How to Test an AC Contactor: Coil, Contacts and Voltage Drop

The contactor is the switch that sends line voltage to the compressor and condenser fan when the thermostat calls. It's also one of the most replaced parts in a condenser, partly because it's often replaced without being tested.

Safety: Contactors carry line voltage. Many residential contactors are single-pole, which means one leg stays energized even when the contactor is open. Always verify both legs are dead with a meter before touching anything.

1. Does the coil get 24 V?

With a call for cooling, measure across the two coil terminals.

• About 24 V and the contactor is pulled in: the coil is working. Move on to the contacts.
• About 24 V but not pulled in: the coil is open, or the armature is stuck. Check coil resistance.
• 0 V: the problem is upstream, in the thermostat, wiring, a safety (pressure switch, float switch) or a board lockout.

2. Coil resistance (power off)

Remove one coil wire and measure across the coil. You should see a reading, usually in the tens of ohms (check the spec if it's listed). OL/open means the coil has failed. A dead short means a shorted coil, which can also blow the low-voltage fuse.

3. The contacts

Look: with power locked out, inspect the contact surfaces. Heavy pitting, burning or welding (the contacts stuck closed) means replace. Ants and debris between contacts are a classic problem in some regions.

Measure voltage drop under load: this is the real test. With the unit running, measure across each set of contacts (line side to load side of the same pole).

• Close to 0 V: good contacts.
• A volt or more: resistance across pitted contacts. That creates heat, low voltage at the compressor and eventually failure. Replace the contactor.

You can also compare line-side and load-side voltage to the compressor. Any meaningful difference is lost across the contacts.

4. Stuck closed

If the outdoor unit keeps running after the thermostat is satisfied, check whether the contactor stays pulled in with no 24 V. If it does, the contacts are welded or the armature is stuck. Replace it.

Replacing

Match coil voltage (usually 24 VAC), number of poles, and a full-load amp rating at or above the original. Move wires one at a time and tighten connections properly.

FAQ

Can I just replace the contactor without testing it?

You can, but test first. A contactor that won't pull in often has a healthy coil and an upstream problem, like a tripped pressure switch, that will show up again.

Why do contactors burn up?

Normal wear from arcing, made worse by low voltage, hard-starting compressors, short cycling and loose connections.

What is a single-pole contactor?

It switches only one leg of the 240 V supply. The other leg goes straight to the load, so the unit isn't fully de-energized when the contactor opens.

Sources

• HVAC School: technical tips and diagnostics