Why Isn't Your AC Contactor Closing?

There's a tiny electrical component inside your air conditioner that's critical to its operation. The contactor is a small switch that operates similarly to a relay. Like a relay, your contactor allows a low-voltage control source to engage and disengage high-voltage power. Unlike a relay, the contactor in your AC can take power from two 120V sources and provide the necessary 240V for your compressor.

Since the contactor's job is to engage and disengage the compressor, your air conditioning system cannot function without it. If your contactor isn't making the necessary connection, your compressor won't turn on, and your air conditioner can't keep your house cool. Fortunately, it's not too hard to get to the bottom of a contactor that refuses to engage.

Contactors Explained

A contactor is a relatively straightforward electrical device that can connect or break an electrical circuit. In this case, the circuit is the high-voltage AC power that allows your air conditioning compressor to run. When your thermostat isn't calling for cooling, the contactor acts as a physical break in the circuit. When it's time to start cooling, the contactor completes the circuit and supplies power to the compressor.

A typical AC contactor has an incoming connection for each leg of your air conditioner's 240V electrical circuit. Additionally, the contactor receives a 24V signal from your furnace control board or your air handler unit if your HVAC system doesn't include a furnace. When the 24V line signals for cooling, the contactor pulls in a plunger and completes the circuit for your condenser fan and compressor.

Common Contactor Issues

Since the contactor is such a simple device, there are usually only three possible reasons that it will fail to start your air conditioner:

  • Physical damage
  • No high-voltage power
  • No low-voltage control signal

Physical damage typically occurs due to poor wiring. Loose high-voltage wires on the contactor can create arcs, physically melting the plastic or causing the plungers to seize. You'll usually see visible evidence of these problems, including burnt areas near the high-voltage terminals. You may be able to fix the problem by adjusting the wiring, but it's usually better to replace the contactor.

On the other hand, a lack of power or control signal often means there's a problem elsewhere in your system. You may have a tripped breaker or fuse, a wiring fault, or any number of other issues interrupting the circuit to your outdoor condenser unit. Since finding these problems is rarely straightforward, it's usually best to rely on an HVAC tech to help you get to the bottom of the issue and take care of your ac repair.