How do I test a refrigerator compressor start relay using a multimeter?

Testing a start relay with a multimeter determines whether its internal contacts or ceramic disk are intact or broken/shorted.

Before starting, unplug the refrigerator for safety. Access the compressor at the bottom rear, pull the plastic relay housing straight off the three compressor pins, and disconnect the wire harness.

1. Physical Inspection & Shake Test

Before using a multimeter, perform a simple physical check on the relay:

• Shake Test: Hold the relay in your hand and shake it gently.
  • Rattle: If you hear a loud rattle (like broken glass or sand inside), the internal ceramic disc of a Solid-State/PTC relay has shattered. The relay is defective and must be replaced; no further electrical testing is required.
  • No Rattle: Proceed to the multimeter tests below.
• Visual Check: Look for burnt plastic, melted casing, or charred terminals, which indicate electrical overheating.

2. Testing a Solid-State (PTC) Relay

Most modern residential refrigerators use a PTC start relay. These contain a ceramic disc that conducts electricity at room temperature.

           [ Terminal N/L ]
                 │
           ┌─────┴─────┐
           │ Ceramic   │
           │  Disc     │
           └─────┬─────┐
                 │
           [ Start Pin ]

Steps:

1. Set your multimeter to the Resistance / Ohms (Ω) setting (or Lowest Resistance scale, e.g., 200Ω).
2. Place one probe on the main power terminal input (where the wire connects) and the other probe on the socket hole that plugs onto the compressor’s Start pin.
3. Check the Reading:
   • Good Reading: Between 10 Ω and 30 Ω at room temperature (around 20°C / 68°F).
   • Bad (Open Circuit / OL): Infinity (∞) or “OL”, indicating the ceramic disc is cracked or burnt open.
   • Bad (Short Circuit): Near 0Ω or significantly lower than 10Ω, meaning current will not properly taper off during startup.

3. Testing an Electromechanical (Current) Relay

Older or heavy-duty units use a coil-driven relay with mechanical contacts. These operate based on gravity and magnetic coil strength.

         ┌─────────────────┐
         │   Relay Coil    │  --> Terminals
         └────────┬────────┘
                  │ Plunger (Closes Start Contacts on High Current)
         [ Compressor Pins ]

Steps:

1. Set your multimeter to Resistance / Ohms (Ω).
2. Test the Coil: Place probes across the two primary line terminals connected to the internal wire coil.
   • Good Reading: Very low resistance, typically 0.5 Ω to 3 Ω.
   • Bad Reading: “OL” / Open Circuit (broken coil wire).
3. Test the Gravity Contacts:
   • Hold the relay right-side up (in its normal mounted orientation) and put probes across the main terminal and the Start pin socket.
     • Expected: Open Circuit (“OL” or infinite resistance) because gravity keeps the contacts resting open.
   • Turn the relay upside down.
     • Expected: Near 0Ω (Continuity), as gravity drops the internal plunger onto the contacts.
   • Faulty Condition: If the contacts read closed when right-side up, or fail to close when turned upside down, the internal plunger is jammed or corroded.

4. Testing the Overload Protector (If Integrated or Combo Unit)

Many modern fridges feature an integrated “Combination Relay” containing both the PTC relay and the thermal overload protector in a single block.

1. Locate the Overload Protector terminals (usually connected to the compressor’s Common pin).
2. Set your multimeter to Continuity (⋅⋅⋅) or lowest Ohms (Ω).
3. Place probes on both sides of the overload protector at room temperature.
4. Good Reading: Continuous tone or near 0Ω (closed circuit).
5. Bad Reading: “OL” / Infinite resistance, indicating the bimetal strip inside has tripped permanently or burned out.