Capacitors are voltage storage devices used in electronic circuits, such as those found in heating and air conditioning fan motors and compressors. Capacitors come in 2 main types: electrolytic, which are used with vacuum tube and transistor power supplies, and non-electrolytic, which are used to regulate direct current surges. Electrolytic capacitors can fail by discharging too much current or by running out of electrolyte and being unable to hold a charge. Non-electrolytic capacitors most often fail by leaking their stored charge. There are several ways to test a capacitor to see if it still functions as it should.
The unit for capacitance is the farad, which is abbreviated with a capital “F.” You may also see the Greek letter mu (µ), which looks like a lowercase “u” with a tail in front of it. (Because the farad is a large unit, most capacitors measure capacitance in microfarads; a microfarad is a millionth of a farad.)
Connect the positive (red) multimeter lead to the capacitor anode lead and the negative (black) lead to the capacitor cathode lead. (On most capacitors, especially electrolytic capacitors, the anode lead is longer than the cathode lead.)
If the capacitance reading on the multimeter is close to the value printed on the capacitor itself, the capacitor is good. If it’s significantly less than the value printed on the capacitor, or zero, the capacitor is dead.