Capacitors are electronic components that store electrical energy. The A/C capacitor provides voltage to enable the motor to spin and thus start the air conditioner. If a capacitor leaks oil or doesn't hold a charge, it likely needs to be replaced. A compressor that hums but doesn't start may also indicate a faulty capacitor. Modern A/C capacitors have a limited lifetime and often require replacement.
Common Types Of A/C Capacitors
Some A/C units use a single capacitor, but dual capacitors are more the norm. Dual capacitors serve both the fan and compressor motors. A/C capacitors are typically intended for use with motors that have specific voltage ranges and horsepower. Capacitors also vary by size. Starting capacitors—which can provide an extra voltage boost--may also be added to an A/C unit as a part of the repair process if the unit's motor is having trouble starting up. This type of problem can occur in unbalanced cooling systems, where the refrigerant pressure on the sides of the unit are unequal, or for motors nearing the end of their usable lives.
Buying Replacement A/C Capacitors
Replacing the A/C capacitor yourself can be hazardous. Even when not connected to a power source, capacitors can hold enough electrical charge to hurt, or even kill a person. Before proceeding, the terminals across the top of the capacitor should be shorted. When purchasing an A/C capacitor, the voltage and horsepower range of the A/C unit's motor must match that of the capacitor. Most starter capacitors will match a range of different motors. Consult the documentation for the capacitor to ensure it will work with your A/C unit's motor.
Who Makes A/C Capacitors?
A/C capacitors are typically available from the manufacturer of the A/C unit as a replacement part. Manufacturers of motor-start capacitors used in A/C units include: Cinco, General Electric, Mars and Philips.