is burned through or snapped.
• Shorted Winding: A pair reads 0Ω or significantly lower than the expected sum. The enamel insulation on the internal wiring has melted, bypassing coil turns.
• Pin Identification Trick: If the pins are completely unmarked, the two pins that yield the highest resistance reading are Start and Run. The remaining third pin is Common.

3. Test Ground Short (Continuity to Ground)

This test checks if internal winding insulation has broken down and touched the steel compressor body, causing a dangerous ground fault or blown circuit breaker.

1. Set your multimeter to Continuity (⋅⋅⋅) or the Highest Resistance setting (e.g., 20MΩ).
2. Scrape a small patch of paint/rust off an unpainted copper line or clean metal surface on the compressor housing to expose bare metal.
3. Place one probe on bare metal frame and touch the other probe to each of the 3 pins (C, S, and R) one at a time.

 [ Bare Metal Frame ]  <────── Multimeter ──────>  [ C, S, or R Pin ]

Fault Conditions:

• Good: Multimeter displays “OL” / infinite resistance on all three pins (no connection between windings and frame).
• Bad (Grounded/Short to Ground): Any reading other than “OL” (a beep, 0Ω, or measurable resistance) indicates an internal electrical leak to ground. The compressor must be replaced.