A contactor is an electric switch that connects power to the compressor or fan when the control calls for cooling. A failing contactor can chatter, fail to close, stay closed or overheat. Symptoms include a compressor that will not start or will not stop.
The contactor is a small, cheap part that takes a lot of abuse. Every start sends a surge of current through its contacts, and over years they pit and weld together. It is a frequent culprit when the box is warm and the compressor is silent.
What it does
A low-voltage coil pulls in a set of heavy contacts when the thermostat or controller calls for cooling. The contacts pass line voltage to the compressor. When the call ends, the coil releases and a spring opens the contacts. Some contactors have two poles for single-phase and three for three-phase equipment.
Signs it is failing
- Chattering or buzzing noise from the electrical box.
- Compressor does not start though the controller calls.
- Compressor keeps running after the call ends.
- Burnt smell or blackened terminals.
- Intermittent starts that get worse with heat.
- Breaker trips on start.
Common causes of failure
| Cause | What happens |
|---|---|
| Pitted contacts | Arcing wears the surface and raises resistance |
| Welded contacts | The compressor cannot switch off |
| Low control voltage | The coil cannot pull in fully |
| Loose wires | Heat at the terminals damages the body |
| Dust, moisture or insects | Blocks movement and causes shorts |
What you can safely check
- Listen. A firm click at start is normal. Chattering is not.
- Look from the outside. Do you see scorch marks or melted plastic through the cover?
- Check the disconnect and breaker. Reset once only.
- Note the pattern. Does it fail more in the afternoon heat?
This part is on live power. Do not open the electrical box, bridge terminals or reset repeatedly. Only a qualified technician should test or replace it.
What a technician does
- Measures coil voltage and contact resistance.
- Inspects contacts for pitting and welding.
- Tightens or replaces damaged terminals and wires.
- Replaces the contactor with the correct rating.
- Checks the compressor and capacitors for damage caused by the fault.
Keeping it healthy
- Have the electrical panel inspected at yearly service.
- Keep the enclosure closed and free of dust and insects.
- Replace early if contacts show pitting.
- Fix short cycling, which wears contacts fast.
A contactor rarely fails alone. If it was stuck, the compressor may have run without protection, so ask the technician to check the motor as well.
When to call for service
Call promptly if the box is warm and the compressor is silent, if you smell burning near the electrical box, or if the compressor will not shut off. A stuck contactor can run a compressor until it fails, and food may be at risk while you wait.
Repair or replace
Contactors are replaced, not repaired, and the swap is quick and inexpensive compared with most refrigeration work. The important question is why it failed. Heat, loose wires or short cycling will kill the new one too. See compressor humming but not starting for related symptoms.
Related reading: the parts and components guide, the troubleshooting guide and tripping the breaker.
Questions about this part
Can a bad contactor cause a warm box?
Yes. If it does not close, the compressor never gets power.
Is a buzzing contactor dangerous?
It can overheat and burn. Have it checked soon.
Does every walk-in have one?
Almost all do, for the compressor and often for fans and heaters.