How to Troubleshoot Air Conditioner Contacts

Air conditioner contacts transfer full voltage to the compressor and blower fans. The full voltage is generally 220 to 240 volts alternating current (VAC). The contactor is made up of two separate distinct parts, the small metal disc contacts and the solenoid coil. The metal contacts act as a switch to turn the voltage on and off. The solenoid coil pulls the contacts together. Typically the solenoid coil is powered by a lower voltage of 24 VAC. The smaller voltage is provided by a low voltage transformer.

Things You'll Need

• Voltmeter
• Wire schematic

Instructions

1. Remove all electrical power from the air conditioning system. Shut off the circuit breakers or pull the fuses supplying the power.

2. Access the contactor that requires troubleshooting. There may be several contactors in an air conditioning system; consult the manufacturer's specifications for location and wiring schematics.

3. Inspect the condition of the contactor and the small metal discs. If the contacts are black and severely pitted, replace the individual contactor as a whole. If any wires connected to the contactor are burnt or charred in any way, contact a certified technician as other parts to the system may be severely damaged. If the contactor and wires appear to be undamaged continue to the next step.

4. Apply power to the air conditioner system. Test the full voltage going to the contactor with the voltmeter. Turn the voltmeter to "volts." Touch the red lead from the voltmeter to one of the wires coming from the power supply on the contactor terminals. Touch the black lead to the other power wire. The voltage should read between 220 and 240 VAC for a two-wire power supply. If the contactor is a one-wire type, touch the red lead to the wire and the black lead to any metal part on the air conditioner frame. The meter display should read between 110 and 120 VAC. If the meter display reads 0 VAC, no power is reaching the contactor terminals. Check the wiring schematic for power source and inline fuses.

5. Test the solenoid coil if the full voltage is correct from the step above. Touch the red lead from the meter to one of the coil terminals and the black lead to the other coil terminal. The meter display should read 24 VAC with the power on and the unit calling for cooling. If the meter display reads 0 VAC, no power is reaching the coil. The low voltage transformer may be bad. If power is reaching the coil terminals, and the contactor still does not operate; the solenoid coil is bad and the contactor requires replacement.

Tips & Warnings

• The air conditioning wiring system is complex and requires an understanding of electrical schematics to trace problems beyond contactor operation.

References

• "Principles of Refrigeration"; R. Warren Marsh, C. Thomas Olivo; 1960
• Inspectapedia: Air Conditioning Diagnosis, Inspection, Repair Guide
• Arnold Service: Troubleshooting Air Conditioning Problems