How do I test a refrigerator compressor start relay and defrost heater using a multimeter?
To test both components safely, unplug the refrigerator from the wall outlet before starting. Both tests measure electrical resistance (Ohms/Ω) and require disconnecting the component from its wiring harness first.
Testing start relay terminal resistance. Source: FixAppLab – Appliance Repair Guides
1
Disconnect the Start Relay
Located at the back bottom of the fridge next to the compressor
Remove the rear access panel. Pull the small plastic relay box directly off the compressor’s 3 metallic terminal pins. Gently detach the wire leads from the relay terminals using needle-nose pliers.
2
Perform a Shake Test (Solid State Relays)
Quick physical check before using the meter
Gently shake the relay near your ear. If it rattles like broken glass or loose sand inside, the internal ceramic disc is shattered. The relay is dead and needs immediate replacement—no multimeter test required.
3
Measure Start Relay Resistance
Set multimeter to Ohms (Ω) or Continuity mode
Insert meter probes into the two terminal slots (Start and Run terminals) where the compressor pins plug in.
- Good Relay: Reads low resistance, typically between 10 Ω and 40 Ω (or beeps for continuity).
- Faulty Relay: Shows OL (Open Loop / Infinite resistance) or 0 Ω (short circuit).
Testing defrost heater element resistance. Source: iFixit
1
Expose the Defrost Heater
Located underneath the evaporator coils in the freezer
Remove the freezer back interior panel. Unplug the two wire harness leads connected directly to the heating element (calrod glass tube or aluminum sheath at the bottom of the cooling coils).
2
Measure Defrost Heater Resistance
Set multimeter to lowest resistance range (200 Ω)
Touch one multimeter probe to each of the heater’s two wiring terminals.
- Good Heater: Reads a steady resistance between 20 Ω and 120 Ω (depending on wattage).
- Faulty Heater: Displays OL (Open Loop), meaning the heating filament inside is broken and cannot complete a circuit.