Air Conditioners - Testing and Replacing The Capacitor and Thermostat

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Air conditioners, window air conditioners, and portable air conditioners, all need the capacitor tested and replaced at one time or another.

Testing and replacing the capacitor - A short or open circuit in the capacitor on an air conditioner, and many portable air conditioner models, will stop the compressor- and the fan motor also, in units where the fan motor is part of the capacitor circuit. Usually there is only one capacitor in window air conditioners, but there may be two of them on Frigidaire air conditioner models.

To reach the capacitor, unplug the unit and remove the front grille. Take out retaining screws and lift off the control panel. If the capacitor is not there, remove the chassis from the cabinet and look for it next to the compressor or the fan motor.

Examine the capacitor. If it bulges, or if its vent plug is blown off and it leaks oil, the capacitor is defective and you should replace it. If nothing is visibly wrong, test the capacitor with a volt-ohm meter. Disconnect one of the leads to the capacitor. Set meter to the RX100 scale and touch its probes to the capacitor terminals. If the needle of the meter jumps toward zero ohms, then slowly return to high ohms, the capacitor is OK. If the needle jumps and remains at zero ohms- or if it does not move at all- the capacitor is faulty. Replace it. To double check, reverse the meter probes and repeat the test.

Some capacitors (those with three terminals) are designed to work in conjunction with the fan motor as well as the compressor. They incorporate two capacitors in one and should be tested twice with the volt-ohm meter. Touch one probe to the terminal labeled "C" (for common), the other probe to each of the other terminals in turn.

When replacing the capacitor, obtain a new one with the same rated capacitance and peak voltage as the indicated on the case of the old one. Remember  to label the leads before disconnecting them from the old capacitor. Connect them to the new one in the same order.

Testing and replacing the thermostat- The thermostat responds to variations in room temperature as measured by the sensing bulb: the bulb is mounted on the evaporator coils and is in turn controlled by a dial on the control panel. The thermostat switches the compressor on or off when the temperature goes above or below that on the dial setting.

To test the thermostat, unplug the air conditioner and remove the front grille. Take out retaining screws and pull the control panel away from the unit to make the thermostat terminals accessible. Test the thermostat with a volt-ohm meter and replace the thermostat if it defective. Some thermostats have more than two leads connected to them. In such cases, unless you are an experienced electrician and have the circuit diagram for your unit, call a repairman.

Caution: Discharge the capacitor before handling it.