Contactor
The low voltage controlled switch that connects line voltage to the outdoor unit.
Safety
What it is
A contactor is an electrically operated switch. A 24 volt coil pulls in a set of line voltage contacts, connecting power to the compressor and condenser fan. Control voltage does the deciding, line voltage does the work, and the two stay separated.
What can go wrong
- Pitted or burned contacts create resistance, heat and voltage drop across the contactor.
- Contacts that weld closed leave the outdoor unit running continuously, even with no call.
- An open coil means the contactor never pulls in, even though the thermostat is calling.
- Insects and debris under the armature prevent full closure and cause chattering.
How to diagnose it
Measure 24 volts across the coil terminals during a call
Confirms whether the fault is in the control circuit or in the contactor itself.
Measure voltage drop across each closed contact set
A meaningful drop across closed contacts indicates worn contacts under load.
Symptoms
Practice & Mastery
- Questions2
- Quiz1
- Contactor Troubleshooting Scenarios2
Practice is part of HVAC Pro — $5/month or $50/year.
AI Tutor
Ask AI about Contactor
The AI Tutor knows the topic you are reading and teaches from the Welcome to HVAC knowledge base. Included with HVAC Pro — $5/month or $50/year.