Modern central air conditioning systems are fairly sophisticated, with numerous potential failure points. Electrical failures are one category that's often easy to spot, although these issues may not be as common as you expect. Since it's a relatively easy part to replace (and many how-to guides are available online), people often mistakenly blame their contactors for AC problems.

While contactors can and do fail, they aren't typically the source of electrical problems with an air conditioner. To understand why this is the case (and where the problem may lie), it's important first to understand the role of this electrical component in your air conditioning system.

How AC Contactors Work

A typical central air conditioning system uses 220-240V. Because of the design of the US electrical grid, 220V appliances require two separate 110V legs. These legs enter your condenser unit on two wires and connect to the contactor. The contactor will also have a low-voltage wire that connects to your HVAC circuit board and acts as a control wire.

When the HVAC control board signals for cooling, the coil in the contactor energizes and produces a magnetic field that pulls a plunger closed. The plunger completes the circuit across the contactor, energizing the side that connects to your air conditioner's capacitor and ultimately powers the compressor.

While the contactor's operation relies on complicated physics, the device is relatively simple. It consists of only a few wire terminals, one or two plungers, and a wire coil. As a result, there aren't many potential failure points.

Why AC Contactors Fails

When a contactor fails, it's typically due to a physical problem with the plunger or damage to the wire terminals. For example, loose wiring connections can cause arcing. Over time, electrical arcs will burn the terminals and the contactor, making a less effective connection or even burning the wire insulation and creating a short. Either way, your system won't turn on since the contactor won't energize.

Physical problems with the plunger are another potential issue. Debris or even bugs can get stuck under the plunger, stopping it from snapping shut. Corrosion can also affect the plunger springs, stopping the plunger from snapping open and closing smoothly. These problems will typically only occur on older units, especially those with contactors that lack cover plates.

However, these situations are relatively rare compared to other problems. Even if you can't hear your contactor closing, there's more likely a problem somewhere else affecting your 220V supply or low-voltage control wires. Since diagnosing these problems requires checking potentially dangerous voltage levels, it's usually best to rely on a professional to find the true cause of your AC issue.

For more information contact an AC repair service in your area such as Turnbull Heating & Air Conditioning.