How To Test The Hard Start Capacitor In A Central AC Unit

A central air conditioning unit turns on when an electrical signal is sent from the thermostat inside your home. The start-up process requires a supply of electricity to get all the parts going. The boost comes from a start capacitor, which is a little piece that stores extra electricity and provides an added boost of electricity. If your unit is older, a previous owner or tech might have installed a hard start capacitor, which provides an even larger boost than a normal start capacitor and can make an older, less efficient unit more efficient.

If your unit has a hard start capacitor and your AC has recently experienced problems starting up properly, you might have a broken capacitor on your hands. You can test to see if the capacitor is working with a multimeter so as to rule in or out this particular problem.

What You Need:

  • Screwdriver
  • Multi-meter

Step 1: Find the Hard Start Capacitor

Switch off the fuse or breaker to cut all electricity to the condensing unit. Use a screwdriver to remove the fasteners holding the condensing unit's grated cover in place. Place the screws in your pocket to reuse them later and set the cover somewhere it won't become dirty or damaged.

Locate the hard start capacitor. Use your owner's manual to find the regular start capacitor, if needed, since the hard capacitor will be in that same area.

Step 2: Discharge and Test the Hard Start Capacitor

Capacitors act as energy stores, which means that the capacitor will hold electricity even when the power is turned off. You will need to remove that electricity before the test.

Turn your multimeter to AC setting. Pull the wires out of each terminal on the end of the capacitor and let the wires dangle in place. Put each probe from your multimeter on one of those terminals. Watch the reading until the number drops to nothing. The capacitor is now drained.

Step 3: Test the Capacitor and Fixing Options

Remove the multimeter probes from the terminals. Turn the multimeter setting to "ohms." Place the probes back on the terminals and check the reading. Look at the side of the capacitor for the appropriate ohms range for that piece. Make sure the reading matches that range.

If the reading does match that range, your air conditioner's problem is elsewhere in the system. If the reading isn't in the range, your hard start capacitor has broken, which also makes it likely that your compressor is near the end of its lifespan. Call in an HVAC technician for help diagnosing further problems or for a new capacitor and compressor. Companies like John Legg's Heating & Air Conditioning may be able to help meet your needs in this area.