How to Test Motor Run Capacitors

A motor run capacitor is a common component of HVAC systems, particularly on fan compressors and fan motors. The capacitor stores an electrical charge and is comprised of one or more pairs of conductors separated by an insulator. Run capacitors maintain a fairly consistent voltage supply and increase the torque of HVAC motors at startup. If you need to run a capacitor test on your system, there are several safety measures that should be taken. The test itself is fairly simple, however.

Before you attempt to test your capacitor, remove all electrical fuses or cords from the unit in which it is housed. To be safe, turn the circuit breaker for the unit off at the breaker box. Inform others that you will be doing electrical work and request that they refrain from flipping any switches until you have finished. Additionally, use insulated gloves and tools whenever possible to protect yourself from shock. If at any point you feel you don't understand the instructions involved in testing your run capacitor, it's best to invoke the help of a professional.

To check your capacitor, you'll need a multimeter. Once the power supply is shut off, you can use a screwdriver across the two terminals on the capacitor to short it and discharge the unit. This will help to prevent shock.

Next, take a photo or make a note of which wire connects to which terminal on the capacitor. Then, use needle-nosed pliers to remove the wires from the capacitor. Apply the two leads of the multimeter to the capacitor. It should give you a digital reading, ideally within 6 percent of the manufacturer specifications for the capacitor. If you find that the reading is more than 6 percent lower than the rating on the unit, it should be replaced.

If you have an analog multimeter, shut off the power supply, discharge the capacitor and remove the wires from the capacitor as outlined above. Then, turn the multimeter resistance to its maximum level and apply its leads to the capacitor terminals. You should see the resistance reading start at zero and rise to the maximum.

Beyond just issues with the capacitor's multimeter readings, it's possible to observe problems with the unit visually. If the unit is bulging, it has gone bad. Similarly, if you see any substance, particularly an oily one, leaking from the capacitor, it will need to be replaced. Remove the capacitor from your HVAC system when the power is turned off safely. You will need to dispose of the capacitor properly and restrictions may vary by jurisdiction.