Capacitor Testing and Replacement Procedure


❶ Understand What a Run Capacitor Does

The run capacitor provides continuous phase-shifted current to the motor start winding allowing the motor to run

  1. With the design efficiency
  2. In the right direction
  3. With the appropriate torque
  4. With near “unity” power factor (power factor near 1.0)

If the run capacitor is failed often the motor won't run in the case of high torque motors like a compressor or in the case of fans they may run backwards or slowly or with high amperage or overheat.


❷ Understand Why a Run Capacitor Fails

A run capacitor fails due to

  1. Poor manufacturing quality
  2. Overtemperature (They got hot)
  3. Over Voltage (The voltage was too high)

Many will say a failed motor “takes out” the capacitor. In actuality a failed or weak capacitor can take out a motor

❸ Visually Inspect

If a capacitor shows physical damage such as the top bubbling or oil leakage then it should be replaced. Normal rust is not a reason to replace a capacitor. Note the MFD or uF rating listed on the capacitor. The voltage rating is also worth noting but you may use a HIGHER voltage rated capacitor but not lower.



❹ Decide if under load or bench testing is best

If the system is currently running then an under load test may be best. Do not do an under load test on blower capacitors due to the risk of the meter leads around a spinning blower wheel. If the system is NOT running then a bench test will be the best bet.


❺ Bench or Under Load Test?

Choose bench for simplicity or if the system is not running. Choose under load because it can be done in real load conditions on a running system.

Bench Test Go to step ❼

Under Load Test Go to step ❻


❻ Test Under Load

In order to test under load, you need to take measurements with the system running. Wear proper PPE and only do so when safe. You need to have an accurate multimeter that can measure Voltage and Amperage reliably. Often under load measurements may come out high if the amp clamp picks up interference from other circuits. Measure the amperage on the start wire with the wire centered in the clamp and multiply by 2652. Now measure the voltage across the capacitor and divide the amperage x 2652 by that voltage to find the capacitance in MFD.If the under load MFD is less than 10% low we suggest replacement. If it is over the rating it is often a mis-measurement

If a capacitor measures weak via under load test go ahead and perform the bench test.


❼ Bench Test

Bench Testing is simply removing both leads from the run capacitor after safely disconnecting power and discharging the capacitor. You then place a meter designed to test capacitance across the terminals and note the reading. Be careful not to touch the meter probes and to get a good solid connection to the metal connection spades on the capacitor.If the measurement is more than 10% we suggest a replacement


❽ Inspect the dual run capacitor and note where each wire is connected

Taking a picture is one of the easiest ways to remember before removing the wires.

❾ Remove disconnect

It is imperative that the disconnect is removed or the unit is off and without potential. Test using a meter that is pre-tested to a known voltage source and check L1 to L2 and L1 – ground and L-2 ground to ensure no voltage is present.


❿ Discharge capacitor

Before testing, touching or removing the capacitor, you need to discharge it. Do this using a high resistance resistor bridging HERM and Fan terminals across to C or across a single capacitor.

NOTE: It is actually very rare for a run capacitor to contain a charge on a normally running system because it bleeds off through the compressor windings UNLESS one of the windings is open. FOr this reason many techs opt to just use a screwdriver to discharge which is controversial but common practice.


⓫ Disconnect wires and remove used dual run capacitor

To remove, disconnect wires on the top of the capacitor and also remove the strap holding the capacitor in place.


⓬ Replace capacitor with same MFD rating

Apples to Apples: You must use the same MFD rating capacitor during this process. This will be located on the box and also the side of the capacitor. Mount the capacitor upright with terminals pointed up.TEST THE NEW CAPACITOR VIA BENCH TEST BEFORE INSTALLING

⓭ Re-sizing of strap

Sometimes the new capacitor might be larger or smaller. At this time, use the metal strapping and create a new strap for the capacitor, by cutting it to the correct size and using self tapping screws to attach to the correct area. Always check before using self tappers to confirm that you are not in any danger of puncturing the coil.


⓮ Attach the wires

At this time reattach the wires onto the top of the dual run capacitor. Make sure that the common is connected, the HERM (compressor) is connected and the FAN is connected. You should always double-check to make sure all wires are in their proper places. Make sure the terminal fit very snug, tighten them by squeezing with a needle nose before installing to make sure they are very tight.


⓯ Test system

Reconnect the disconnect or flip the breaker back on. Check to make sure that all aspects of the system are running and the compressor and fan are running at the proper amperage.Using a power meter and testing motor power factor to ensure it is near unity is a good additional practice.


⓰ Clean up and reattach panel on the condenser

Make sure you clean up the area and reattach the panel to the condenser

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One response to “Capacitor Testing and Replacement Procedure”

  1. Good morning,
    Thank you for doing what you do, you are doing a great service for our industry. One question about today’s article. It says: “If the under load MFD is less than 10% low we suggest replacement.” Do you mean if the under load MFD is more than 10% low?
    Thanks again

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