Small PCB relay with its cover removed, showing the coil and switching contacts

Generic photo: Raimond Spekking · CC BY-SA 4.0

Contactor (HVAC): coil, contacts and voltage drop

The contactor closes line voltage to the compressor and fan when it gets 24 V on its coil. Check coil voltage, coil resistance, contact condition and voltage drop across closed contacts.

Cross-checked ~25 min moderateBrands: All

Before you start

Line side of the contactor stays live until the disconnect is pulled
Live-voltage tests: use a CAT III-rated meter, insulated probes and one hand; keep the other hand away from the chassis
Never push the contactor in by hand with a tool to force a start

Diagnostic steps

Unlock all 5 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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